Virtual University of Pakistan Pakistan...1 ๐ Energy of S.H.M If T is the kinetic energy, V the...
Transcript of Virtual University of Pakistan Pakistan...1 ๐ Energy of S.H.M If T is the kinetic energy, V the...
Lecture Handouts
on
VECTORS AND CLASSICAL MECHANICS
(MTH-622)
Virtual University of Pakistan
Pakistan
ii
About the Handouts
The following books have been mainly followed to prepare the slides and handouts:
1. Spiegel, M.R., Theory and Problems of Vector Analysis: And an Introduction to Tensor
Analysis. 1959: McGraw-Hill.
2. Spiegel, M.S., Theory and problems of theoretical mechanics. 1967: Schaum.
3. Taylor, J.R., Classical Mechanics. 2005: University Science Books.
4. DiBenedetto, E., Classical Mechanics: Theory and Mathematical Modeling. 2010:
Birkhรคuser Boston.
5. Fowles, G.R. and G.L. Cassiday, Analytical Mechanics. 2005: Thomson Brooks/Cole.
The first two books were considered as main text books. Therefore the students are advised to
read the first two books in addition to these handouts. In addition to the above mentioned books,
some other reference book and material was used to get these handouts prepared.
3
Contents Module No. 101............................................................................................................................ 10
Example of Non- Conservative Field ..................................................................................... 10
Module No. 102............................................................................................................................ 11
Introduction to Simple Harmonic Motion and Oscillator ................................................... 11
Module No. 103............................................................................................................................ 13
Amplitude, Time period, Frequency and Energy of S.H.M ................................................. 13
Module No. 104............................................................................................................................ 14
Example of S.H.M ................................................................................................................... 14
Module No. 105............................................................................................................................ 16
Example of Energy of S.H.M .................................................................................................. 16
Module No. 106............................................................................................................................ 18
Damped Harmonic Oscillator ................................................................................................ 18
Module No. 107............................................................................................................................ 19
Example of Damped Harmonic Oscillator ........................................................................... 19
Module No. 108............................................................................................................................ 21
Eulerโs Theorem โ Derivation ............................................................................................... 21
Module No. 109............................................................................................................................ 23
Chaslesโ Theorem.................................................................................................................... 23
Module No. 110............................................................................................................................ 24
Kinematics of a System of Particles ...................................................................................... 24
Module No. 111............................................................................................................................ 25
The Concept of Rectilinear Motion of Particles, Uniform Rectilinear Motion, Uniformly
Accelerated Rectilinear Motion ............................................................................................. 25
Module No. 112............................................................................................................................ 27
The Concept of Curvilinear Motion of Particles .................................................................. 27
Module No. 113............................................................................................................................ 28
Example Related to Curvilinear Coordinates ...................................................................... 28
Module No. 114............................................................................................................................ 30
Introduction to Projectile, Motion of a Projectile ................................................................ 30
Module No. 115............................................................................................................................ 33
Conservation of Energy for a System of Particles ............................................................... 33
Module No. 116............................................................................................................................ 34
Conservation of Energy .......................................................................................................... 34
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Module No. 117............................................................................................................................ 36
Introduction to Impulse โ Derivation ................................................................................... 36
Module No. 118............................................................................................................................ 37
Example of Impulse ................................................................................................................ 37
Module No. 119............................................................................................................................ 38
Torque ...................................................................................................................................... 38
Module No. 120............................................................................................................................ 39
Example of Torque ................................................................................................................. 39
Module No. 121............................................................................................................................ 41
Introduction to Rigid Bodies and Elastic Bodies ................................................................. 41
Module No. 122............................................................................................................................ 42
Properties of Rigid Bodies ...................................................................................................... 42
Module No. 123............................................................................................................................ 43
Instantaneous Axis and Center of Rotation.......................................................................... 43
Module No. 124............................................................................................................................ 44
Centre of Mass & Motion of the Center of Mass ................................................................. 44
Module No. 125............................................................................................................................ 46
Centre of Mass & Motion of the Center of Mass ................................................................. 46
Module No. 126............................................................................................................................ 48
Example of moment of Inertia and Product of Inertia ........................................................ 48
Module No. 127............................................................................................................................ 49
Radius of Gyration .................................................................................................................. 49
Module No. 128............................................................................................................................ 50
Principal Axes for the Inertia Matrix ................................................................................... 50
Module No. 129............................................................................................................................ 51
Introduction to the Dynamics of a System of Particles ....................................................... 51
Module No. 130............................................................................................................................ 54
Introduction to Center of Mass and Linear Momentum .................................................... 54
Module No. 131............................................................................................................................ 55
Law of Conservation of Momentum for Multiple Particles ................................................ 55
Module No. 132............................................................................................................................ 57
Example of Conservation of Momentum .............................................................................. 57
Module No. 133............................................................................................................................ 60
Angular Momentum โ Derivation ......................................................................................... 60
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Module No. 134............................................................................................................................ 62
Angular Momentum in Case of Continuous Distribution of Mass..................................... 62
Module No. 135............................................................................................................................ 64
Law of Conservation of Angular Momentum ...................................................................... 64
Module No. 136............................................................................................................................ 66
Example Related to Angular Momentum ............................................................................. 66
Module No. 137............................................................................................................................ 68
Kinetic Energy of a System about Principal Axes โ Derivation ......................................... 68
Module No. 138............................................................................................................................ 70
Moment of Inertia of a Rigid Body about a Given Line ...................................................... 70
Module No. 139............................................................................................................................ 72
Example of M.I of a Rigid Body About Given Line............................................................. 72
Module No. 140............................................................................................................................ 75
Ellipsoid of Inertia .................................................................................................................. 75
Module No. 141............................................................................................................................ 76
Rotational Kinetic Energy ...................................................................................................... 76
Module No. 142............................................................................................................................ 79
Moment of Inertia & Angular Momentum in Tensor Notation ......................................... 79
Module No. 143............................................................................................................................ 81
Introduction to Special Moments of Inertia ......................................................................... 81
Module No. 144............................................................................................................................ 83
M.I. of the Thin Rod โ Derivation ......................................................................................... 83
Module No. 145............................................................................................................................ 85
M.I. of Hoop or Circular Ring โ Derivation ......................................................................... 85
Module No. 146............................................................................................................................ 87
M.I. of Annular Disk - Derivation ......................................................................................... 87
Module No. 147............................................................................................................................ 89
M.I. of a Circular Disk - Derivation ...................................................................................... 89
Module No. 148............................................................................................................................ 91
Rectangular Plate โ Derivation.............................................................................................. 91
Module No. 149............................................................................................................................ 92
M.I. of Square Plate โ Derivation .......................................................................................... 92
Module No. 150............................................................................................................................ 94
M.I. of Triangular Lamina โ Derivation .............................................................................. 94
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Module No. 151............................................................................................................................ 96
M.I. of Elliptical Plate along its Major Axis โ Derivation ................................................... 96
Module No. 152............................................................................................................................ 98
M.I. of a Solid Circular Cylinder - Derivation ..................................................................... 98
Module No. 153.......................................................................................................................... 100
M.I. of Hollow Cylindrical Shell - Derivation .................................................................... 100
Module No. 154.......................................................................................................................... 102
M.I of Solid Sphere - Derivation .......................................................................................... 102
Module No. 155.......................................................................................................................... 104
M.I. of the Hollow Sphere โ Derivation .............................................................................. 104
Module No. 156.......................................................................................................................... 107
Inertia Matrix / Tensor of solid Cuboid .............................................................................. 107
Module No. 157.......................................................................................................................... 110
Inertia Matrix / Tensor of solid Cuboid .............................................................................. 110
Module No. 158.......................................................................................................................... 113
M.I of Hemi-Sphere โ Derivation ........................................................................................ 113
Module No. 159.......................................................................................................................... 116
M.I. of Ellipsoid โDerivation................................................................................................ 116
Module No. 160.......................................................................................................................... 120
Example 1 of Moment of Inertia.......................................................................................... 120
Module No. 161.......................................................................................................................... 122
Example 2 of Moment of Inertia.......................................................................................... 122
Module No. 162.......................................................................................................................... 124
Example 3 of Moment of Inertia.......................................................................................... 124
Module No. 163.......................................................................................................................... 126
Example 4 of Moment of Inertia.......................................................................................... 126
Module No. 164.......................................................................................................................... 129
Example 5 of Moment of Inertia.......................................................................................... 129
Module No. 165.......................................................................................................................... 131
Example 5 of Moment of Inertia.......................................................................................... 131
Module No. 166.......................................................................................................................... 133
Example 1 of Parallel Axis Theorem ................................................................................... 133
Module No. 167.......................................................................................................................... 135
Example 2 of Parallel Axis Theorem ................................................................................... 135
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Module No. 168.......................................................................................................................... 138
Example 3 of Parallel Axis Theorem ................................................................................... 138
Module No. 169.......................................................................................................................... 140
Example 4 of Parallel Axis Theorem ................................................................................... 140
Module No. 170.......................................................................................................................... 141
Perpendicular Axis Theorem ............................................................................................... 141
Module No. 171.......................................................................................................................... 143
Example 1 of Perpendicular Axis Theorem........................................................................ 143
Module No. 172.......................................................................................................................... 145
Example 2 of Perpendicular Axis Theorem........................................................................ 145
Module No. 173.......................................................................................................................... 149
Example 3 of Perpendicular Axis Theorem........................................................................ 149
Module No. 174.......................................................................................................................... 152
Problem of Moment of Inertia ............................................................................................. 152
Module No. 175.......................................................................................................................... 154
Existence of Principle Axes .................................................................................................. 154
Module No. 176.......................................................................................................................... 157
Determination of Principal Axes of Other Two When One is known .............................. 157
Module No. 177.......................................................................................................................... 159
Determination of Principal Axes by Diagonalizing the Inertia Matrix ........................... 159
Module No. 178.......................................................................................................................... 162
Relation of Fixed and Rotating Frames of Reference........................................................ 162
Module No. 179.......................................................................................................................... 165
Equation of Motion in Rotating Frame of Reference ........................................................ 165
Module No. 180.......................................................................................................................... 168
Example 1 of Equation of Motion in Rotating Frame of Reference................................. 168
Module No. 181.......................................................................................................................... 171
Example 2 of Equation of Motion in Rotating Frame of Reference................................. 171
Module No. 182.......................................................................................................................... 174
Example 3 of Equation of Motion in Rotating Frame of Reference................................. 174
Module No. 183.......................................................................................................................... 177
Example 4 of Equation of Motion in Rotating Frame of Reference................................. 177
Module No. 184.......................................................................................................................... 181
Example 5 of Equation of Motion in Rotating Frame of Reference................................. 181
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Module No. 185.......................................................................................................................... 183
General Motion of a Rigid Body .......................................................................................... 183
Module No. 186.......................................................................................................................... 185
Equation of Motion Relative to Coordinate System Fixed on Earth ............................... 185
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Module No. 101
Example of Non- Conservative Field
Problem:
Show that the force field given by
๏ฟฝ๏ฟฝ = ๐ฅ2๐ฆ๐ง๐ โ ๐ฅ๐ฆ๐ง2๐ + 2๐ฅ๐ง๏ฟฝ๏ฟฝ
is non-conservative.
Solution:
We have
๏ฟฝ๏ฟฝ = ๐ฅ2๐ฆ๐ง๐ โ ๐ฅ๐ฆ๐ง2๐ + 2๐ฅ๐ง๏ฟฝ๏ฟฝ
To verify whether the force field F is conservative or not, we will check whether ๐ป ร ๏ฟฝ๏ฟฝ = 0 or
not.
โ ร ๏ฟฝ๏ฟฝ = |
๐ ๐ ๏ฟฝ๏ฟฝ๐
๐๐ฅโ ๐๐๐ฆโ ๐
๐๐งโ
๐ฅ2๐ฆ๐ง โ๐ฅ๐ฆ๐ง2 2๐ฅ๐ง
|
= ๐ [๐
๐๐ฆ(2๐ฅ๐ง) โ
๐
๐๐ง(โ๐ฅ๐ฆ๐ง2)] โ ๐ [
๐
๐๐ง(๐ฅ2๐ฆ๐ง) โ
๐
๐๐ฅ(2๐ฅ๐ง)] + ๏ฟฝ๏ฟฝ [
๐
๐๐ฅ(โ๐ฅ๐ฆ๐ง2) โ
๐
๐๐ฆ(๐ฅ2๐ฆ๐ง)]
= (2๐ฅ๐ฆ๐ง)๐ โ (๐ฅ2๐ฆ โ 2๐ง)๐ + (๐ฆ๐ง2 + ๐ฅ2๐ง)๏ฟฝ๏ฟฝ
โ 0
As we obtain โ ร ๏ฟฝ๏ฟฝ โ 0
We conclude that ๏ฟฝ๏ฟฝ is non-conservative.
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Module No. 102
Introduction to Simple Harmonic Motion and Oscillator
Simple Harmonic Motion (SHM) is a particular type of oscillation and periodic motion in which
restoring force of an object is directly proportional to the displacement of the object acting in
opposite direction of displacement.
Mathematically, the restoring force ๐น is given by
๐น๐ = โ๐๐ฅ
where the subscript ๐ represents the restoring force and ๐ is the constant of proportionality often
called the spring constant or modulus of elasticity.
In Newtonian mechanics, by Newton's second law we have eq. of S.H.M
๐น = ๐๐ = ๐๐2๐ฅ
๐๐ก2= โ๐๐ฅ
or
๐๏ฟฝ๏ฟฝ + ๐๐ฅ = 0
This vibrating system is called a simple harmonic oscillator or linear harmonic oscillator.
This type of motion is often called simple harmonic motion.
The following physical systems are some examples of simple harmonic oscillator
Mass on a spring
An object of mass m linked to a spring of spring constant k represents the simple harmonic
motion in closed region.
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The equation representing the period for the mass attached on a spring
๐ป = ๐๐ โ๐
๐
The above equation expresses that the time period of oscillation is independent of amplitude as
well as the acceleration.
Simple pendulum
The movement of the mass attached to a simple pendulum is considered as simple harmonic
motion.
The time period of a mass m attached to a pendulum of length l with gravitational
acceleration g is given by
๐ป = ๐๐ โ๐
๐
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Module No. 103
Amplitude, Time period, Frequency and Energy of S.H.M
Amplitude
Amplitude is defined as the maximum distance covered by the oscillating body in one oscillation
or length of a wave measured from its mean position.
The amplitude of a pendulum is one-half the distance that the mass covered in moving from one
terminal to the other. The vibrating sources generate waves, whose amplitude is proportional to
the amplitude of the vibrating source.
Time Period
Time period is minimum time required by a oscillation system to complete its one cycle of
oscillation of the specific system.
It is denoted by T and measured in seconds.
Frequency
The frequency (f) of an oscillatory system is the number of oscillations pass through a specific
point in one second.
It is measure in hertz (Hz).
The frequency of S.H.M can be calculate by using the following relation
๐ =1
๐
Energy of S.H.M
If T is the kinetic energy, V the potential energy and ๐ธ = ๐ + ๐ the total energy of a simple
harmonic oscillator, then we have
๐พ. ๐ธ = ๐ =1
2๐๐ฃ2
and
๐ =1
2๐๐ฅ2
Then the total energy of S.H.M will be
๐ธ =1
2๐๐ฃ2 +
1
2๐๐ฅ2
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Module No. 104
Example of S.H.M
Problem Statement
A particle of mass 4 moves along ๐ฆ axis attracted towards the origin by a force whose magnitude
is numerically equal to 6๐ฆ. If the particle is initially at rest at ๐ฆ = 10,
i. The differential equation and initial conditions describing the motion,
ii. The position of the particle at any time,
iii. The speed and velocity of the particle at any time,
iv. The amplitude, time period and frequency of the vibration.
Solution:
i. Let ๐ = ๐ฆ be the position vector of the particle. The acceleration of the particle is
๐2๐
๐๐ก2=
๐2๐ฆ
๐๐ก2
The net force acting on the particle is โ6๐ฆ. Then by using the Newton's second law,
โ6๐ฆ = 4๐2๐ฆ
๐๐ก2
or ๐2๐ฆ
๐๐ก2 +3
2๐ฆ = 0
which is the required differential equation. The required initial conditions (I. Cs) are
๐ฆ = 10, ๐๐ฆ/๐๐ก = 0 ๐๐ก ๐ก = 0
ii. Since the general solution the diff. eq. is
When ๐ก = 0, ๐ฅ = 20 ๐ ๐ ๐กโ๐๐ก ๐ด = 20. Thus
๐ฆ = ๐ด cosโ3/2๐ก + ๐ต sinโ3/2 ๐ก (1)
Using I. Cs
At ๐ก = 0, ๐ฆ = 10. Thus ๐ด = 10. So,
๐ฆ = 10 cosโ3/2๐ก + Bsin โ3/2 ๐ก (2)
15
๐๐ฆ
๐๐ก= โ10โ3/2 sinโ3/2 ๐ก + โ3/2Bcosโ3/2๐ก
(3)
Since at ๐ก = 0,โ ๐๐ฆ
๐๐ก= 0
Thus, we get ๐ต = 0. Hence (2) becomes
๐ฆ = 10 cosโ3/2๐ก (4)
This gives the position at any time.
iii. Speed at any time?
From (6) ๐๐ฆ
๐๐ก= โ10โ3/2 sinโ3/2๐ก
This gives the speed at any time.
iv. Amplitude, T, F ?
General form for the position of a particle moving to and fro is
๐ฆ(๐ก) = A cos(2๐๐ก/T)
Thus, ๐ฆ = 10 cosโ3/2๐ก
Amplitude = 10
Period = T = 2โ2
3๐
Frequency = 1 ๐โ
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Module No. 105
Example of Energy of S.H.M
ENERGY OF A SIMPLE HARMONIC OSCILLATOR
Example:
Find the total energy of the force ๏ฟฝ๏ฟฝ = โ3๐ฅ๐ acting on a simple harmonic oscillator, where ๐
represents the direction.
Solution:
Since the total energy of SHM
๐ธ = ๐ + ๐
By Newton's second law,
๐น = ๐๐
therefore
๐น = ๐๐๐ฃ
๐๐ก= โ3๐ฅ
๐๐ฃ
๐๐ก= โ
3
๐๐ฅ
Integrating with respect to ๐ก, we have
๐ฃ = โ3
2๐๐ฅ2 + ๐
๐ can be calculated if the initial condition is given. Assuming ๐ฃ = 0 initially, which gives ๐ = 0.
Thus ๐ฃ = โ3
2๐๐ฅ2
So,
๐ =9
8๐๐ฅ4
Now, as the potential energy is given by V where ๏ฟฝ๏ฟฝ = โ๐ป๐
or ๐น = โ3๐ฅ๐ = โ(๐๐
๐๐ฅ๐ +
๐๐
๐๐ฆ๐ +
๐๐
๐๐ง๐)
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Then
๐๐
๐๐ฅ= 3๐ฅ ,
๐๐
๐๐ฆ= 0,
๐๐
๐๐ง= 0
By integrating, we obtain
๐ =3
2๐ฅ2 + ๐1
Assuming ๐ = 0, corresponding to ๐ฅ = 0, we get ๐1 = 0.
So the potential energy is ๐ =1
2๐๐ฅ2
Hence
๐ธ =9
8๐๐ฅ4 +
1
2๐๐ฅ2
18
Module No. 106
Damped Harmonic Oscillator
The forces acting on a harmonic oscillator are called damping forces which tend to decrease the
amplitude of the successive oscillations or simply force apposing the motion. Since the damping
force is proportional to the velocity. Thus, mathematically,
๐น๐ = โ๐๐ฃ
= โ๐๏ฟฝ๏ฟฝ
where ๐ represents the damping force and ๐ is the damping coefficient.
The negative sign shows that that direction of ๐น๐ is opposite to the velocity ๐ฃ.
As we know that for the restoring force:
๐๏ฟฝ๏ฟฝ + ๐๐ฅ = 0
Adding the restoring force with the damping force, the equation of motion of the damped
harmonic oscillator will be,
๐๏ฟฝ๏ฟฝ + ๐๐ฅ = โ๐๏ฟฝ๏ฟฝ
or
๐๏ฟฝ๏ฟฝ + ๐๏ฟฝ๏ฟฝ + ๐๐ฅ = 0
Since the mass is not equal to zero, therefore ๏ฟฝ๏ฟฝ +๐
๐๏ฟฝ๏ฟฝ +
๐
๐๐ฅ = 0
let
๐
๐= 2๐, โ
๐
๐= ๐0
then the equation can be written as
๏ฟฝ๏ฟฝ + 2๐๏ฟฝ๏ฟฝ + ๐02๐ฅ = 0
19
Module No. 107
Example of Damped Harmonic Oscillator
Problem Statement
A particle of mass 4 moves along ๐ฆ axis attracted towards the origin by a force whose magnitude
is numerically equal to 6๐ฆ. If the particle is initially at rest at ๐ฆ = 10. Consider that the particle has
also a damping force whose magnitude is numerically equal to 6 times the instantaneous speed.
Find
i. Position of the particle
ii. Velocity of the particle at any time
Solution
i. Since we have by Newton's second law
๐น = ๐๐ = 4๏ฟฝ๏ฟฝ
According to the given information, the damping force is โ6๏ฟฝ๏ฟฝ.
So the net force is
โ6๐ฆ โ 6๏ฟฝ๏ฟฝ
Hence,
4๏ฟฝ๏ฟฝ = โ6๐ฆ โ 6๏ฟฝ๏ฟฝ
or
๏ฟฝ๏ฟฝ +2
3๏ฟฝ๏ฟฝ +
2
3๐ฆ = 0
The solution of the above equation is
๐ฆ = ๐โโ2 3โ ๐ก(๐ด + ๐ต๐ก)
and
๏ฟฝ๏ฟฝ = ๐ต๐โโ2 3โ ๐ก โ โ2 3โ (๐ด + ๐ต๐ก)๐โโ2 3โ ๐ก
To find the values of constants, we use I. Cs
At ๐ก = 0, ๐ฆ = 10 and ๐๐ฆ/๐๐ก = 0; thus,
๐ด = 10
and 0 = ๐ต(1) โ โ2 3โ (10)
๐ต = 10โ2 3โ
and the solution gives
๐ฆ = ๐โโ2 3โ ๐ก(๐ด + ๐ต๐ก)
= 10๐โโ2 3โ ๐ก(1 + โ2 3โ ๐ก)
20
the position at any time t.
21
Module No. 108
Eulerโs Theorem โ Derivation
The following theorem, called Euler's theorem, is fundamental in the motion of rigid bodies.
Theorem Statement
โA rotation of a rigid body about a fixed point of the body is equivalent to a rotation about a line
which passes through the point.โ
The line referred to is called the instantaneous axis of rotation. Rotations can be considered as
finite or infinitesimal. Finite rotations cannot be represented by vectors since the commutative
law fails. However, infinitesimal rotations can be represented by vectors.
Proof:
Let O be the fixed point in the body, which we take as a sphere S. Further, we take O at the
center of the sphere. Let A, B be two distinct points on the sphere. As the body moves, the point
O (on the axis) remains foxed and A and B suffer displacement.
Let ๐ดโ and ๐ตโ bet the new locations of the points A and B after an infinitesimal time interval ๐ฟ๐ก
respectively.We join (A, B) and (๐ดโ, ๐ตโ) by great circular areas.
Also we join (๐ด, ๐ดโ) and (๐ต, ๐ตโ) by mean of great circular arcs. Let Aโ and Bโ draw axes at right
angles, which meat at the point C on the sphere.
We join C with A, B, ๐ดโ, ๐ตโ by means of great circular arcs.
Consider the spherical triangles โ๐ถ๐ดโฒ๐ด" and โ๐ถ๐ด๐ด". Obviously
22
i. ๐ < ๐ถ๐ด"A โ CA"๐ดโฒ
ii. ๐ด๐ด"=A"๐ดโฒ
iii. ๐ถ๐ด" is common to triangle
Therefore โ๐ถ๐ด๐ด" โ โ๐ถ๐ดโฒ๐ด" and it follows that ๐ถ๐ด = ๐ถ๐ดโ
Similarly for the triangle โ๐ถ๐ด๐ต ๐๐๐ โ๐ถ๐ดโฒ๐ตโฒ, we have
๐ถ๐ต = ๐ถ๐ตโ, ๐ถ๐ดโ = ๐ถ๐ด ๐๐๐ ๐ด๐ต = ๐ดโ๐ตโ
The last relation is due to the fact that by the definition of rigid body, the distance between a pair
of particles remains unchanged. Hence
โ๐ถ๐ด๐ต โ โ๐ถ๐ดโฒ๐ตโฒ
The portion of rigid body lying in โ๐ถ๐ด๐ต has moved to โ๐ถ๐ดโฒ๐ตโฒ.
In this process the point O and C have remained fixed, Although the later was at rest only
instantaneously. Therefore the body has under gone a rotation about the axis OC.
Hence the theorem.
23
Module No. 109
Chaslesโ Theorem
Theorem Statement:
Chasleโs theorem states that the most general rigid body displacement can be produced by a
translation along a line (called its screw axis) followed (or preceded) by a rotation about that line.
Explanation:
A rigid body has six degrees of freedom.
By Eulerโs theorem, three of these are associated with pure rotation.
The remaining three must be associated with translation.
To describe the general motion of a rigid body, think of the general motion as translation
of a fixed point ๐ in the body to a point ๐โฒ followed by the rotation about an axis through
๐โฒ.
24
Module No. 110
Kinematics of a System of Particles (Space, time & matter)
Kinematics is the branch of mechanics deals with the moving objects without reference to the
forces which cause the motion.
In other words we can say those kinematics are the features or properties of motion of concerned
with system of particles (rigid bodies).
Here some features of rigid body motion are
Displacement
Position
Velocity
Linear Velocity & Angular Velocity
Linear Acceleration & Angular Acceleration
Motion of a Rigid Body (Translation & Rotation)
From everyday experience, we all have some idea as to the meaning of each of the following terms
or concepts. However, we would certainly find it difficult to formulate completely satisfactory
definitions. We take them as undefined concepts.
I. Space. This is closely related to the concepts of point, position,' direction and displacement.
Measurement in space involves the concepts of length or distance, with which we assume
familiarity. Units of length are feet, meters, miles, etc.
II. Time. This concept is derived from our experience of having one event taking place after, before
or simultaneous with another event. Measurement of time is achieved, for example, by use of
clocks. Units of time are seconds, hours, years, etc.
III. Matter. Physical objects are composed of "small bits of matter" such as atoms and molecules.
From this we arrive at the concept of a material object called a particle which can be considered as
occupying a point in space and perhaps moving as time goes by. A measure of the "quantity of
matter" associated with a particle is called its mass. Units of mass are grams, kilograms, etc. Unless
otherwise stated we shall assume that the mass of a particle does not change with time.
25
Module No. 111
The Concept of Rectilinear Motion of Particles, Uniform Rectilinear Motion,
Uniformly Accelerated Rectilinear Motion
When a moving particle remains on a single straight line, the motion is said to be rectilinear. In
this case, without loss of generality we can choose the ๐ฅ-axis as the line of motion.
The general equation of motion is then
๐น = ๐๐ โน ๐น(๐ฅ, ๏ฟฝ๏ฟฝ, ๏ฟฝ๏ฟฝ) = ๐๏ฟฝ๏ฟฝ
Rectilinear motion for a particle:
Rectilinear motion of a body is defined by considering the two point of a body covered the same
distance in the parallel direction. The figures below illustrate rectilinear motion for a particle and
body.
Rectilinear motion for a body:
In the above figures, ๐ฅ(๐ก) represents the position of the particles along the direction of motion, as
a function of time t.
An example of linear motion is an athlete running g along a straight track.
The rectilinear motion can be of two types:
i. Uniform rectilinear motion
ii. Non uniform rectilinear motion
Uniform Rectilinear Motion:
Uniform rectilinear motion is a type of motion in which the body moves with uniform velocity or
zero acceleration.
26
In contrast, Non uniform rectilinear motion is such type of motion with variable velocity or non-
zero acceleration.
Uniformly accelerated rectilinear motion:
Uniformly accelerated rectilinear motion is a special case of non-uniform rectilinear motion along
a line is that which arises when an object is subjected to constant acceleration. This kind of motion
is called uniformly accelerated motion.
Uniformly accelerated motion is a type of motion in which the velocity of an object changes by an
equal amount in every equal intervals of time.
An example of uniformly accelerated body is freely falling object in which the amount of
gravitational acceleration remains same.
๐น = ๐๐
27
Module No. 112
The Concept of Curvilinear Motion of Particles
The motion of a particle moving in a curved path is called curvilinear motion. Example: A stone
thrown into the air at an angle.
Curvilinear motion describes the motion of a moving particle that conforms to a known or fixed
curve. The study of such motion involves the use of two co-ordinate systems, the first being
planar motion and the latter being cylindrical motion.
Tangential and normal unit vectors are usually denoted by ๐๐ก and ๐๐ respectively.
Velocity of Curvilinear motion
If the tangential and normal unit vectors are ๐๐ก and ๐๐ respectively, then the velocity will be
๏ฟฝ๏ฟฝ =๐๐
๐๐ก๐๐ก
You have already learnt that
๐ฃ = vT
Acceleration of Curvilinear motion
If the tangential and normal unit vectors are ๐๐ก and ๐๐ respectively, then the acceleration will be
๏ฟฝ๏ฟฝ =๐2๐
๐๐ก2๐๐ก +
(๐s๐๐กโ )
2
๐๐๐
You have already learnt that
๐ = T๐v
๐๐ก+
v2
rN
Example
A stone thrown into the air at an angle.
A car driving along a curved road.
Throwing paper airplanes or paper darts is an example of curvilinear motion.
28
Module No. 113
Example Related to Curvilinear Coordinates
Example 1
Problem Statement
Prove that a cylindrical coordinate system is orthogonal.
Solution
The position vector of any point in cylindrical coordinates is
๐ = ๐ฅ๐ + ๐ฆ๐ + ๐ง๏ฟฝ๏ฟฝ
We studied that the in cylindrical coordinates
๐ฅ = ๐ cos๐ , ๐ฆ = ๐ sin ๐, ๐ง = ๐ง
this implies
๐ = ๐ cos๐ ๐ + ๐ sin๐ ๐ + ๐ง๏ฟฝ๏ฟฝ
The tangent vectors to the ฯ, ฯ and ะณ curves are given respectively by ๐๐
๐๐ ,
๐๐
๐๐,๐๐
๐๐ง and where
๐๐
๐๐= cos๐ ๐ + sin๐ ๐
๐๐
๐๐= โ๐ sin๐ ๐ + ๐ cos๐ ๐
๐๐
๐๐ง= ๏ฟฝ๏ฟฝ
The unit vectors in these directions are
๐1 = ๐๐ =๐๐ ๐๐โ
|๐๐ ๐๐โ |=
cos๐ ๐ + sin๐ ๐
|cos ๐ ๐ + sin๐ ๐|
=cos๐ ๐ + sin ๐ ๐
โcos2 ๐ + sin2 ๐ = cos๐ ๐ + sin๐ ๐
๐2 = ๐๐ =๐๐ ๐๐โ
|๐๐ ๐๐โ |=
โ๐ sin๐ ๐ + ๐ cos๐ ๐
|โ๐ sin๐ ๐ + ๐ cos๐ ๐|
=๐(โ sin ๐ ๐ + cos๐)
โ๐2 cos2 ๐ + ๐2 sin2 ๐ = โsin๐ ๐ + cos๐ ๐
๐3 = ๐๐ง =๐๐ ๐๐งโ
|๐๐ ๐๐งโ |= ๏ฟฝ๏ฟฝ
Then
29
๐1. ๐2 = (cos๐ ๐ + sin๐ ๐). (โ sin๐ ๐ + cos๐ ๐)
= โsin๐ cos๐ +sin๐ cos๐ = 0
๐2. ๐3 = (โsin๐ ๐ + cos๐ ๐). ๏ฟฝ๏ฟฝ = 0
๐3. ๐1 = ๏ฟฝ๏ฟฝ. (cos๐ ๐ + sin๐ ๐) = 0
and so ๐1, ๐2 and ๐3 are mutually perpendicular and the coordinate system is orthogonal.
Example 2
Problem Statement
Prove
๐๐๐
๐๐ก= ๐๐๐
๐๐๐
๐๐ก= โ๐๐๐
where dots denote differentiation with respect to time t.
Solution
We have
๐๐ = cos๐ ๐ + sin๐ ๐
and
๐๐ = โsin๐ ๐ + cos๐ ๐
Then
๐๐๐
๐๐ก=
๐
๐๐ก(cos๐ ๐ + sin๐ ๐)
= โsin ๐ ๏ฟฝ๏ฟฝ๐ + cos๐ ๏ฟฝ๏ฟฝ๐ = ๏ฟฝ๏ฟฝ(โ sin๐ ๐ + cos๐ ๐) = ๏ฟฝ๏ฟฝ๐๐
๐๐๐
๐๐ก=
๐
๐๐ก(โ sin๐ ๐ + cos๐ ๐)
= โcos๐ ๏ฟฝ๏ฟฝ๐ โ sin ๐ ๏ฟฝ๏ฟฝ๐ = โ๏ฟฝ๏ฟฝ(cos๐ ๏ฟฝ๏ฟฝ๐ + sin๐ ๏ฟฝ๏ฟฝ๐) = โ๏ฟฝ๏ฟฝ๐๐
30
Module No. 114
Introduction to Projectile, Motion of a Projectile
Introduction to Projectile
If a ball is thrown from one person to another or an object is dropped from a moving plane, then
their path of traveling/motion is often called a projectile. If air resistance is negligible, a projectile
can be considered as a freely falling body so the Eq of motion will be
md2r
dt2= โmg
or
d2r
dt2= โg
with appropriate I.Cs
Motion of a Projectile with Resistance
Earlier, we studied the motion of a projectile under the assumption that air resistance is negligible.
If we further assume that the motion takes place in the vertical plane, (๐๐-plane), and the only
force acting on the projectile is the gravity, then the equation of motion will be
d2r
dt2= g = โgj
Here we will discuss the problem of projectile motion when air resistance is taken into account.
Assuming the model of retarding force in which
๐น โ ๐ฃ
or
๐น = โ๐1๐ฃ
Let the initial velocity of the projectile be ๐ฃ0 and angle of projection be ๐
The initial conditions can be taken as
๐ฅ(๐ก = 0) = ๐ฆ(๐ก = 0) = 0
๏ฟฝ๏ฟฝ(๐ก = 0) = ๐ฃ0 cos ๐ = ๐ข1
๏ฟฝ๏ฟฝ(๐ก = 0) = ๐ฃ0 sin ๐ = ๐ฃ1
The equation of motion in the horizontal and vertical direction can be written as
๐๏ฟฝ๏ฟฝ = โ๐1๏ฟฝ๏ฟฝ
or ๏ฟฝ๏ฟฝ = โ๐1
๐๏ฟฝ๏ฟฝ = โ๐๏ฟฝ๏ฟฝ (1)
๐๏ฟฝ๏ฟฝ = โ๐๏ฟฝ๏ฟฝ โ ๐๐ (2)
By the substitution ๏ฟฝ๏ฟฝ = ๐ง we can solve eq (1) as
31
๐ง = ๏ฟฝ๏ฟฝ = ๐ด๐โ๐๐ก
With the initial condition ๏ฟฝ๏ฟฝ = ๐ข1, when ๐ก = 0, we obtain the constant of integration ๐ด = ๐ข1
so that
๏ฟฝ๏ฟฝ = ๐ข1๐โ๐๐ก
Another integration gives
๐ฅ = โ๐ข1๐
โ๐๐ก
๐+ ๐ต
The initial condition ๐ฅ(0) = 0 gives ๐ต =๐ข1
๐โ . Hence the solution of equation (1) can be
written as
๐ฅ =๐ข1
๐(1 โ ๐โ๐๐ก) (3)
We can solve equation (2) in the same manner and obtain
๐ฆ = โ๐๐ก
๐+
๐๐ฃ1+๐
๐2(1 โ ๐โ๐๐ก) (4)
The path of motion can be obtained from (3) and (4) by eliminating ๐ก
๐ฆ =๐
๐2ln (1 โ
๐๐ฅ
๐ข1) +
๐๐ฃ1 + ๐
๐2
๐๐ฅ
๐ข1
which is no longer a parabola.
We can solve equation (2) in the same manner and obtain
๐ฆ = โ๐๐ก
๐+
๐๐ฃ1+๐
๐2(1 โ ๐โ๐๐ก) (4)
The path of motion can be obtained from (3) and (4) by eliminating ๐ก
๐ฆ =๐
๐2ln (1 โ
๐๐ฅ
๐ข1) +
๐๐ฃ1 + ๐
๐2
๐๐ฅ
๐ข1
which is no longer a parabola.
32
Time of Flight ๐ Time of flight can be found by putting in equation (4) ๐ฆ = 0 and ๐ก = ๐
0 = โ๐๐
๐+
๐๐ฃ1 + ๐
๐2(1 โ ๐โ๐๐)
๐ =๐๐ฃ1+๐
๐๐(1 โ ๐โ๐๐) (5)
This equation can be solved exactly for ๐. To find an approximate solution, we expand the
exponential factor in equation (5), using
๐๐ฅ = 1 + ๐ฅ +๐ฅ2
2!+
๐ฅ3
3!+ โฏโฆโฆ ..
Form equation (5)
๐ =๐๐ฃ1 + ๐
๐๐(๐๐ โ
๐2๐2
๐๐+
๐3๐3
3!+ โฏ . . )
1 =๐๐ฃ1 + ๐
๐๐(๐ โ
๐2๐
๐๐+
๐3๐2
3!+ โฏ . . )
which on simplification gives
๐ =2๐ฃ1
๐๐ฃ1 + ๐+
1
3๐๐2 + โฏ . .
In the limit of small air resistance (i.e. ๐ โ 0), the above expression reduces to
๐ =2๐ฃ1
๐๐ฃ1+๐+
๐
3๐2 (6)
When there is no resistance, (๐ = 0), obtain from (6)
๐0 =2๐ฃ1
๐=
2๐ฃ0 sin ๐
๐
If ๐ is non-zero small number, then the time of flight ๐ will be very close to ๐0. Substituting ๐0 for
๐ on the R.H.S of (6), and simplifying we get
๐ =2๐ฃ1
๐(1 โ
๐๐ฃ1
3๐)
The above equation gives a formula for approximate time of flight in the presence of a weak
retarding force.
33
Module No. 115
Conservation of Energy for a System of Particles
Statement
โThe law of conservation of energy describes that the net energy of an isolated system remains
conserved. Energy can neither be created nor destroyed; rather, it transforms from one form to
another.โ
Theorem Statement
(Principle of conservation of energy)
In case of conservative force field, the total energy is a constant. i.e.,
If ๐ is for kinetic energy and ๐ is for potential energy, then the total energy ๐ธ is
๐ธ = ๐ + ๐ = constant
Explanation
For a conservative force field, we have already learned that the work done by the system of
particles is
Work done = change in kinetic energy = ๐ = ๐2 โ ๐1
Also Work done = change in potential energy = ๐ = ๐1 โ ๐2
By comparing, we get
๐2 โ ๐1 = ๐1 โ ๐2
or
๐1 + ๐1 = ๐2 + ๐2
which can be written as
1
2๐๐ฃ1
2 + ๐1 =1
2๐๐ฃ2
2 + ๐2
.
34
Module No. 116
Conservation of Energy
Example
Consider the force field ๐น = โ๐๐3๐
i. Find whether the given field is conservative or not.
ii. Find the potential energy of the given force field of part (i).
iii. For the particle moving in ๐ฅ๐ฆ โ plane, find the work done by the force in moving the
particle from A to B, where A is the point where ๐ = ๐, and B where ๐ = ๐.
iv. If the particle of mass ๐ moves with velocity ๐ฃ =๐๐
๐๐ก in this field, show that if ๐ธ is the
constant, total energy then
v. 1
2 ๐ (
๐๐
๐๐ก)2
+1
5 ๐๐5 = ๐ธ
Solution
i. As ๏ฟฝ๏ฟฝ = โ๐๐3๐ = โ๐(๐ฅ2 + ๐ฆ2)3/2(๐ฅ๐ + ๐ฆ๐)
โ ร ๏ฟฝ๏ฟฝ = |
๐ ๐ ๐
๐ ๐๐ฅโ ๐ ๐๐ฆโ ๐ ๐๐งโ
โ๐(๐ฅ2 + ๐ฆ2)3/2๐ฅ โ๐(๐ฅ2 + ๐ฆ2)3/2๐ฆ 0
|
= ๐ [๐
๐๐ฆ(0) โ
๐
๐๐ง(โ๐(๐ฅ2 + ๐ฆ2)3/2๐ฆ)] + ๐ [
๐
๐๐ง(โ๐(๐ฅ2 + ๐ฆ2)3/2๐ฅ) โ
๐
๐๐ฅ(0)]
+ ๐ [๐
๐๐ฅ(โ๐(๐ฅ2 + ๐ฆ2)3/2๐ฆ) โ
๐
๐๐ฆ(โ๐(๐ฅ2 + ๐ฆ2)3/2๐ฅ)]
= โ3๐(๐ฅ2 + ๐ฆ2)12๐ฅ๐ฆ + 3๐(๐ฅ2 + ๐ฆ2)
12๐ฅ๐ฆ = 0
Thus the given field is conservative.
ii. Since the field is conservative there exists a potential V such that
๐น = โ๐ป๐
๐น = โ๐๐3๐ = โ๐(๐ฅ2 + ๐ฆ2)32(๐ฅ๐ + ๐ฆ๐)
= โ๐(๐ฅ2 + ๐ฆ2)32๐ฅ๐ โ ๐(๐ฅ2 + ๐ฆ2)
32๐ฆ๐ = โโ๐
= โ๐๐
๐๐ฅ๐ โ
๐๐
๐๐ฆ๐ โ
๐๐
๐๐ง๐
35
By comparing, we get
๐๐
๐๐ฅ= ๐(๐ฅ2 + ๐ฆ2)3/2๐ฅ,
๐๐
๐๐ฆ=๐(๐ฅ2 + ๐ฆ2)3/2๐ฆ and
๐๐
๐๐ง= 0
From which, by omitting the constants, we get
๐ =1
5 ๐(๐ฅ2 + ๐ฆ2)5/2 =
1
5 ๐๐5
is required potential.
iii. The velocity potential at point A
=1
5 ๐๐5
and the velocity potential at point B
=1
5๐๐5
So, the work done from A to B is
= Potential at A - Potential at B
=1
5 ๐๐5 โ
1
5 ๐๐5 =
1
2 ๐(๐5 โ ๐5)
which is required work done by the given force field.
iv. Since the kinetic energy of a particle of mass ๐ moving with velocity v =๐๐
๐๐ก is
๐ =1
2 ๐๐ฃ2 =
1
2 ๐(
๐๐
๐๐ก)2
From part (ii), we have the potential energy
๐ =1
5 ๐๐5
Thus the total energy ๐ธ will be
๐ธ = ๐ + ๐
=1
2 ๐ (
๐๐
๐๐ก)2
+1
5 ๐๐5
Hence proved.
36
Module No. 117
Introduction to Impulse โ Derivation
Impulse is a special type of force defined by applying the integral of a force ๐น, over the time
interval, t, for which it acts on the body.
Impulse is a directional (vector) quantity in the same direction of force as force is also a directional
quantity. When Impulse is applied to a rigid body, it results a corresponding vector change in its
linear momentum along the same direction. The SI unit of impulse is the newton second (๐ ยท ๐ ),
and the dimensionally equivalent unit of momentum is the kilogram meter per second (๐๐๐๐ โ1).
The particle is located at ๐1 and ๐2 at times ๐ก1 and ๐ก2 where it has velocities ๐ฃ1 and ๐ฃ2 respectively.
The time integral of the force F given by
โซ ๐น๐๐ก
๐ก2
๐ก1
is called the impulse of the force ๐น.
Theorem Statement
The impulse is equal to the change in momentum; or, in symbols,
โซ ๐น๐๐ก = ๐๐ฃ2 โ ๐๐ฃ1 = ๐2 โ ๐1
๐ก2
๐ก1
Proof
We have to prove that the impulse of a force is equal to the change in momentum.
By definition of impulse and Newton's second law, we have
โซ ๐น๐๐ก =
๐ก2
๐ก1
โซ ๐๐๐ฃ
๐๐ก๐๐ก =
๐ก2
๐ก1
โซ ๐๐๐ฃ = ๐|๐ฃ|๐ก1๐ก2 =
๐ก2
๐ก1
๐๐ฃ2 โ ๐๐ฃ1 = ๐2 โ ๐1
where we use the conditions
๐ฃ(๐ก1) = ๐ฃ1 and ๐ฃ(๐ก2) = ๐ฃ2
The theorem is true even when the mass is variable and the force is non-conservative.
37
Module No. 118
Example of Impulse
Example:
What is the magnitude of the impulse developed by a mass of 200 gm which changes its velocity
from 5๐ โ 3๐ + 7๐ m/sec to 2๐ + 3๐ + ๐ m/sec?
Solution:
Since we have the following information:
๐ = 200 gm
๐ฃ1 = 5๐ โ 3๐ + 7๐
๐ฃ2 = 2๐ + 3๐ + ๐
As we know that
Impulse = ๐2 โ ๐1
= ๐๐ฃ2 โ ๐๐ฃ1
= ๐(๐ฃ2 โ ๐ฃ1)
Substituting the values, we get
Impulse = 200(2๐ + 3๐ + ๐ โ (5๐ โ 3๐ + 7๐))
= 200(โ3๐ + 6๐ โ 6๐)
The magnitude of the Impulse will be
Impulse magnitude = 200โ9 + 36 + 36
= 200โ81
= 200(9)
= 1800mgm/sec
= 1.8 N sec
38
Module No. 119
Torque
Definition
Torque is defined as the turning effect of a body. It is trend of an acting force due to which the
rotational motion of a body changes. It is also called twist and rotational force on an object.
Mathematically, torque is defined as the cross product of the force vector to the distance vector,
which causes rotational motion of the body.
๐ = ๏ฟฝ๏ฟฝ ร ๏ฟฝ๏ฟฝ
The magnitude of torque depends upon the applied force, the length of the lever arm connecting
the axis to the point where the force applied, and the angle between the force vector and the length
of lever arm. Symbolically we can write it as:
๐ = |๐||๐ญ| ๐ฌ๐ข๐ง ๐ฝ
Torque is a vector quantity implies that it has direction as well as magnitude.
The SI unit for torque is the newton meter (Nm).
The direction of torque can be approximate using Right Hand Rule.
Theorem:
The torque acting on a particle equals the time rate of change in its angular momentum, i.e.,
๐ =๐ฮฉ
๐๐ก
where ฮฉ = r ร p is defined angular momentum of the system.
Proof:
As we know torque is defined as
๐ = ๐ ร ๐น = ๐ ร ๐๐
as m is constant, therefore
= ๐(๐ ร ๐)
= ๐(๐ ร๐๐ฃ
๐๐ก)
= ๐๐
๐๐ก(๐ ร ๐ฃ)
=๐
๐๐ก(๐ ร ๐๐ฃ)
=๐
๐๐ก(๐ ร ๐)
where ๐ = ๐๐ฃ is defined as linear momentum and hence the theorem.
39
Module No. 120
Example of Torque
Example
A particle of mass ๐ moves Along a space curve defined by ๐ = acos๐๐ก ๐ + ๐ sin๐๐ก ๐.
Find
(i) The Torque
(ii) The angular momentum about the origin.
Solution
(i) As we know that the torque acting on a particle is equal to the time rate of change of its
angular momentum, i.e.
๐ =๐ฮฉ
๐๐ก=
๐
๐๐ก(๐ ร ๐)
where ๐ = ๐๐ฃ.
As ๐ = acos๐๐ก ๐ + ๐ sin๐๐ก ๐. Therefore
๐ฃ =๐๐
๐๐ก
๐ฃ = โ๐๐ sin๐๐ก ๐ + ๐๐ cos๐๐ก ๐
So,
๐ = โ๐๐๐ sin๐๐ก ๐ + ๐๐๐ cos๐๐ก ๐
Now
๐ ร ๐
= |๐ ๐ ๏ฟฝ๏ฟฝ
acos๐๐ก ๐ sin๐๐ก 0โ๐๐๐ sin๐๐ก ๐๐๐ cos๐๐ก 0
|
= [๐๐๐๐cos2๐๐ก + ๐๐๐๐sin2๐๐ก]๏ฟฝ๏ฟฝ
= ๐๐๐๐๏ฟฝ๏ฟฝ
40
๐ =๐ฮฉ
๐๐ก=
๐
๐๐ก(๐ ร ๐) = 0
(ii) From part (i), we already have
ฮฉ = ๐ ร ๐
= ๐๐๐๐๏ฟฝ๏ฟฝ
is the required angular momentum.
41
Module No. 121
Introduction to Rigid Bodies and Elastic Bodies
Definition of Rigid Bodies
When a force is applied to an object/ system of particles, and if the object maintains its
overall shape, then the object is called a rigid body.
Gap between two fixed points on the rigid body remains same regardless of external
forces exerted on it.
We can neglect the deformation of such bodies.
A rigid body usually has continuous distribution of mass.
Definition of Elastic Bodies
When a force is applied to a system of particles, it changes the distance between
individual particles. Such systems are often called deformable or elastic bodies.
Examples
A spring and rubber band are some common examples of elastic bodies.
A wheel is a common example of rigid body.
42
Module No. 122
Properties of Rigid Bodies
Following are some of the properties of the rigid bodies.
Degree of freedom
The number of coordinates required to specify the position of a system of one or more particles is
called the number of degrees of freedom of the system.
For example a particle moving freely in space requires 3 coordinates, e.g. (x, y, z), to specify its
position. Thus the number of degrees of freedom is 3.
Similarly, a system consisting of N particles moving freely in space requires 3N coordinates to
specify its position. Thus the number of degrees of freedom is 3N.
Translations
A displacement of a rigid body is a direct change of position of its particles. Translational motion
is the displacement of all particles of the body by the same amount and the line segment joining
the initial and the final position of the particles represented by parallel vectors.
Examples of translational motion are particles freely falling down to earth and the motion of a
bullet fired from a gun.
Rotations
Circular motion of a body about a fixed point or axis is called rotation. If during a displacement
the points of the rigid body on some line remains fixed and all other are displaced through the
same angle, then this displacement is called rotation. A rigid performs rotations around an
imaginary line called a rotation axis.
If the axis of rotation passes through the center of mass of the rigid body then body is said to be
spin or rotate upon itself. If a body rotates about some external fixed point is called revolution or
orbital motion of the rigid body. The example of revolution is the rotation of earth around sun
and motion of moon around sun.
Rotational motion concerns only with rigid bodies. The reverse rotation of a body (inverse
rotation) is also a rotation.
A wheel is common examples of rotation.
43
Module No. 123
Instantaneous Axis and Center of Rotation
Introduction to General Plane Motion
The general plane motion of a rigid body can be considered as:
Translational motion along the given fixed plane and rotational motion about a suitable
axis perpendicular to the plane.
This fixed axis is specifically chosen to pass through the center of mass of the rigid body.
Instantaneous Axis of Rotation
The axis about which the rigid body rotates is called instantaneous axis of rotation, where
this axis is perpendicular to the plane.
Instantaneous Center of Rotation
The point where instantaneous axis meets the fixed plane along which the body performs
translation motion is described as the instantaneous center of rotation.
44
Module No. 124
Centre of Mass & Motion of the Center of Mass
The center of mass (c.m.) or centroid of system of particles is a hypothetical particle such that if
the entire mass of the system were concentrated there, the mechanical properties would remain the
same. In particular expression of linear momentum, angular momentum and kinetic energy assume
simpler or more convenient forms when referred to the coordinated of this hypothetical particle
and the equation of motion can be reduced to simpler equation of a single particle.
Let ๐1, ๐2, ๐3, โฆโฆโฆ . , ๐๐be the position vectors of a system of N particles of masses
๐1, ๐2, ๐3, โฆโฆโฆ . ,๐๐ respectively [see Fig.].
The center of mass or centroid of the system of particles is defined as that point C having position
vector
๐๐ =โ ๐๐๐๐๐
โ ๐๐๐
When the system is moving the position vector will depend on time t and therefore ๐๐will also be
a function of t. the velocity and acceleration of center of mass will be then given by
๏ฟฝ๏ฟฝ๐ = ๐๐ =โ ๐๐๐๏ฟฝ๏ฟฝ๐
โ ๐๐๐
๏ฟฝ๏ฟฝ๐ = ๐๐ =โ ๐๐๐๏ฟฝ๏ฟฝ๐
โ ๐๐๐
Motion of Center of Mass Motion of center of mass can be examined by considering the following points:
45
1. If a system experiences no external force, the center-of-mass of the system will remain at
rest, or will move at constant velocity if it is already moving.
2. If there is an external force, the center of mass accelerates according to ๐น = ๐๐.
3. Basically, the center-of-mass of a system can be treated as a point mass, following
Newton's Laws.
4. If an object is thrown into the air, different parts of the object can follow quite complicated
paths, but the center-of-mass will follow a parabola.
5. If an object explodes, the different pieces of the object will follow seemingly independent
paths after the explosion. The center of mass, however, will keep doing what it was doing
before the explosion. This is because an explosion involves only internal forces.
46
Module No. 125
Centre of Mass & Motion of the Center of Mass
The moment of inertia of a rigid body is a property which depends upon its mass and shape, (i.e.
the mass distribution of the body) and determines its behavior in rotational motion.
In rotational motion, the moment of inertia plays the same role as the mass in linear motion.
Definition of Moment of Inertia
Formally the moment of inertia ๐ผ of the particle of mass ๐ about a line is defined by
๐ผ = ๐๐2
where ๐ is the perpendicular distance between the particle and the line (called the axis).
Moment of Inertia of System of particles
The moment of inertia of a system of particles, with masses ๐1, ๐2, ๐3, โฆ ,๐๐ about the line or
axis AB is defined as
๐ผ = โ ๐๐
๐
๐=1๐๐
2
= ๐1๐12 + ๐2๐2
2 + โฏ+ ๐๐๐๐2
In dimensions, the moment of inertia can be expressed as
[๐ผ] = [๐][๐ฟ2]
Moment of Inertia in Coordinate System
The moment of inertia of a particle of mass ๐ with coordinates (๐ฅ, ๐ฆ, ๐ง) relative to the
orthogonal Cartesian coordinate system๐๐๐๐ about ๐, ๐, ๐ axes will be
๐ผ๐ฅ๐ฅ = ๐(๐ฆ2 + ๐ง2)
๐ผ๐ฆ๐ฆ = ๐(๐ง2 + ๐ฅ2)
๐ผ๐ง๐ง = ๐(๐ฅ2 + ๐ฆ2)
Product of Inertia
The product of inertia for the same particle w.r.to the pair of coordinate axes are defined as
๐ผ๐ฅ๐ฆ = โ๐๐ฅ๐ฆ
๐ผ๐ฆ๐ง = โ๐๐ฆ๐ง
๐ผ๐ง๐ฅ = โ๐๐ง๐ฅ
These definitions can be easily generalized to a system of particle and a rigid body.
47
48
Module No. 126
Example of moment of Inertia and Product of Inertia
Problem Statement:
Find the moments of inertia of a ring of radius ๐ about an axis through center.
Solution:
Let ๐ be the mass and ๐ the radius of the ring. Then the mass per unit length will be ๐ 2๐๐โ .
We regard this ring to be composed of small elements of mass (๐ฟ๐) each of length ๐ฟ๐ ,
we can write it as
๐ฟ๐
๐ฟ๐ =
๐
2๐๐โน ๐ฟ๐ =
๐
2๐๐๐ฟ๐
Moment of inertia of the element about an axis through center O and the perpendicular to the
plane of the ring equals ๐
2๐๐๐ฟ๐ ๐2.
Therefore the M.I of the whole ring will be
๐ผ๐๐๐๐ =๐
2๐๐๐2 โ ๐ฟ๐
๐๐๐๐๐๐๐ก๐ =
๐๐
2๐โซ๐๐ =
๐๐
2๐ร 2๐๐ = ๐๐2
Hence ๐๐2 is the required moment of inertia of the ring.
49
Module No. 127
Radius of Gyration
Radius of gyration specifies the distribution of the elements of body around the axis in terms of
the mass moment of inertia, As it is the perpendicular distance from the axis of rotation to a point
mass m that gives an equivalent inertia to the original object m The nature of the object does not
affect the concept, which applies equally to a surface bulk mass.
Mathematically the radius of gyration is the root mean square distance of the object's parts from
either its center of mass or the given axis, depending on the relevant application.
Let ๐ผ = โ ๐๐๐๐=1 ๐๐
2 be the moment of inertia of a system of particles about AB, and
๐ = โ ๐๐๐๐=1 be the total mass of the system.
Then the quantity K such that
๐พ2 =๐ผ
๐=
โ ๐๐๐๐=1 ๐๐
2
โ ๐๐๐๐=1
or
๐พ = โ๐ผ
๐
is called the radius of gyration of the system AB.
Example:
Find the radius of gyration, K, of the triangular lamina of mass M and moment of inertia ๐ผ =1
6โ ๐โ2.
Solution:
Since formula for radius of gyration is given by
๐พ2 =๐ผ
๐
by substituting value in above , we get
๐พ2 =1
6โ ๐โ2
๐= 1
6โ โ2
or ๐พ =โ
โ6
50
Module No. 128
Principal Axes for the Inertia Matrix
Principal Axes
When a rigid body is rotating about a fixed point O, the angular velocity vector ๐ and the
angular momentum vector L (about O) are not in general in the same direction. However
it can be proved that at each point in the body there exists distinct directions, which are
fixed relative to the body, along which the two vectors are aligned i.e. coincident. Such
directions are called principal directions and the axes along them are referred to as
principal axes of inertia. The corresponding moments of inertia are called principal
moments of inertia.
Orthogonality of Principal Axes
If the principal axes at each point of the body exist, then their orthogonality can be proved by
stating that axes relative to which product of inertia are zero are the principal axes.
51
Module No. 129
Introduction to the Dynamics of a System of Particles
Dynamics
Dynamics is the branch of mechanics deals with forces and relationship fundamentally to the
motion but sometimes also to the equilibrium of bodies.
The Dynamics of a System of Particles
Suppose we have a system of N particles in motion under forces. These will be two types of
forces, internal and external.
Internal forces act between particles of the system; all other are external. We suppose further that
the internal forces satisfy Newtonโs third law of motion. i.e. Internal forces between a pair of
particles are equal and opposite If ๐น๐๐(๐๐๐ก)
denotes the internal force on the ith particle due to the
jth particle, then in the view of this assumption we can write
๐น๐๐(๐๐๐ก) = โ๐น๐๐
(๐๐๐ก) (1)
The equation of motion for the ith particle of system can therefore be written as
๐น๐(๐๐ฅ๐ก)
+ โ ๐น๐๐(๐๐๐ก)๐
๐=1 = ๐๐๏ฟฝ๏ฟฝ๐ (2)
where ๐น๐(๐๐ฅ๐ก)
denotes the external force on the ith particle, and ๏ฟฝ๏ฟฝ๐, the acceleration of the ith
particle. It is clear from equation (1) that the internal force on a given particle due to itself is zero
and therefore the term ๐ = ๐ does not contribute to the sum.
To obtain an equation of motion for the whole system we sum over i, (from 1 to N), on the both
sides of equation (2), and obtain
โ๐น๐(๐๐ฅ๐ก)
๐
+ โโ๐น๐๐(๐๐๐ก)
๐
= โ๐๐๏ฟฝ๏ฟฝ๐
๐๐
or
๐น(๐๐ฅ๐ก) + โ ๐น๐๐(๐๐๐ก)
๐,๐ = โ ๐๐๏ฟฝ๏ฟฝ๐๐ (3)
Where ๐น(๐๐ฅ๐ก) denotes the total external force on the system. Now we will show that because of
condition (1), the second term on L.H.S of (3) is zero.
โ๐น๐๐(๐๐๐ก)
๐,๐
=1
2โ(๐น๐๐
(๐๐๐ก) + ๐น๐๐(๐๐๐ก))
๐,๐
Since the indices I and j are dummy and vary over the same set of integers, we can interchange
them in the second sum on the R.H.S of the last equation. Therefore we have
52
โ๐น๐๐(๐๐๐ก)
๐,๐
=1
2โ(๐น๐๐
(๐๐๐ก) + ๐น๐๐(๐๐๐ก))
๐,๐
=1
2โ(๐น๐๐
(๐๐๐ก) โ ๐น๐๐(๐๐๐ก))
๐,๐
= 0
Therefore the equation (3) becomes
๐น(๐๐ฅ๐ก) = โ ๐๐๏ฟฝ๏ฟฝ๐๐ (4)
In case of centroid, the relation becomes
โ๐๐๏ฟฝ๏ฟฝ๐
๐
= ๐๏ฟฝ๏ฟฝ๐
where ๏ฟฝ๏ฟฝ๐ = ๏ฟฝ๏ฟฝ๐ is the acceleration of center of mass (c.m.) and M is the total mass of the system.
Hence equation (4) can be written as
๐น(๐๐ฅ๐ก) = ๐๏ฟฝ๏ฟฝ๐ =๐
๐๐ก(๐๏ฟฝ๏ฟฝ๐) (5)
If ๐น(๐๐ฅ๐ก) = 0 then ๐
๐๐ก(๐๏ฟฝ๏ฟฝ๐) = 0, which implies that ๐๏ฟฝ๏ฟฝ๐ =constant.
We conclude that if external forces are on a system of particles are zero, then its momentum will
be a constant of motions or a conserved quantity. Equation (5) describes the translational motion
of the system and may be referred to as the translational equation of motion which describes the
rotational motion of the system, we take the cross product of both sides of equation (2) with
๐๐, and obtain
๐๐ร๐น๐(๐๐ฅ๐ก)
+ โ ๐๐ ร ๐น๐๐(๐๐๐ก)
๐
๐=1
= ๐๐๐๐ ร ๏ฟฝ๏ฟฝ๐
summing over i
โ ๐๐ร๐น๐(๐๐ฅ๐ก)
+ โ ๐๐ ร ๐น๐๐(๐๐๐ก)๐
๐,๐=1 =๐ โ ๐๐๐๐ ร ๏ฟฝ๏ฟฝ๐๐ (6)
Now we will show that in view of our assumption (1) about internal forces, the second term on
L.H.S of equation (6) is zero. On interchanging the dummy indices and using (1) we have
โ ๐๐ ร ๐น๐๐(๐๐๐ก)
๐,๐
=1
2โ(๐๐ ร ๐น๐๐
(๐๐๐ก)
๐,๐
+ ๐๐ ร ๐น๐๐(๐๐๐ก))
=1
2โ(๐๐ ร ๐น๐๐
(๐๐๐ก) โ
๐,๐
๐๐ ร ๐น๐๐(๐๐๐ก))
=1
2โ (๐๐ โ ๐๐) ร ๐น๐๐
(๐๐๐ก)๐,๐ (7)
The vector ๐๐ โ ๐๐ is in the direction of the line segment joining the particles I and j and therefore
it is parallel to ๐น๐๐ . Therefore the last term on R.H.S of equation (7) is zero.
Hence we finally obtain
โ๐๐ ร ๐น๐๐(๐๐ฅ๐ก)
๐,๐
= โ๐๐๐๐ ร ๏ฟฝ๏ฟฝ๐
๐
or
53
โ ๐บ๐ = ๐บ๐๐ฅ๐ก =
๐
โ๐๐๐๐ ร ๏ฟฝ๏ฟฝ๐
๐
=๐
๐๐ก(โ๐๐๐๐ ร ๏ฟฝ๏ฟฝ๐
๐
)
=๐
๐๐ก(โ๐๐ ร ๐๐๏ฟฝ๏ฟฝ๐
๐
)
=๐
๐๐กโ๐ฟ๐ =
๐๐ฟ
๐๐ก๐
which is usually written as
๐๐ฟ
๐๐ก= ๐บ(๐๐ฅ๐ก)
where ๐บ๐(๐๐ฅ๐ก)
is the torque or momentum due to external force on the ith particle, and ๐บ๐(๐๐ฅ๐ก) is
the torque due to all external forces on the system.
Similarly L is the total angular momentum of the system.
โ๐๐ ร ๐น๐๐(๐๐ฅ๐ก)
๐,๐
= โ๐บ๐ = ๐บ
๐
Thus we have obtained the following two equations of motion for a system of particles. ๐
๐๐ก(๐๏ฟฝ๏ฟฝ๐) = ๐น(๐๐ฅ๐ก) (8)
and ๐๐ฟ
๐๐ก= ๐บ(๐๐ฅ๐ก) (9)
It follows from equation (8) that if the total external torque on the system is zero, then its angular
momentum will be a constant of motion or a conserved quantity.
54
Module No. 130
Introduction to Center of Mass and Linear Momentum
The rigid bodies are a system of particles in which the position of particles is relatively fixed. We
consider such a system of particles in this article and will discuss its center of mass and linear
momentum.
Center of Mass
Our general system consists of ฮท particles of masses ๐1, ๐2, โฆโฆ . . , ๐๐whose position vectors
are, respectively, ๐1, ๐2, โฆโฆ . . , ๐๐. We define the center of mass of the system as the point whose
position vector ๐๐๐ is given by
๐๐๐ =๐1๐1+๐2๐2+๐3๐3+โฏโฆโฆ.+๐๐๐๐
๐1+๐2+โฏโฆ..+๐๐=
โ ๐๐๐๐๐๐=1
๐ (1)
where ๐ = โ ๐๐ ๐๐=1 is the total mass of the system.
The above definition of center of mass leads us to three equations
๐ฅ๐๐ =โ ๐๐๐ฅ๐
๐๐=1
๐, ๐ฆ๐๐ =
โ ๐๐๐ฆ๐๐๐=1
๐, ๐ง๐๐ =
โ ๐๐๐ง๐๐๐=1
๐
which are the rectangular coordinates of the center of mass of the system.
Linear Momentum
We define the linear momentum ฯ of the system as the vector sum of the linear momentum of the
individual particles, namely,
๐ = โ๐๐ = โ๐๐๐ฃ๐
๐๐
From equation (1)
๐๐๐ =โ ๐๐๐๐
๐๐=1
๐ (1)
๏ฟฝ๏ฟฝ๐๐ = ๐๐๐ =โ ๐๐๐ฃ๐
๐๐=1
๐
it follows that
๐ = ๐๐๐๐
that is, the linear momentum of a system of particles is the product of the velocity of the center
of mass and the total mass of the system.
55
Module No. 131
Law of Conservation of Momentum for Multiple Particles
Statement
In the absence of the external force, the momentum of the system of particles will be conserved.
Proof
Suppose we have a system of N particles in motion under forces. These will be two types of
forces, internal and external.
Suppose now that there are external forces ๐น1๐น2, . . . , ๐น๐, , . . . , ๐น๐ acting on the respective particles.
In addition, there may be internal forces of interaction between any two particles of the system.
We denote these internal forces by ๐น๐๐(๐๐๐ก), meaning the force exerted on particle i by particle j.
Internal Forces act between particles of the system; all over the external. We suppose further that
the internal force satisfy Newtonโs third law of motion. i.e. Internal forces between a pair of
particles are equal and opposite.
If ๐น๐๐(๐๐๐ก) denotes the internal force on the ith particle due to the jth particle, then in the view of
this assumption we can write
๐น๐๐(๐๐๐ก) = โ๐น๐๐
(๐๐๐ก) (1)
The equation of motion for the ith particle of the system can therefore be written as
๐น๐(๐๐ฅ๐ก) + โ ๐น๐๐
(๐๐๐ก)๐๐=1 = ๐๐๐๐ = ๐๏ฟฝ๏ฟฝ (2)
where ๐น๐(๐๐ฅ๐ก) denotes the total external force on the ith particle, and ๐๐ the acceleration of the ith
particle.
To obtain an equation of motion for the whole system, we sum over I (from 1 to N), on both
sides of the equation (2).
โ๐น๐(๐๐ฅ๐ก)
๐
+ โโ๐น๐๐(๐๐๐ก)
๐
๐=1๐
= โ๐๐๐๐
๐
= ๐๏ฟฝ๏ฟฝ
๐น(๐๐ฅ๐ก) + โ ๐น๐๐(๐๐๐ก)
๐,๐ = โ ๐๐๐๐ = ๐๏ฟฝ๏ฟฝ๐ (3)
where ๐น(๐๐ฅ๐ก)denotes the total force of the system.
now we will show that because of equation 1, L.H.S of equation 3 is zero.
56
๐๏ฟฝ๏ฟฝ = โ๐น๐๐(๐๐๐ก)
๐,๐
= (1
2โ๐น๐๐
(๐๐๐ก)
๐,๐
+1
2โ๐น๐๐
(๐๐๐ก)
๐,๐
)
since the indices I and j are dummy and vary over the same set of integers, we can interchange
them in the second sum on R.H.S of the last equation. Therefore we have
๐๏ฟฝ๏ฟฝ = โ๐น๐๐(๐๐๐ก)
๐,๐
= (1
2โ๐น๐๐
(๐๐๐ก)
๐,๐
+1
2โ๐น๐๐
(๐๐๐ก)
๐,๐
)
using equation (1), we have
โ ๐น๐๐(๐๐๐ก)
๐,๐ = (1
2โ ๐น๐๐
(๐๐๐ก)๐,๐ โ
1
2โ ๐น๐๐
(๐๐๐ก)๐,๐ )=0
therefore equation (3) becomes
๐๏ฟฝ๏ฟฝ = ๐น(๐๐ฅ๐ก) = โ๐๐๐๐
๐
for centroid, we know that โ ๐๐๐๐๐ = ๐๐๐where ๐๐is the acceleration of centroid.
Hence equation can be written as
๐๏ฟฝ๏ฟฝ = ๐น(๐๐ฅ๐ก) = ๐๐๐ =๐
๐๐ก(๐๐ฃ๐)
If ๐น(๐๐ฅ๐ก) = 0 then ๐๏ฟฝ๏ฟฝ =๐
๐๐ก(๐๐ฃ๐) = 0 ๐๐ ๐ = ๐๐ฃ๐ = constant
We conclude that if external forces are on a system of particles are zero, then its momentum will
be a constant of motion or a conserved quantity.
57
Module No. 132
Example of Conservation of Momentum
Problem Statement
A particle ๐ด, of mass 6 kg, travelling in a straight line at 5 msโ1 collides with a particle ๐ต, of
mass 4 kg, travelling in the same straight line, but in the opposite direction, with a speed of 3
msโ1. Given that after the collision particle ๐ด continues to move in the same direction at 1.5
msโ1, what speed does particle ๐ต move with after the collision? (ยฉ mathcentre 2009)
Given Data
Mass of particle ๐ด = ๐1 = 6kg
Velocity of ๐ด before collision = ๐ข1 =5msโ1
Velocity of ๐ด after collision = ๐ฃ1 =1.5msโ1
Mass of Particle ๐ต = ๐2 = 4kg
Velocity of ๐ต before collision = ๐ข2 = 3msโ1
Required
Velocity of ๐ต after collision = ๐ฃ2 = ?
Solution
It is always useful to depict the collision with the velocities both before and after.
Using the principle of conservation of momentum:
๐1๐ข1 + ๐2๐ข2 = ๐1๐ฃ1 + ๐2๐ฃ2
6 ร 5 + 4 ร (โ3) = 6 ร 1.5 + 4 ร ๐ฃ2
9 = 4 ร ๐ฃ2
๐ฃ2 = 9 4โ = 2.3 ๐ ๐ โ1
So after the collision particle B moves with a speed of 2.3 m sโ1, in the same direction as A.
Example 2
58
Problem Statement
A particle A, of mass 8 kg, collides with a particle B, of mass m2 kg. The velocity of particle
A before the collision was (โ1๐ + 4๐) ๐ ๐ โ1 and the velocity of particle B before the collision
was (โ0.8๐ + 1.4๐) ๐ ๐ โ1. Given the velocity of particle A after the collision was
(โ2๐ + 2๐) ๐ ๐ โ1, and the velocity of particle B was 3๐ ๐ ๐ โ1, what was the mass of particle
B?
Given Data
Mass of particle A= ๐1 = 8kg
Velocity of A before collision= ๐ข1 = (โ1๐ + 4๐) ๐ ๐ โ1
Velocity of A after collision= ๐ฃ1 = (โ2๐ + 2๐) ๐ ๐ โ1
Velocity of B before collision= ๐ข2 = (โ0.8๐ + 1.4๐) ๐ ๐ โ1
Velocity of B after collision= ๐ฃ2 =?3๐ ๐ ๐ โ1
Required
Mass of Particle B= ๐2 =?
Solution
It is always useful to depict the collision with the velocities both before and after.
Using the principle of conservation of momentum:
๐1๐ข1 + ๐2๐ข2 = ๐1๐ฃ1 + ๐2๐ฃ2
6 ร 5 + 4 ร (โ3) = 6 ร 1.5 + 4 ร ๐ฃ2
9 = 4 ร ๐ฃ2
๐ฃ2 = 9 4โ = 2.3msโ1
So after the collision particle ๐ต moves with a speed of 2.3 msโ1, in the same direction as ๐ด.
Example 2
Problem Statement
A particle ๐ด, of mass 8 kg, collides with a particle ๐ต, of mass 2 kg. The velocity of particle ๐ด
before the collision was (โ1๐ + 4๐) msโ1 and the velocity of particle ๐ต before the collision was
(โ0.8๐ + 1.4๐) msโ1. Given the velocity of particle ๐ด after the collision was (โ2๐ + 2๐) msโ1,
and the velocity of particle ๐ต was 3๐ msโ1, what was the mass of particle ๐ต?
Given Data
59
Mass of particle ๐ด = ๐1 = 8kg
Velocity of ๐ด before collision = ๐ข1 = (โ1๐ + 4๐)msโ1
Velocity of ๐ด after collision = ๐ฃ1 = (โ2๐ + 2๐)msโ1
Velocity of ๐ต before collision = ๐ข2 = (โ0.8๐ + 1.4๐)msโ1
Velocity of ๐ต after collision = ๐ฃ2 = 3๐ msโ1
Required
Mass of Particle ๐ต = ๐2 = ?
Solution
By making use of principle of conservation of momentum:
๐1๐ข1 + ๐2๐ข2 = ๐1๐ฃ1 + ๐2๐ฃ2
8(โ1๐ + 4๐) + ๐2(โ0.8๐ + 1.4๐)
= 8(โ2๐ + 2๐) + ๐2(3๐)
โ8๐ + 32๐ + 16๐ โ 16๐ + ๐2(โ0.8๐ + 1.4๐) โ ๐2(3๐) = 0
8๐ + 16๐ + ๐2(โ0.8๐ โ 1.6๐) = 0
8๐ + 16๐ โ ๐2(+0.8๐ + 1.6๐) = 0
8๐ + 16๐ = ๐2(0.8๐ + 1.6๐)
๐2 =0.8๐ + 1.6๐
8๐ + 16๐
By rationalizing and solving the above fraction, we obtain
๐2 = 10๐๐
60
Module No. 133
Angular Momentum โ Derivation
Angular momentum of a particle of mass ๐, position vector ๐ and linear momentum ๐ is defined
as ๐ ร ๐. It is also called moment of momentum.
Let there be a system consists of N particles with positon vector ๐๐ and the momentum ๐๐ (๐ =
1,2,3, โฆโฆ . , ๐) then the total angular momentum L is given by
๐ฟ = โ๐๐ ร ๐๐ = โ๐๐ ร (๐๐ฃ๐)
๐๐
To find the relation between L and ๐, we proceed as follows:
๐ฟ = โ๐๐ ร (๐๐๐ฃ๐)
๐
= โ๐๐ ร ๐๐(๏ฟฝ๏ฟฝ ร ๐๐)
๐
= โ๐๐[๐๐ ร (๏ฟฝ๏ฟฝ ร ๐๐)
๐
]
= โ ๐๐[(๐๐. ๐๐)๏ฟฝ๏ฟฝ โ (๐๐. ๏ฟฝ๏ฟฝ)๐๐๐ ] (1)
Here (๐๐. ๐๐)๏ฟฝ๏ฟฝ and (๐๐. ๏ฟฝ๏ฟฝ)๐๐ are not parallel vectors. If they were parallel then we could easily say
that L is parallel to ๏ฟฝ๏ฟฝ.
We conclude that in general L and ๏ฟฝ๏ฟฝ are not parallel to each other. To find the relationship
between the vectors L and ๏ฟฝ๏ฟฝ we proceed as follows:
Since
๐๐ = ๐ฅ๐๐ + ๐ฆ๐๐ + ๐ง๐๏ฟฝ๏ฟฝ
therefore
๐๐. ๐๐ = ๐ฅ๐2 + ๐ฆ๐
2 + ๐ง๐2
and
๏ฟฝ๏ฟฝ. ๐๐ = ๐๐ฅ๐ฅ๐ + ๐๐ฆ๐ฆ๐ + ๐๐ง๐ง๐
Therefore substituting these values in equation (1), we have
๐ฟ = โ ๐๐(๐ ๐ฅ๐2 + ๐ฆ๐
2 + ๐ง๐2)(๐๐ฅ๐ + ๐๐ฆ๐ + ๐๐ง๏ฟฝ๏ฟฝ) โ (๐๐ฅ๐ฅ๐ + ๐๐ฆ๐ฆ๐ + ๐๐ง๐ง๐)(๐ฅ๐๐ + ๐ฆ๐๐ + ๐ง๐๏ฟฝ๏ฟฝ)
(2)
Writing
๐ฟ = ๐ฟ๐ฅ๐ + ๐ฟ๐ฆ๐ + ๐ฟ๐ง๏ฟฝ๏ฟฝ โก ๐ฟ1๐ + ๐ฟ2๐ + ๐ฟ3๏ฟฝ๏ฟฝ
and
๐ = ๐๐ฅ๐ + ๐๐ฆ๐ + ๐๐ง๏ฟฝ๏ฟฝ โก ๐1๐ + ๐2๐ + ๐3๏ฟฝ๏ฟฝ
and comparing the coefficient of ๐ on both sides of equation (2), we obtain
61
๐ฟ๐ฅ = โ๐๐(
๐
๐ฅ๐2 + ๐ฆ๐
2 + ๐ง๐2)๐๐ฅ โ (๐๐ฅ๐ฅ๐ + ๐๐ฆ๐ฆ๐ + ๐๐ง๐ง๐)๐ฅ๐
= โ๐๐[
๐
๐ฅ๐2๐๐ฅ + (๐ฆ๐
2 + ๐ง๐2)๐๐ฅ โ ๐๐ฅ๐ฅ๐
2 โ ๐๐ฆ๐ฅ๐๐ฆ๐ โ ๐๐ง๐ฅ๐๐ง๐]
= ๐๐ฅ โ ๐๐๐ (๐ฆ๐2 + ๐ง๐
2) โ ๐๐ฅ๐ฅ๐2 โ ๐๐ฆ โ ๐๐๐ ๐ฅ๐๐ฆ๐ โ ๐๐ง โ ๐๐๐ ๐ฅ๐๐ง๐] (3)
As we studied the definitions of moment of inertia and product of inertia be defined as
๐ผ๐ฅ๐ฅ = โ๐๐(๐ฆ๐2 + ๐ง๐
2)
๐
๐ผ๐ฆ๐ฆ = โ๐๐(๐ง๐2 + ๐ฅ๐
2)
๐
๐ผ๐ง๐ง = โ๐๐ (๐ฅ๐2 + ๐ฆ๐
2)
๐
and
๐ผ๐ฅ๐ฆ = โโ๐๐๐ฅ๐๐ฆ๐
๐
๐ผ๐ฆ๐ง = โโ๐๐๐ฆ๐๐ง๐
๐
๐ผ๐ง๐ฅ = โโ๐๐๐ฅ๐๐ง๐
๐
Using these definitions and noting that ๐ผ๐ฅ๐ฆ = ๐ผ๐ฆ๐ฅ etc. we can write
๐ฟ๐ฅ = ๐ผ๐ฅ๐ฅ๐๐ฅ + ๐ผ๐ฅ๐ฆ๐๐ฆ + ๐ผ๐ฅ๐ง๐๐ง
Similarly we obtain
๐ฟ๐ฆ = ๐ผ๐ฆ๐ฅ๐๐ฅ + ๐ผ๐ฆ๐ฆ๐๐ฆ + ๐ผ๐ฆ๐ง๐๐ง
๐ฟ๐ง = ๐ผ๐ง๐ฅ๐๐ฅ + ๐ผ๐ฆ๐ง๐๐ฆ + ๐ผ๐ง๐ง๐๐ง
62
Module No. 134
Angular Momentum in Case of Continuous Distribution of Mass
An actual rigid body consists of very large number of particles and therefore we may suppose that
there is a continuous distribution of mass. If ๐(๐) denotes density of a volume element ๐๐,
surrounding the point ๐, then the mass of this element will be ๐(๐)๐๐.
Hence we can write the expression of angular momentum in case of continuous distribution of
mass by
๐ผ11 โก ๐ผ๐ฅ๐ฅ = โซ๐(๐)(๐ฆ2 + ๐ง2)๐๐
where
๐ = (๐ฅ, ๐ฆ, ๐ง) โก (๐ฅ1, ๐ฅ2, ๐ฅ3), ๐๐ = ๐๐ฅ๐๐ฆ๐๐ง
Similarly the other two moment of inertia are defined as
๐ผ22 โก ๐ผ๐ฆ๐ฆ = โซ๐(๐)(๐ง2 + ๐ฅ2)๐๐
and
๐ผ33 โก ๐ผ๐ง๐ง = โซ๐(๐)(๐ฅ2 + ๐ฆ2)๐๐
Similarly the product of inertia are defined as
๐ผ12 โก ๐ผ๐ฅ๐ฆ = โโซ๐(๐)๐ฅ๐ฆ๐๐,
๐ผ23 โก ๐ผ๐ฆ๐ง = โโซ๐(๐)๐ฆ๐ง๐๐,
๐ผ31 โก ๐ผ๐ง๐ฅ = โโซ๐(๐)๐ง๐ฅ๐๐
By using these definitions of moment of inertia, we can find the expression of angular momentum
by using the following result
๐ฟ = ๐ฟ๐ฅ๐ + ๐ฟ๐ฆ๐ + ๐ฟ๐ง๏ฟฝ๏ฟฝ
๐ฟ = ๐ผ๐ฅ๐ฅ๐๐ฅ + ๐ผ๐ฆ๐ฆ๐๐ฆ + ๐ผ๐ง๐ง๐๐ง + ๐ผ๐ฅ๐ฆ(๐๐ฅ + ๐๐ฆ) + ๐ผ๐ฆ๐ง(๐๐ฆ + ๐๐ง) + ๐ผ๐ง๐ฅ(๐๐ง + ๐๐ฅ)
where
๐ฟ๐ฅ = ๐ผ๐ฅ๐ฅ๐๐ฅ + ๐ผ๐ฅ๐ฆ๐๐ฆ + ๐ผ๐ฅ๐ง๐๐ง
๐ฟ๐ฆ = ๐ผ๐ฆ๐ฅ๐๐ฅ + ๐ผ๐ฆ๐ฆ๐๐ฆ + ๐ผ๐ฆ๐ง๐๐ง
๐ฟ๐ง = ๐ผ๐ง๐ฅ๐๐ฅ + ๐ผ๐ง๐ฆ๐๐ฆ + ๐ผ๐ง๐ง๐๐ง
and ๐๐ฅ, ๐๐ฆ, ๐๐ง are the components of angular velocity along (๐ฅ, ๐ฆ, ๐ง).
or
63
[
๐ฟ๐ฅ
๐ฟ๐ฆ
๐ฟ๐ง
] = [
๐ผ๐ฅ๐ฅ ๐ผ๐ฅ๐ฆ ๐ผ๐ฅ๐ง
๐ผ๐ฅ๐ฆ ๐ผ๐ฆ๐ฆ ๐ผ๐ฆ๐ง
๐ผ๐ฅ๐ง ๐ผ๐ฆ๐ง ๐ผ๐ง๐ง
] [
๐๐ฅ
๐๐ฆ
๐๐ง
]
where we have used the property of products of inertia that
๐ผ๐ฅ๐ฆ = ๐ผ๐ฆ๐ฅ, ๐ผ๐ฆ๐ง = ๐ผ๐ง๐ฆ and
๐ผ๐ฅ๐ง = ๐ผ๐ง๐ฅ
The above matrix form can be written as
๐ฟ = ๐ผ๐
64
Module No. 135
Law of Conservation of Angular Momentum
Angular Momentum
The angular momentum of a single particle is defined as the cross product of linear momentum
and position vector of concerned particle.
Mathematically ๐ฟ = ๐ ร ๐๐ฃ.
Angular Momentum of System of Particles
The angular momentum L of a system of particles is defined accordingly, as the vector sum of the
individual angular momentum, namely,
๐ฟ = โ๐๐ ร ๐๐๐ฃ๐ (1)
๐
๐=1
Law of Conservation of Angular Momentum
The time rate change of angular momentum in the absence of some external forces is zero.
Mathematically, we can write
๐ ๐ณ
๐ ๐= ๐ โน ๐ณ = ๐๐จ๐ง๐ฌ๐ญ๐๐ง๐ญ
Let us calculate the time derivative of the angular momentum. Using the rule for differentiating
the cross product, we find
๐๐ฟ
๐๐ก=
๐
๐๐ก(โ๐๐ ร ๐๐๐ฃ๐
๐
๐=1
) = โ(๐ฃ๐ ร
๐
๐=1
๐๐๐ฃ๐) + โ(๐๐ ร
๐
๐=1
๐๐๐๐)
Now the first term on the right vanishes, because,๐ฃ๐ ร ๐ฃ๐ = 0 and, because ๐๐๐๐ is equal to the
total force acting on particle i, we can write
๐๐ฟ
๐๐ก= โ(๐๐ ร
๐
๐=1
๐๐๐๐)
๐๐ฟ
๐๐ก= โ [๐๐ ร (๐
๐=1 โ ๐น๐(๐๐ฅ๐ก)
๐ + โ โ ๐น๐๐(๐๐๐ก)๐
๐=1๐๐=1 )] (2)
๐๐ฟ
๐๐ก= โ ๐๐ ร ๐น๐
(๐๐ฅ๐ก)๐๐=1 + โ โ ๐น๐๐
(๐๐๐ก)๐๐=1
๐๐=1 ) (3)
where ๐น๐ denotes the total external force on particle ๐, and ๐น๐๐ denotes the (internal) force exerted
on particle i by any other particle ๐. Now the double summation on the right consists of pairs of
terms of the form
65
(๐๐ ร ๐น๐) + (๐๐ ร ๐น๐๐) (4)
Denoting the vector displacement of particle ๐ relative to particle ๐ by ๐๐๐, we see from the triangle
shown in Figure that
๐๐๐ = ๐๐ โ ๐๐ (5)
Therefore, because ๐น๐๐ = -๐น๐๐, expression (3) reduces to
โ๐๐๐ ร ๐น๐๐ (6)
which clearly vanishes if the internal forces are central, that is, if they act along the lines
connecting pairs of particles. Hence, the double sum in Equation (3) vanishes. Now the cross
product (๐๐ ร ๐น๐)is the moment of the external force F. The sum โ(๐๐ ร ๐น๐) is, therefore, the
total moment of all the external forces acting on the system. If we denote the total external
torque, or moment of force, by N, Equation (3) takes the form
๐๐ฟ
๐๐ก= ๐
That is, the time rate of change of the angular momentum of a system is equal to the total
moment of all the external forces acting on the system.
If a system is isolated, then ๐ = 0, and the angular momentum remains constant in both
magnitude and direction:
๐ฟ = โ๐๐ ร ๐๐๐ฃ๐
๐
๐=1
= Constant vector (8)
This is a statement of the principle of conservation of angular momentum. It is a generalization
for a single particle in a central field.
66
Module No. 136
Example Related to Angular Momentum
Problem Statement
The moon revolves around the earth so that we always see the same face of the moon.
i. Find how the spin angular momentum and orbital angular momentum of the moon w.r.t.
the earth are related?
ii. Find the change in spin angular momentum of moon so that we could see the entire
moonโs surface during a month.
Solution
i. Let ๐, ๐ ๐ denotes the mass and the radius of the moon respectively.
Then its spin angular momentum i.e. angular momentum about its axis of rotation is given by
๐ฟ๐ = ๐ผ๐๐
where ๐ผ is the moment of inertia and ๐๐ angular velocity of the moon about its own axis
The moment of inertia of the sphere is
๐ผ =2
5๐๐2
by generalizing it for moon, we obtain the moment of inertia for the moon
๐ผ =2
5๐๐ ๐
2
then the angular momentum of moon will be
๐ฟ๐ =2
5๐๐ ๐
2๐๐ (1)
In addition to spinning about its own axis, the moon is also performing orbital motion about the
earth. If we denote the orbital angular momentum by ๐ฟ0, then
๐ฟ0 = ๐ ร (๐๐) = ๐ ร (๐๐ ๐0)
= ๐๐ 2๐0 (2)
โด ๐ฃ = ๐๐
where we have treated the moon as a particle and R is the distance between the moon and the
earth. Since we always see the same face of the moon, the moon makes on rotation about its axis
in the same time as it makes on revolution around the earth i.e. ๐๐ = ๐0. Form equation (1) and
(2) we have
๐ฟ๐
๐ฟ0=
25
๐๐ ๐ 2๐๐
๐๐ 2๐0
67
=2
5(๐ ๐
๐ )2
ii. If we have to see the entire moonโs surface during a month, then ๐ฟ๐ must either increase
or decrease by one-half of its present value.
68
Module No. 137
Kinetic Energy of a System about Principal Axes โ Derivation
Kinetic Energy
The kinetic energy for a particle is given by the following scalar equation:
๐ =1
2๐๐ฃ2
Where:
T is the kinetic energy of the particle with respect to ground (an inertial reference frame)
m is the mass of the particle
v is the velocity of the particle, with respect to ground
Kinetic Energy of a Rigid Body
For a rigid body experiencing planar (two-dimensional) motion, the kinetic energy is given by
the following general scalar equation:
๐ =1
2๐๐ฃ๐
2 +1
2๐ผ๐๐
2 (1)
where the first term in equation (1) shows the kinetic energy due to the motion of center of mass
and second term shows the rotational kinetic energy and subscript c denotes the center of mass
and ๐ฃ๐ denotes the velocity of c.m and ๐ผ๐ denotes the inertia matrix w.r.t c.m.
Kinetic Energy of a Rigid Body w.r.t Origin
If the rigid body is rotating about a fixed point O that is attached to ground, we can express the
kinetic energy as:
๐ =1
2๐ผ0๐
2
Where:
๐ผ0 is the moment of inertia of the rigid body about an axis passing through the fixed point O, and
perpendicular to the plane of motion and ๐ is the angular velocity.
Kinetic Energy of a Rigid Body about Principal Axes
69
If the center of mass is oriented along with origin then the coordinate axes (xyz axes) and the
principal axes of the rigid body are aligned. In this case, the inertia matrix reduces, i.e. products
of inertia are zero along the principal axes, (๐ผ๐ฅ๐ฆ = ๐ผ๐ฆ๐ง = ๐ผ๐ง๐ฅ = 0) .
For general three-dimensional motion, the kinetic energy of a rigid body about principal axes is
given by the following general scalar equation:
๐ =1
2๐๐ฃ0
2 +1
2๐ผ๐ฅ๐๐ฅ
2 +1
2๐ผ๐ฆ๐๐ฆ
2 +1
2๐ผ๐ง๐๐ง
2 (3)
where ๐ผ๐ฅ, ๐ผ๐ฆ, ๐ผ๐ง are the moment of inertia along principal axes (xyz axes) also called principal
moment of inertia and products of inertia ๐ผ๐ฅ๐ฆ, ๐ผ๐ฆ๐ง , ๐ผ๐ง๐ฅ are zero along the axes
Equation (3) is the required expression of Kinetic Energy along principal axes.
If the rigid body has a fixed point O that is attached to ground, we can give an alternate scalar
equation for the kinetic energy of the rigid body:
๐ =1
2๐ผ๐ฅ๐๐ฅ
2 +1
2๐ผ๐ฆ๐๐ฆ
2 +1
2๐ผ๐ง๐๐ง
2
in this case, the kinetic energy due to the motion of c.m. vanishes.
70
Module No. 138
Moment of Inertia of a Rigid Body about a Given Line
Let M be the mass of the system and ๏ฟฝ๏ฟฝ a unit vector along the line ๐. Then ๏ฟฝ๏ฟฝ = ๐๐ + ๐๐ + ๐๏ฟฝ๏ฟฝ,
where (๐, ๐, ๐)are direction cosines of the line.
If ๐ผ๐ denotes the record moment of inertia, then
๐ผ๐ = โ๐๐๐๐2
๐
Form the figure
๐๐ = |๐๐| sin ๐๐ = |๐๐| sin ๐๐ = ๐๐ sin ๐๐ = |๐๐ ร ๐๐|
Therefore
๐ผ๐ = โ๐๐|๐๐ ร ๐๐|2
๐
(1)
Now
|๐๐ ร ๐๐| = |๐ ๐ ๏ฟฝ๏ฟฝ๐ ๐ ๐๐ฅ๐ ๐ฆ๐ ๐ง๐
|
= (๐๐ง๐ โ ๐๐ฆ๐)๐ + (๐๐ฅ๐ โ ๐๐ง๐)๐ + (๐๐ง๐ โ ๐๐ฅ๐)๏ฟฝ๏ฟฝ
|๐๐ ร ๐๐|2 = (๐๐ง๐ โ ๐๐ฆ๐)
2 + (๐๐ฅ๐ โ ๐๐ง๐)2 + (๐๐ฆ๐ โ ๐๐ฅ๐)
2
Hence on substitution in equation (1), we have
๐ผ๐ = โ ๐๐[
๐
(๐๐ง๐ โ ๐๐ฆ๐)2 + (๐๐ฅ๐ โ ๐๐ง๐)
2 + (๐๐ฆ๐ โ ๐๐ฅ๐)2]
๐ผ๐ = โ๐๐[
๐
๐2๐ง๐2 + ๐2๐ฆ๐
2 โ 2๐๐๐ฆ๐๐ง๐ + ๐2๐ฅ๐2 + ๐2๐ง๐
2 โ 2๐๐๐ฅ๐๐ง๐ + ๐๐ฆ๐ + ๐๐ฅ๐ โ 2๐๐๐ฅ๐๐ฆ๐]
= โ๐๐[
๐
๐2(๐ฅ๐2 + ๐ง๐
2) + ๐2(๐ฅ๐2 + ๐ฆ๐
2) + ๐2(๐ฆ๐2 + ๐ง๐
2) โ 2๐๐๐ฅ๐๐ฆ๐ โ 2๐๐๐ฆ๐๐ง๐ โ 2๐๐๐ฅ๐๐ง๐
= โ๐๐[
๐
๐2(๐ฆ๐2 + ๐ง๐
2) + ๐2(๐ฅ๐2 + ๐ง๐
2) + ๐2(๐ฅ๐2 + ๐ฆ๐
2) โ 2๐๐๐ฅ๐๐ฆ๐ โ 2๐๐๐ฆ๐๐ง๐ โ 2๐๐๐ฅ๐๐ง๐
= ๐2 โ๐๐[
๐
(๐ฆ๐2 + ๐ง๐
2) + ๐2 โ๐๐
๐
(๐ฅ๐2 + ๐ง๐
2) + ๐2 โ๐๐
๐
(๐ฅ๐2 + ๐ฆ๐
2) + 2๐๐(โโ๐๐
๐
๐ฅ๐๐ฆ๐)
+ 2๐๐(โโ๐๐
๐
๐ฆ๐๐ง๐) + 2๐๐(โโ๐๐
๐
๐ฅ๐๐ง๐)
71
or finally,
๐ผ๐ = ๐2๐ผ๐ฅ๐ฅ + ๐2๐ผ๐ฆ๐ฆ + ๐2๐ผ๐ง๐ง + 2๐๐๐ผ๐ฅ๐ฆ + 2๐๐๐ผ๐ฆ๐ง + 2๐๐๐ผ๐ง๐ฅ
which may also be written as
๐ผ๐ = ๐2๐ผ11 + ๐2๐ผ22 + ๐2๐ผ33 + 2๐๐๐ผ12 + 2๐๐๐ผ23 + 2๐๐๐ผ31
72
Module No. 139
Example of M.I of a Rigid Body About Given Line
Problem Statement
Calculate the moment of inertia of a right circular cone about its axis of symmetry.
Solution
Let M be the mass, a the radius and h the height of right circular cone. We regard the cone as
composed of elementary circular discs of small thickness each parallel to the base of the cone.
We choose the z-axis along the axis of symmetry, and consider a typical disc of radius r and
width ๐ฟ๐ง at a distance z from the base.
Mass of the disc is given by
๐ฟ๐ = ๐๐๐2๐ฟ๐ง
We regard the disk to be composed of concentric elementary circular rings of varying radii say ๐โฒ
then the mass ๐ฟ๐โฒ of one circular ring with height ๐ฟ๐ง will be
๐ฟ๐โฒ = ๐2๐๐โฒ๐ฟ๐ง
Then the M.I of one circular ring will be
๐ผ๐.๐ = ๐2๐๐โฒ๐ฟ๐ง(๐โฒ)2 = 2๐๐(๐โฒ)3๐ฟ๐ง
Hence the M.I of the whole disk will be
๐ฟ๐ผ = โ 2๐๐(๐โฒ)3๐ฟ๐ง
๐๐๐๐๐
= โซ2
๐
0
๐(๐ฟ๐
๐๐2๐ฟ๐ง)(๐โฒ)3๐ฟ๐ง๐๐โฒ
=2๐ฟ๐
๐2โซ(๐โฒ)3๐๐โฒ
๐
0
๐ฟ๐ผ =2๐ฟ๐
4๐2๐4 =
1
2๐ฟ๐๐2
From the similar triangles,
we have
73
๐
๐=
โ โ ๐ง
โ or ๐ = ๐
โ โ ๐ง
โ
Therefore
๐ฟ๐ = ๐๐ (๐โ โ ๐ง
โ)2
๐ฟ๐ง
Since the M.I of the disk is
๐ฟ๐ผ =1
2๐ฟ๐๐2
On substituting for ๐ฟ๐ and ๐, the M.I of the cone about its axis of symmetry will be
๐ผ =1
2โซ ๐๐ (๐
โ โ ๐ง
โ)2
(๐โ โ ๐ง
โ)2
โ
0
๐๐ง
=1
2
๐๐๐4โ
โ4โซ(โ โ ๐ง)4๐๐ง
โ
0
=1
2
๐๐๐4โ
โ4โซ(๐ง โ โ)4๐๐ง
โ
0
Since the M.I of the disk is
๐ฟ๐ผ =1
2๐ฟ๐๐2
On substituting for ๐ฟ๐ and ๐, the M.I of the cone about its axis of symmetry will be
๐ผ =1
2โซ ๐๐ (๐
โ โ ๐ง
โ)2
(๐โ โ ๐ง
โ)2
โ
0
๐๐ง
=1
2
๐๐๐4โ
โ4โซ(โ โ ๐ง)4๐๐ง
โ
0
=1
2
๐๐๐4โ
โ4โซ(๐ง โ โ)4๐๐ง
โ
0
๐ผ =๐๐๐4โ
10โ4|(๐ง โ โ)5|0
โ
=๐๐๐4โ6
10โ4=
๐๐๐4โ2
10
Since we know that
๐ = density of the cone
74
=๐
(1 3โ )๐๐2โ
So,
๐ผ =3
10๐๐2
75
Module No. 140
Ellipsoid of Inertia
We have obtained the expression of moment of inertia of the given line l in terms of moment and
product of inertia w.r.t the coordinate axes OXYZ coordinate system whose origin O lies on the
line l.
๐ผ๐ = ๐ผ = ๐2๐ผ11 + ๐2๐ผ22 + ๐2๐ผ33 + 2๐๐๐ผ12 + 2๐๐๐ผ23 + 2๐๐๐ผ31
(1)
For the ellipsoid of inertia, we chose a point P such that |๐๐| = 1โ๐ผ
โ . If (๐ฅ, ๐ฆ, ๐ง) are coordinates
of point P then
๐ =๐ฅ
|๐๐|= ๐ฅโ๐ผ, ๐ =
๐ฆ
|๐๐|= ๐ฆโ๐ผ , ๐ =
๐ง
|๐๐|= ๐งโ๐ผ
On eliminating ๐, ๐, ๐ from equation (1), we obtain
๐ผ = (๐ฅโ๐ผ)2๐ผ11 + (๐ฆโ๐ผ)2๐ผ22 + (๐งโ๐ผ)2๐ผ33 + 2(๐ฅ๐ฆโ๐ผโ๐ผ)๐ผ12 + 2(๐ฆ๐งโ๐ผโ๐ผ)๐ผ23 + 2(๐ฅ๐งโ๐ผโ๐ผ)๐ผ31
๐ผ = ๐ผ[๐ฅ2๐ผ11 + ๐ฆ2๐ผ22 + ๐ง2๐ผ33 + 2๐ฅ๐ฆ๐ผ12 + 2๐ฆ๐ง๐ผ23 + 2๐ง๐ฅ๐ผ31]
1 = ๐ฅ2๐ผ11 + ๐ฆ2๐ผ22 + ๐ง2๐ผ33 + 2๐ฅ๐ฆ๐ผ12 + 2๐ฆ๐ง๐ผ23 + 2๐ง๐ฅ๐ผ31
which can also be written as
๐ผ11๐ฅ2 + ๐ผ22๐ฆ
2 + ๐ผ33๐ง2 + 2๐ผ12๐ฅ๐ฆ + 2๐ผ23๐ฆ๐ง + 2๐ผ31๐ง๐ฅ = 1
Since ๐ผ11, ๐ผ22, ๐ผ33 are all positive, equation (2) represents an ellipsoid. This ellipsoid is called
ellipsoid of inertia or momental ellipsoid. The momental ellipsoid contains information about
moments or product of inertia at given point. If P is any point on the ellipsoid of inertia, then
|๐๐| = 1โ๐ผ
โ or ๐ผ = 1๐๐2โ i.e. the moment of inertia of a rigid body about any line ๐๐ is equals
to the reciprocal of the square of the length of |๐๐ |.
76
Module No. 141
Rotational Kinetic Energy
Introduction to Kinetic Energy
Kinetic energy is the energy produced in any body during its motion. It is equal to the half of the
product of mass and square of the velocity of the moving body;
๐พ. ๐ธ.= ๐ =1
2๐๐ฃ2
We consider a rigid body in a general state of motion in which it has both translation and rotation
w.r.t a fixed coordinate system. We suppose that it has an instantaneous angular velocity ๐ about
a reference point C. We will use a model of a rigid body in which it is considered a collection of
large number N of particles which satisfy the constraint of rigidity.
If v is the velocity of C, then the velocity ๐ฃ๐ of the ith particle is given by
๐ฃ๐ = ๐ฃ + ๐ ร ๐๐
If ๐๐ is the mass and ๐ฃ๐ the velocity of the ith particle, then the kinetic energy of the ith particle
will be
๐๐ =1
2๐๐๐ฃ๐
2
therefore the total kinetic energy of the system will be given by
๐ = โ๐๐
๐
๐=1
= โ1
2๐๐๐ฃ๐
2
๐
๐=1
= โ1
2๐๐(
๐
๐=1
๐ฃ + ๐ ร ๐๐)2
=1
2โ๐๐[
๐
๐=1
๐ฃ2 + 2๐ฃ.๐ ร ๐๐ + (๐ ร ๐๐)2]
=1
2(โ ๐๐
๐
๐=1
)๐ฃ2 +1
2โ๐๐
๐
๐=1
2๐ฃ.๐ ร ๐๐ +1
2โ๐๐(๐ ร ๐๐)
2
๐
๐=1
=1
2๐๐ฃ2 + ๐ฃ. (๐ ร โ๐๐๐๐) +
๐
๐=1
1
2โ๐๐(๐ ร ๐๐)
2
๐
๐=1
=1
2๐๐ฃ2 + ๐ฃ.๐ ร โ ๐๐๐๐ +๐
๐=11
2โ ๐๐(๐ ร ๐๐)
2๐๐=1 (1)
where โ ๐๐ = ๐ ๐๐=1 is the total mass of the system.
Now by using the definition of position vector center of mass (c.m.), we have
๐๐ =โ ๐๐๐๐๐
โ ๐๐๐ (2)
Now we will discuss the consequence of referring of the position vectors of particle of the system
to the c.m. If the position vector of the ith particle of the system w.r.t the c.s. , then
๐๐ = ๐โฒ๐ + ๐๐
77
Then by substitution this expression in the definition of c.m.in (2) , we obtain
๐๐ =โ ๐๐(๐ ๐โฒ
๐ + ๐๐)
โ ๐๐๐
๐๐ =โ ๐๐๐ ๐โฒ
๐
โ ๐๐๐+
๐๐๐๐
๐๐ =โ ๐๐๐ ๐โฒ
๐
โ ๐๐๐+ ๐๐
or we can write
๐๐ =โ ๐๐๐ ๐โฒ
๐
๐+ ๐๐
which gives
โ ๐๐๐
๐โฒ๐ = 0
Hence if the reference point C is identified with the center of the mass and origin is taken thereat,
the expression โ ๐๐๐๐ = 0 ๐๐=1 and therefore
๐ =1
2๐๐ฃ2 +
1
2โ ๐๐(๐ ร ๐๐)
2
๐
๐=1
๐ = ๐๐ก๐ + ๐๐๐๐ก
where ๐๐ก๐ =1
2๐๐ฃ2 is the translational kinetic energy is also equals to the K.E of the center of the
mass and ๐ก๐๐๐ก =1
2โ ๐๐(๐ ร ๐๐)
2๐๐=1 is the rotational kinetic energy of the system.
But we have
(๐ ร ๐๐)2 = (๐ ร ๐๐). (๐ ร ๐๐)
= ๐. ๐๐ ร (๐ ร ๐๐)
therefore on substitution
78
๐๐๐๐ก =1
2โ๐๐[๐. ๐๐ ร (๐ ร ๐๐)] =
1
2๐. ๐ฟ
๐
๐=1
Thus we have obtained the following formulas for translational and rotational K.E. of a rigid
body
๐๐ก๐ =1
2๐๐ฃ2
๐๐๐๐ก =1
2๐. ๐ฟ (1)
As we studied the relation ๐ฟ = ๐ผ๐
By using this value in relation (1) we obtain the rotational kinetic energy in terms of moment of
inertia.
๐๐๐๐ก =1
2๐. ๐ผ๐ =
1
2๐ผ๐2 (2)
where ๐ผ is the moment of inertia of the rigid body about the origin and equation (2) is the
required expression for Rotational Kinetic Energy.
79
Module No. 142
Moment of Inertia & Angular Momentum in Tensor Notation
To show that the 3 ร 3 inertia matrix (๐ผ๐๐ ) is also a Cartesian tensor of rank 3, we proceed as
follows. Here we have to distinguish between the particle index and the component index, which
denotes particle number, will be denoted by Greek letter ๐ผ, and will take the value
1,2, โฆโฆ . , ๐. On the other hand, the component index, which denotes the component number
will be denotes by the Latin letters such as i and will take the value 1,2,3.
Using this notation, we can write the angular momentum of the system of N particles as
๐ฟ = โ๐๐ผ ร ๐๐ผ =
๐ผ
โ๐๐ผ ร (๐๐ผ๐ฃ๐ผ)
๐ผ
where ๐ฃ๐ผ is the velocity of ๐ผ๐กโ particle. Continuing we have
โ๐๐ผ๐๐ผ ร ๐ฃ๐ผ =
๐ผ
โ๐๐ผ๐๐ผ ร (๐ ร ๐๐ผ) , โด ๐ฃ๐ผ = ๐ ร ๐๐ผ๐ผ
Recall that every particle of the rigid body has the same angular velocity at a given ๐ก.
Simplification of the above reduces to
๐ฟ = โ๐๐ผ((๐๐ผ. ๐๐ผ)๐ โ (๐. ๐๐ผ)๐๐ผ)
๐ผ
= โ๐๐ผ(๐๐ผ2๐ โ (๐. ๐๐ผ)๐๐ผ)
๐ผ
But
๐. ๐๐ผ = โ๐๐๐๐ผ,๐ =
3
๐=1
โ๐๐๐ฅ๐ผ,๐ =
3
๐=1
โ๐๐๐ฅ๐ผ,๐
3
๐=1
where
๐๐ผ = (๐ฅ๐ผ , ๐ฆ๐ผ, ๐ง๐ผ) = (๐ฅ๐ผ,1, ๐ฅ๐ผ,2, ๐ฅ๐ผ,3) โก ๐ฅ๐ผ,๐
Making substitution and taking the ith component of the expression of angular momentum,
we have
๐ฟ๐ = โ๐๐ผ (๐๐ผ2๐๐ โ (โ๐๐
๐
๐ฅ๐ผ,๐)๐ฅ๐ผ,๐)
๐ผ
But ๐๐ = โ ๐๐๐ ๐ฟ๐๐; where ๐ฟ๐๐ = {0, ๐ โ ๐1, ๐ = ๐
, thus
80
therefore
๐ฟ๐ = โ๐๐ผ (๐๐ผ2 โ๐๐
๐
๐ฟ๐๐ โ โ๐๐
๐
๐ฅ๐ผ,๐๐ฅ๐ผ,๐)
๐ผ
๐ฟ๐ = โ๐๐ผ โ(๐๐ผ2๐ฟ๐๐ โ ๐ฅ๐ผ,๐๐ฅ๐ผ,๐)๐๐
๐๐ผ
๐ฟ๐ = โ๐๐
๐
โ๐๐ผ(๐๐ผ2๐ฟ๐๐ โ ๐ฅ๐ผ,๐๐ฅ๐ผ,๐)
๐ผ
which can also be expressed as
๐ฟ๐ = โ ๐๐๐ ๐ผ๐๐ (1)
where
๐ผ๐๐ = โ ๐๐ผ(๐๐ผ2๐ฟ๐๐ โ ๐ฅ๐ผ,๐๐ฅ๐ผ,๐)๐ผ (2)
Equation (1) shows that the component of angular momentum depends not only on the angular
velocity ๐ but also on the inertia tensor ๐ผ๐๐.
Since the angular velocity (๐๐) and the angular momentum (๐ฟ๐) are known to be cartesion tensor
of rank 1, it follows from the quotient theorem ๐ผ๐๐ that ๐ผ๐๐ is a tensor of rank 2. It is called the
inertia tensor.
81
Module No. 143
Introduction to Special Moments of Inertia
In this module, we will study some special moment of inertia. In order to introduce the moment
of inertia of some specific shapes and special rigid bodies, we assume that the rigid body is of
uniform density of particles.
Solid Circular Cylinder
We assume the radius of cylinder is ๐ and mass ๐ about axis of cylinder.
๐ผ =1
2๐๐2
Hollow Circular Cylinder
We assume the radius of cylinder is ๐ and mass ๐ about axis of cylinder.
We consider the thickness of Wall of cylinder is negligible.
๐ผ = ๐๐2
Solid Sphere
We assume the radius of sphere is ๐ and mass ๐ about a diameter.
๐ผ =2
5๐๐2
Hollow Sphere
We assume the radius of sphere is ๐ and mass ๐ about a diameter.
We consider the thickness of sphere is negligible.
๐ผ = ๐๐2
Rectangular Plate
We consider sides of length ๐ and ๐, and mass M about an axis perpendicular to the plate
through the center of mass.
๐ผ =1
12๐(๐2 + ๐2)
Thin Rod
We assume the length of rod is ๐ and mass M about an axis perpendicular to the rod through the
center of mass.
82
๐ผ =1
12๐๐2
Triangular Lamina
We assume the height ๐ and mass M of lamina.
๐ผ =1
6๐โ2
Right Circular Cone
We assume the radius of the circular cone is ๐ and mass M.
๐ผ =3
10๐๐2
83
Module No. 144
M.I. of the Thin Rod โ Derivation
Problem Statement
Calculate the moment of inertia of a uniform rod of length ๐ about an axis perpendicular to the
rod and passing through an end point.
Solution
Let the ๐ โ ๐๐ฅ๐๐ be chosen along the length of the rod, with origin at one end point as shown in
the figure. Let ๐ and ๐ be the mass and length of rod respectively. We suppose the rod to be
composed of small elements.
Let ๐๐ and ๐๐ฅ be the mass and the length of the specific element of the rod at a distance ๐ฅ from
the end point O.
Then
๐๐
๐๐ฅ=
๐
๐
โน ๐๐ = ๐
๐๐๐ฅ
Then the moment of inertia of this element about the given axis is
๐ผ๐๐๐๐๐๐๐ก = ๐๐๐ฅ2 =๐
๐๐ฅ2๐๐ฅ
84
Hence the moment of inertia of the whole rod will be
๐ผ = โ๐
๐๐๐๐ ๐๐๐๐๐๐๐ก๐
๐ฅ2๐๐ฅ
=๐
๐โซ ๐ฅ2๐๐ฅ
๐
0
=๐
๐. |
๐ฅ3
3|0
๐
=๐
๐
๐3
3
=1
3๐๐2
85
Module No. 145
M.I. of Hoop or Circular Ring โ Derivation
Problem Statement
Calculate the moment of inertia of a hoop of mass ๐ and radius ๐ about an axis passing through
its center.
Proof
Let ๐ be the mass and ๐ the radius of the hoop. Then we can define the density of the hoop by
๐ =mass
area=
๐
2๐๐๐๐
We consider this hoop to be composed of small masses (๐ฟ๐) each of length ๐ฟ๐ .
We can write it as
๐ =๐
2๐๐๐๐=
๐ฟ๐
๐๐๐ฟ๐
โน ๐ฟ๐ =๐
2๐๐๐ฟ๐
Moment of inertia of the small portion of the hoop of mass ๐ฟ๐ about an axis through center and
perpendicular to the plane of the ring equals
๐ผ๐๐๐๐ก๐๐๐๐ = ๐ฟ๐๐2
=๐
2๐๐๐ฟ๐ ๐2 =
๐๐
2๐๐ฟ๐
Therefore the ๐. ๐ผ of the
whole ring/hoop will be
86
๐ผ =๐๐
2๐โ ๐ฟ๐
๐๐๐๐ก๐๐๐๐๐
๐ผ =๐๐
2๐โซ๐๐
=๐๐
2๐2๐๐
= ๐๐2
Hence we obtain ๐๐2 as the moment of inertia of the hoop.
87
Module No. 146
M.I. of Annular Disk - Derivation Problem
Calculate the moment of inertia of annular disk of mass ๐. The inner radius of the annulus is ๐ 1
and the outer radius is ๐ 2 about an axis passing through its center.
Solution
Subdivide the annulur disk into concentric rings one of which is shown in the fig.
Let the mass of the ring is ๐๐, and the radius be ๐, then the moment of inertia of the ring will be:
๐ผ๐๐๐๐ = ๐2๐๐
The Surface area of the ring is
Area = (2๐๐)๐๐ = 2๐๐๐๐
Since the surface area of the
annulus is
๐(๐ 22 โ ๐ 1
2)
Therefore, we can have
88
๐๐
๐=
2๐๐๐๐
๐(๐ 22 โ ๐ 1
2)
๐๐ =2๐๐๐
(๐ 22 โ ๐ 1
2)๐
Since the moment of Inertia of the ring is:
๐ผ๐๐๐๐ = ๐2๐๐
or
๐ผ๐๐๐๐ = ๐22๐๐๐
๐ 22 โ ๐ 1
2 ๐ =2๐๐3๐๐
๐ 22 โ ๐ 1
2
Thus the total M.I of the annulur disk will be
๐ผ = โซ ๐ผ๐๐๐๐
๐ 2
๐=๐ 1
๐ผ = โซ2๐๐3๐๐
๐ 22 โ ๐ 1
2
๐ 2
๐=๐ 1
=2๐
๐ 22 โ ๐ 1
2 โซ ๐3๐๐
๐ 2
๐=๐ 1
=2๐
๐ 22 โ ๐ 1
2 |๐4
4|๐ 1
๐ 2
=2๐
๐ 22 โ ๐ 1
2
๐ 24 โ ๐ 1
4
4
๐ผ =1
2๐(๐ 2
2 + ๐ 12)
89
Module No. 147
M.I. of a Circular Disk - Derivation
Problem
To find the moment of inertia of a circular disk of radius ๐, and mass ๐ about the axis of the disk.
Solution
Subdivide the disk into concentric rings one of which is the element (ring) shown in the fig.
Let the mass the of the ring is ๐๐, then the moment of inertia of the ring will be:
๐ผ๐๐๐๐ = ๐2๐๐
The Surface area this element is
Area = (2๐๐)๐๐ = 2๐๐๐๐
Since we have
๐๐
๐=
2๐๐๐๐
๐๐2
๐๐ =2๐๐๐
๐2๐
Since the moment of Inertia of the ring is:
90
๐ผ๐๐๐๐ = ๐2๐๐
or
๐ผ๐๐๐๐ = ๐22๐๐๐
๐2๐ =
2๐๐3๐๐
๐2
Thus the total moment of inertia of the circular disk will be
๐ผ๐๐๐ ๐ = โซ ๐ผ๐๐๐๐
๐
๐=0
๐ผ๐๐๐ ๐ = โซ2๐๐3๐๐
๐2
๐
๐=0
=2๐
๐2โซ ๐3๐๐
๐
๐=0
=2๐
๐2|๐4
4|0
๐
=2๐
๐2
๐4
4
๐ผ๐๐๐ ๐ =1
2๐๐2
which is M.I of a disk.
91
Module No. 148
Rectangular Plate โ Derivation
Problem Statement
Calculate the inertia matrix of a uniform rectangular plate with sides ๐ and ๐ about its side.
Solution
We consider a rectangular plate (lamina) of sides of length a and b. We consider an element of
length ๐๐ฅ and ๐๐ฆ.
The mass of selected element will be
๐๐ = ๐๐๐ฅ๐๐ฆ
Its moment of inertia about the y-axis is
๐๐๐ฅ๐๐ฆ๐ฅ2 = ๐๐ฅ2๐๐ฅ๐๐ฆ.
Thus the total moment of inertia is
๐ผ = โซ โซ ๐๐ฅ2๐๐ฅ๐๐ฆ
๐
๐ฆ=0
๐
๐ฅ=0
= ๐๐ โซ๐ฅ2
๐
0
= ๐๐ |๐ฅ3
3|0
๐
=1
3๐๐๐3
Since the total mass of rectangular plate is
๐ = ๐๐๐
the moment o inertia will be
๐ผ =1
3๐๐2
92
Module No. 149
M.I. of Square Plate โ Derivation
Problem Statement
Calculate the moment of inertia of a uniform square plate with sides ๐ about any axis through its
center and lying in the plane of plate.
Solution
Consider a uniform square plate with length of its side to be ๐, we have to find out M.I. of this
plate about any axis, through its center.
We consider this axis as ๐ฆ โ ๐๐ฅ๐๐ .
Let ๐ be the density of plate and the total area of square plate is ๐2.
So density will be
๐ =๐
๐2
We assume that the plate has been divided into vertical strips. Let us consider a strip from the
whole square as shown in figure.
The strips are chosen in this way because each point on a particular strip is approximately the
same distance from axis of rotation i.e. y-axis, the mass of the strip is ๐ฟ๐ and the width of each
strip is ๐ฟ๐ฅ, then the area of the strip will be ๐๐ฟ๐ฅ, so
๐ฟ๐ = ๐๐๐ฟ๐ฅ
Let the distance of the strip from y-axis is ๐ฅ, then the moment of inertia of strip will be
๐ฟ๐ผ = ๐ฟ๐๐ฅ2 = ๐ฅ2๐๐๐ฟ๐ฅ
So the moment of inertia of square is
๐ผ๐ ๐๐ข๐๐๐ = โ ๐ฅ2๐๐๐ฟ๐ฅ
๐๐๐ ๐ ๐ก๐๐๐๐
93
๐ผ๐ ๐๐ข๐๐๐ = โซ ๐ฅ2๐๐๐๐ฅ
๐2โ
โ๐2โ
= ๐๐ โซ ๐ฅ2๐๐ฅ
๐2โ
โ๐2โ
= ๐๐ |๐ฅ3
3|โ๐
2โ
๐2โ
=๐๐
3|๐3
8โ
(โ๐3)
8|
=๐๐
3(๐3
4) =
๐๐4
12
Now substitute ๐ =๐
๐2, we obtain
๐ผ๐ ๐๐ข๐๐๐ =๐๐2
12
94
Module No. 150
M.I. of Triangular Lamina โ Derivation
Problem Statement
Find the moment of inertia of a uniform triangular lamina of mass ๐ about one of its sides.
Solution
Let ABC be the lamina. We will find its M.I. about one of its side BC. We choose the X-axis and
the origin as shown in figure.
At a distance x from BC we consider a strip of lamina of width dx. If the mass of the strip is dm
then its moment of inertia about BC is = ๐ฅ2๐๐.
To find dm we note that the triangles ABC and ADE are similar. Therefore the ratios of sides are
the same.
Hence
๐ท๐ธ
๐ต๐ถ=
โ๐๐๐โ๐ก ๐๐ ๐ด๐ท๐ธ
โ๐๐๐โ๐ก ๐๐ ๐ด๐ต๐ถ=
โ โ ๐ฅ
โ
which gives
๐ท๐ธ =โ โ ๐ฅ
โร ๐ต๐ถ =
โ โ ๐ฅ
โร ๐
where h is the height of the triangle ABC at base BC and a is the length of BC. Now
๐ฟ๐ = ๐(๐๐๐๐) = ๐๐ท๐ธ๐ฟ๐ฅ
= ๐ (โ โ ๐ฅ
โ)๐ฟ๐ฅ๐
95
where ๐ is the density.
Therefore the moment of inertia of triangular lamina about the side BC is
๐ผ = โซ๐ฅ2๐๐ =โซ๐ (โ โ ๐ฅ
โ) ๐ฅ2๐๐ฅ๐
โ
0
=๐๐
โโซ๐ฅ2(โ โ ๐ฅ)๐๐ฅ
โ
0
=๐๐
โโซ(โ๐ฅ2 โ ๐ฅ3)๐๐ฅ
โ
0
=๐๐
โ|โ
๐ฅ3
3โ
๐ฅ4
4|0
โ
=๐๐
โ(โ4
3โ
โ4
4) =
๐๐โ3
12
Now substitute density= ๐ =๐๐๐ ๐
๐ด๐๐๐=
๐
1 2โ ๐โ
we obtain
๐ผ =1
6๐โ2
Hence the required expression for moment of inertia of the triangular lamina is 1
6๐โ2.
96
Module No. 151
M.I. of Elliptical Plate along its Major Axis โ Derivation
Problem Statement
Find the M.I. of a uniform elliptical plate with semi major axes and semi minor axes a, b
respectively about its major axes.
Solution
We consider the elliptical plate as
๐ฅ2
๐2 +๐ฆ2
๐2 = 1 (1)
with semi major axes along x-axis as shown in figure.
From (1) we have
๐ฆ = ยฑ๐
๐โ๐2 โ ๐ฅ2
So, let ๐ฆ1 =๐
๐โ๐2 โ ๐ฅ2,
We consider a small element of plate of mass ๐ฟ๐ of the elliptical plate will be ๐๐๐ which will be
equal to ๐๐๐ฅ๐๐ฆ. The moment of inertia if this element along x-axis will be equal to ๐ผ = ๐ฟ๐๐ฆ2.
Then the moment of inertia of whole plate will be
๐ผ๐ฅ = โซ(๐ฟ๐)๐ฆ2 = โซ ๐๐๐
๐๐๐๐ก๐
๐ฆ2
97
= ๐ โซ
(
โซ ๐ฆ2๐๐ฆ
๐๐โ๐2โ๐ฅ2
โ๐๐โ๐2โ๐ฅ2
)
๐๐ฅ
๐
โ๐
= ๐ โซ2๐ฆ1
2
3๐๐ฅ
๐
โ๐
=2๐
3โซ
๐3
๐3(๐2 โ ๐ฅ2)
32โ ๐๐ฅ
๐
โ๐
due to symmetry, we can write it as
=4๐๐3
3๐3โซ(๐2 โ ๐ฅ2)
32โ
๐
0
๐๐ฅ
By making use of polar coordinates, substitute ๐ฅ = ๐ sin ๐, then ๐๐ฅ = ๐ cos ๐ this integral
becomes
๐ผ๐ฅ =4๐๐3
3๐3โซ ๐3 cos3 ๐ (๐ cos ๐
๐2โ
0
)๐๐
=4๐๐๐3
3โซ cos4 ๐
๐2โ
0
๐๐
=4๐๐๐3
3
1 ร 3
2 ร 4ร
๐
2
=1
4๐๐๐3๐
=1
4๐๐3๐ ร
๐
๐๐๐=
1
4๐๐2
where for elliptical plate, we have
๐ =๐
๐๐๐
is the required expression for M.I.
98
Module No. 152
M.I. of a Solid Circular Cylinder - Derivation
Problem Statement
To find the moment of inertia of a solid circular cylinder of radius ๐, mass ๐ and the height of
the cylinder โ about the axis of the cylinder.
Solution
Letโs subdivide the solid circular cylinder into concentric cylindrical shells/ hollow cylinders,
one of which is shown in the fig.
Let the mass of one shell is๐๐, height is same as โ, thickness be ๐๐ and the radius be ๐ then the
density of the shell will be
๐ =๐๐
2๐๐๐๐โ
or
๐๐ = 2๐๐๐๐๐โ
Since we have
99
๐ =๐
๐๐๐ฆ๐๐๐๐๐๐=
๐
๐๐2โ=
๐๐
2๐๐๐๐โ
or
๐๐ =2๐
๐2๐๐๐
Since the moment of inertia of the shell will be:
๐ผ๐ โ๐๐๐ = ๐2๐๐
or
๐ผ๐ โ๐๐๐ = ๐22๐
๐2๐๐๐ =
2๐
๐2๐3๐๐
Thus the total moment of inertia of the solid circular cylinder will be
๐ผ๐๐ฆ๐๐๐๐๐๐ = โซ ๐ผ๐ โ๐๐๐
๐
๐=0
๐ผ๐๐ฆ๐๐๐๐๐๐ = โซ2๐
๐2๐3๐๐
๐
๐=0
=2๐
๐2โซ ๐3๐๐
๐
๐=0
=2๐
๐2|๐4
4|0
๐
=2๐
๐2
๐4
4
๐ผ๐๐ฆ๐๐๐๐๐๐ =1
2๐๐2
is the M.I of solid cylinder.
100
Module No. 153
M.I. of Hollow Cylindrical Shell - Derivation
Problem
To find the moment of inertia of a hollow open cylindrical shell of radius ๐ , thickness ๐๐ , mass
๐ and height of shell โ about the axis of shell.
Solution
Letโs subdivide the hollow shell into small hoops/ rings, one of which is shown in the figure.
Let the mass of one ring is๐๐, height is ๐โ, thickness be ๐๐ and the radius be ๐ , then the mass
of one ring will be
101
๐๐ = ๐2๐๐ ๐๐ ๐โ
Hence the moment of inertia of the ring of radius ๐ will be
๐ผ๐๐๐๐ = ๐๐๐ 2
or
๐ผ๐๐๐๐ = (๐2๐๐ ๐๐ ๐โ)๐ 2 = 2๐๐๐ 3๐๐ ๐โ
In order to obtain the moment of inertia for the whole hollow cylindrical open shell, we will
integrate
๐ผ = โซ ๐ผ๐๐๐๐ = โซ2๐๐๐ 3๐๐ ๐โ
โ
0
๐ผ = 2๐๐๐ 3๐๐ โซ๐โ
โ
0
= 2๐๐๐ 3๐๐ โ
Since the density of the hollow cylindrical shell is
๐ =๐
2๐๐ ๐๐ โ
Therefore
๐ผ = ๐๐ 2
is the M.I of hollow cylindrical open shell.
102
Module No. 154
M.I of Solid Sphere - Derivation
Problem
Find the moment of inertia of a uniform solid sphere of radius ๐ and mass ๐ด about an axis (the
z-axis) passing through the center.
Solution
For a uniform solid sphere, due to symmetry, we have
๐ผ๐ฅ๐ฅ = ๐ผ๐ฆ๐ฆ = ๐ผ๐ง๐ง
In order to calculate the moment of Inertia of the sphere, we split the sphere into thin circular
discs, one of which is shown in Figure.
We have already derived the expression for the moment of inertia of a representative disc of
radius ๐ฅ, which is
๐ผ๐๐๐ ๐ =1
2๐ฅ2๐๐
of an elementary disc of mass ๐๐ and the radius ๐ฅ.
As we know the mass = (density)(Area of disc)
therefore
103
๐๐ = ๐๐๐ฅ2
Hence moment of inertia of the sphere along z-axis will be
๐ผ๐ง๐ง = โซ1
2๐๐๐ฅ4๐๐ง
๐
โ๐
Now, to write โ๐ฅโin
terms of ๐ง, we make
a triangle as shown in fig,
where
๐2 = ๐ฅ2 + ๐ง2, โน ๐ฅ2 = ๐2 โ ๐ง2
๐ผ๐ง๐ง = โซ1
2๐๐(๐2 โ ๐ง2)2๐๐ง
๐
โ๐
=8
15๐๐๐5
Since the mass of the sphere is
๐ =4
3๐๐3๐
Therefore
๐ผ๐ง๐ง =2
5๐๐2
Also
๐ผ๐ฅ๐ฅ = ๐ผ๐ฆ๐ฆ = ๐ผ๐ง๐ง =2
5๐๐2
is the required moment of inertia of the sphere.
104
Module No. 155
M.I. of the Hollow Sphere โ Derivation
Problem
A thin uniform hollow sphere has a radius ๐น and mass ๐ด. Calculate its moment of inertia about
any axis through its center.
Solution
In order to calculate the moment of inertia of the hollow sphere, we split the hollow sphere into
thin hoops (rings), as shown in Figure.
We have already derived the expression for the moment of inertia of a representative hoop of
radius ๐ฅ, which is
๐ผ = ๐๐๐ฅ2
of an elementary ring of mass ๐๐ and the radius ๐ฅ.
The volume of the elementary ring is
๐๐ = 2๐๐ฅ๐ ๐๐๐๐
As we know the mass = (density)(volume)
105
๐๐ = ๐๐๐
therefore
๐๐ = ๐2๐๐ฅ๐ ๐๐ ๐๐
Hence the moment of inertia of the small ring of radius ๐ฅ will be
๐ผ๐๐๐๐ = ๐๐๐ฅ2
or
๐ผ๐๐๐๐ = (2๐๐ฅ๐๐ ๐๐ ๐๐)๐ฅ2 = 2๐๐๐ ๐๐ ๐ฅ3๐๐
which is the moment of inertia of ring of radius ๐ฅ chosen from the hollow sphere.
In order to obtain the moment of inertia for the whole hollow sphere, we will integrate
๐ผ = โซ ๐ผ๐๐๐๐ = โซ 2๐๐๐ ๐๐ ๐ฅ3๐๐
๐2โ
โ๐2โ
due to symmetry, we can write
๐ผ = 4๐๐๐ ๐๐ โซ ๐ฅ3๐๐
๐2โ
0
To solve the integral, we need to write ๐ฅ in terms of ๐. From fig we have
๐ฅ = ๐ cos ๐
The integral becomes,
๐ผ = 4๐๐๐ ๐๐ โซ (๐ cos ๐)3๐๐
๐2โ
0
๐ผ = 4๐๐๐ 4๐๐ โซ cos3 ๐ ๐๐
๐2โ
0
๐ผ = 4๐๐๐ 4๐๐ โซ cos ๐ cos2 ๐ ๐๐
๐2โ
0
106
๐ผ = 4๐๐๐ 4๐๐ โซ cos ๐ (1 โ sin2 ๐)๐๐
๐2โ
0
๐ผ = 4๐๐๐ 4๐๐ โซ cos ๐ (1 โ sin2 ๐)๐๐
๐2โ
0
๐ผ = 4๐๐๐ 4๐๐ (โซ cos ๐ ๐๐
๐2โ
0
โ โซ sin2 ๐ cos ๐ ๐๐)
๐2โ
0
๐ผ = 4๐๐๐ 4๐๐ |sin ๐ โsin3 ๐
3|0
๐2โ
= 4๐๐๐ 4๐๐ (1 โ1
3)
= 4๐๐๐ 4๐๐ ร2
3
=8
3๐๐๐ 4๐๐
Since the density of the hollow sphere is
๐ =๐
๐=
๐
4๐๐ 2๐๐
Hence M.I of hollow sphere will be
๐ผ =2
3๐๐ 2
107
Module No. 156
Inertia Matrix / Tensor of solid Cuboid
Problem Statement
Calculate the inertia matrix / inertia tensor of uniform solid cube at one of its corners.
Solution
Let the length of the edges be ๐ of each side and let the axes be chosenalong the edges as shown
in the figure.
By definition
๐ผ11 โก ๐ผ๐ฅ๐ฅ = โซ๐(๐)(๐ฆ2 + ๐ง2)๐๐
Since the box is made of uniform material, the density ๐ must be constant. Therefore
๐ผ๐ฅ๐ฅ = ๐ โซ โซโซ(๐ฆ2 + ๐ง2)๐๐ฅ๐๐ฆ๐๐ง
๐
0
๐
0
๐
0
= ๐ โซ โซ(๐ฆ2 + ๐ง2)๐๐ฆ๐๐ง
๐
0
๐
0
โซ๐๐ฅ
๐
0
= ๐๐ โซโซ(๐ฆ2 + ๐ง2)๐๐ฆ๐๐ง
๐
0
๐
0
108
= ๐๐ [โซ โซ๐ฆ2๐๐ฆ๐๐ง
๐
0
๐
0
+ โซ โซ๐ง2๐๐ฆ๐๐ง
๐
0
๐
0
]
= ๐ (๐4
3+
๐4
3)
๐ผ๐ฅ๐ฅ = ๐๐ (2๐4
3) = ๐ (
2๐5
3)
Since we know that for the cube
๐ =๐
๐3
We obtain
๐ผ๐ฅ๐ฅ =2๐
3๐2
Similarly, due to symmetry, we can write
๐ผ๐ฆ๐ฆ =2๐
3๐2
๐ผ๐ง๐ง =2๐
3๐2
Now for the product of inertia, we have
๐ผ12 = โโซ๐๐ฅ๐ฆ๐๐
= โ๐ โซโซ โซ๐ฅ๐ฆ๐๐ฅ๐๐ฆ๐๐ง
๐
0
๐
0
๐
0
= โ๐ โซ๐ฅ๐๐ฅ โซ ๐ฆ๐๐ฆ โซ ๐๐ง
๐
0
๐
0
๐
0
= โ๐๐๐2
2
๐2
2= โ๐
๐5
4
Again using
๐ =๐
๐3
109
We obtain
๐ผ12 = โ๐๐2
4
Similarly,
๐ผ12 = โ๐๐2
4
๐ผ12 = โ๐๐2
4
The required inertia matrix / inertia tensor will be
๐ผ๐๐ =
[ 2๐
3๐2 โ
๐๐2
4โ
๐๐2
4
โ๐๐2
4
2๐
3๐2 โ
๐๐2
4
โ๐๐2
4โ
๐๐2
4
2๐
3๐2
]
110
Module No. 157
Inertia Matrix / Tensor of solid Cuboid
Problem Statement
Calculate the inertia matrix of uniform solid cuboid (parallelepiped) at one of its corner.
Solution
Let the length of the edges be ๐, ๐, ๐ and let the axes be chosen along the edges as shown in the
figure.
By definition
๐ผ11 โก ๐ผ๐ฅ๐ฅ
= โซ๐(๐)(๐ฆ2 + ๐ง2)๐๐
Since the box is made of uniform material, the density ๐ must be constant. Therefore
๐ผ๐ฅ๐ฅ = ๐ โซโซโซ(๐ฆ2 + ๐ง2)๐๐ฅ๐๐ฆ๐๐ง
๐
0
๐
0
๐
0
= ๐ โซโซ(๐ฆ2 + ๐ง2)๐๐ฆ๐๐ง
๐
0
๐
0
โซ๐๐ฅ
๐
0
111
= ๐๐ โซโซ(๐ฆ2 + ๐ง2)๐๐ฆ๐๐ง
๐
0
๐
0
= ๐๐ [โซโซ๐ฆ2๐๐ฆ๐๐ง
๐
0
๐
0
+ โซโซ๐ง2๐๐ฆ๐๐ง
๐
0
๐
0
]
= ๐๐ (๐๐3
3+ ๐
๐3
3)
๐ผ๐ฅ๐ฅ = ๐๐๐๐
3(๐2 + ๐2)
using relation
๐ =๐
๐๐๐
๐ผ๐ฅ๐ฅ =๐
3(๐2 + ๐2)
Similarly, due to symmetry, we can write
๐ผ๐ฆ๐ฆ =๐
3(๐2 + ๐2)
๐ผ๐ง๐ง =๐
3(๐2 + ๐2)
Now for the product of inertia, we have
๐ผ12 = โโซ๐๐ฅ๐ฆ๐๐
= โ๐ โซโซโซ๐ฅ๐ฆ๐๐ฅ๐๐ฆ๐๐ง
๐
0
๐
0
๐
0
= โ๐ โซ๐ฅ๐๐ฅ โซ๐ฆ๐๐ฆ โซ๐๐ง
๐
0
๐
0
๐
0
โ๐๐๐2
2
๐2
2= โ๐
๐2๐2๐
4
Again using
112
๐ =๐
๐๐๐
we get
๐ผ12 = โ๐๐๐
4
Similarly,
๐ผ23 = โ๐๐๐
4
๐ผ31 = โ๐๐๐
4
The required inertia matrix / inertia tensor will be
๐ผ๐๐ =
[ ๐
3(๐2 + ๐2) โ
๐๐๐
4โ
๐๐๐
4
โ๐๐๐
4
๐
3(๐2 + ๐2) โ
๐๐๐
4
โ๐๐๐
4โ
๐๐๐
4
๐
3(๐2 + ๐2)]
113
Module No. 158
M.I of Hemi-Sphere โ Derivation
Problem Statement
Find the moment of inertia of a uniform hemisphere of radius ๐ about its axis of symmetry.
Solution
We will use the spherical polar coordinates (๐, ๐, ๐). Their use makes computational work
simpler.
Their range of variation for hemisphere will be
0 โค ๐ < ๐
0 โค ๐ โค ๐2โ
0 โค ๐ โค 2๐
๐ฅ = ๐ sin ๐ cos๐ ,
๐ฆ = ๐ sin ๐ sin๐ , ๐ง = ๐ cos ๐
We choose the z-axis as the axis of symmetry.
Hence
114
๐ผ๐ง๐ง = โซ๐(๐)(๐ฅ2 + ๐ฆ2)๐๐
= ๐ โซ(๐ฅ2 + ๐ฆ2)๐๐
Now calculate ๐ฅ2 + ๐ฆ2 in terms of sp. coordinates
๐ฅ2 + ๐ฆ2 = ๐2(sin2 ๐ cos2 ๐ + sin2 ๐ sin2 ๐)
= ๐2 sin2 ๐ (cos2 ๐ + sin2 ๐)
= ๐2 sin2 ๐
and the element of volume in spherical polar coordinates is given by
๐๐ = ๐๐(๐๐๐)(๐ sin ๐๐๐) = ๐2 sin ๐ ๐๐๐๐๐๐
Therefore
๐ผ๐ง๐ง = ๐ โซ โซ โซ ๐4 sin3 ๐
2๐
0
๐2โ
0
๐
0
๐๐๐๐๐๐
= โซ ๐4๐๐
๐
0
โซ sin3 ๐ ๐๐
๐2โ
0
โซ ๐๐
2๐
0
= 2๐๐๐5
5โซ sin3 ๐ ๐๐
๐2โ
0
= 2๐๐๐5
5โซ
3 sin ๐ โ sin 3๐
4๐๐
๐2โ
0
= (2๐๐๐5
5)1
4|โ3 cos ๐ +
cos 3๐
3|0
๐2โ
= ๐๐๐5
10[(0) โ (โ3 + 1/3)]
= ๐๐๐5
10(8
3)
115
= 4๐๐๐5
15
By using the relation for density
๐ =๐
23โ ๐๐3
we obtain
๐ผ๐ง๐ง =2
5๐๐2
which is required expression for moment of inertia of hemisphere.
116
Module No. 159
M.I. of Ellipsoid โDerivation
Problem
Find the moment of inertia and product of inertia for the ellipsoid
๐ฅ2
๐2+
๐ฆ2
๐2+
๐ง2
๐2= 1
w.r.to its axes of symmetry.
Solution
By definition
๐ผ11 โก ๐ผ๐ฅ๐ฅ
= โซ ๐(๐)(๐ฆ2 + ๐ง2)๐๐
๐๐๐๐๐๐ ๐๐๐
If we put
๐ฅ
๐= ๐ฅโฒ,
๐ฆ
๐= ๐ฆโฒ,
๐ง
๐= ๐งโฒ
then
๐ฅ = ๐๐ฅโฒ, ๐ฆ = ๐๐ฆโฒ, ๐ง = ๐๐งโฒ
โน ๐๐ฅ = ๐๐๐ฅโฒ, ๐๐ฆ = ๐๐๐ฆโฒ, ๐๐ง = ๐๐๐งโฒ
and
๐๐ = ๐๐ฅ๐๐ฆ๐๐ง = ๐๐๐๐๐ฅโฒ๐๐ฆโฒ๐๐งโฒ
Now under the above transformation, the given ellipsoid is transformed into the unit sphere S:
๐ฅโฒ2 + ๐ฆโฒ2 + ๐งโฒ2 = 1
The integration is now over the region enclosed by the unit sphere.
117
๐ผ11 = ๐ โซ(๐2๐ฆโฒ2 + ๐2๐งโฒ2)๐๐๐๐๐ฅโฒ๐๐ฆโฒ๐๐งโฒ
๐
or
๐ผ11 = ๐๐๐3๐ โซ ๐ฆโฒ2๐๐โฒ + ๐๐๐๐3 โซ ๐งโฒ2๐๐โฒ
๐ ๐
where ๐๐โฒ = ๐๐ฅโฒ๐๐ฆโฒ๐๐งโฒ
Now because of symmetry
โซ ๐ฆโฒ2
๐
๐๐โฒ = โซ ๐งโฒ2
๐
๐๐โฒ
Now we solve one integral.
We use the spherical polar coordinates (๐, ๐, ๐).
Their range of variation will be
0 โค ๐ < 1, 0 โค ๐ โค ๐, 0 โค ๐ โค 2๐
๐ฅ = ๐ sin ๐ cos๐ , ๐ฆ = ๐ sin ๐ sin ๐ , ๐ง = ๐ cos ๐
and
๐๐โฒ = ๐๐(๐๐๐)(๐ sin ๐๐๐) = ๐2 sin ๐ ๐๐๐๐๐๐
Thus
โซ ๐ฆโฒ2
๐
๐๐โฒ = โซโซ โซ ๐4 sin3 ๐
2๐
0
๐
0
1
0
sin2 ๐ ๐๐๐๐๐๐
= โซ๐4๐๐
1
0
โซ sin3 ๐ ๐๐
๐
0
โซ sin2 ๐ ๐๐
2๐
0
=1
5
1
4|โ3 cos ๐ +
cos 3๐
3|0
๐ 1
2|๐ โ
sin 2๐
2|0
2๐
Thus
118
โซ ๐ฆโฒ2
๐
๐๐โฒ =2๐
15
Therefore on substitution
๐ผ11 = ๐๐๐3๐2๐
15+ ๐๐๐๐3
2๐
15
๐ผ11 = ๐๐๐๐2๐
15(๐2 + ๐2)
=๐
(4 3โ )๐๐๐๐๐๐๐ ร
2๐
15(๐2 + ๐2)
๐ผ11 =๐
10(๐2 + ๐2)
Similarly,
๐ผ22 =๐
10(๐2 + ๐2)
๐ผ33 =๐
10(๐2 + ๐2)
For product of inertia
๐ผ12 โก ๐ผ๐ฅ๐ฆ = โ โซ ๐ฅ๐ฆ๐๐
๐๐๐๐๐๐ ๐๐๐
= โ๐ โซ ๐๐ฅโฒ๐๐ฆโฒ(๐๐๐๐๐ฅโฒ๐๐ฆโฒ๐๐งโฒ
๐
)
= โ๐๐2๐2๐ โญ๐ฅโฒ๐ฆโฒ๐๐ฅโฒ๐๐ฆโฒ๐๐งโฒ
๐
Using the polar coordinates (๐, ๐, ๐)
๐ผ12 = โ๐๐2๐2๐ โซ โซ โซ๐ sin ๐ cos๐(
1
0
๐
0
2๐
0
rsin ๐ sin๐)๐2 sin ๐ ๐๐๐๐๐๐
= 0
119
Similarly, we can obtain
๐ผ23 = 0
๐ผ31 = 0
120
Module No. 160
Example 1 of Moment of Inertia
Problem
Particles of masses 2๐, 3๐ and 4๐ are held in a rigid light framework at points (0,1,1), (1,1,0)
and (โ1, 0, 1) resp.
Show that the M. I. of the system about the ๐ฅ, ๐ฆ, ๐ง โ axis are 11, 13, 12 respectively.
Solution
Since the masses are 2, 3 and 4 i.e. ๐1 = 2, ๐2 = 3 and ๐3 = 4, and the points are given by
๐1(0,1,1), ๐2(1,1,0) and ๐3(โ1,0,1).
We are required to find out the moment of inertia about x-axis, y-axis and z-axis i.e. ๐ผ๐ฅ๐ฅ, ๐ผ๐ฆ๐ฆ, ๐ผ๐ง๐ง .
๐ผ๐ฅ๐ฅ = โ๐๐(
๐
๐ฆ๐2 + ๐ง๐
2)
= ๐1(๐ฆ12 + ๐ง1
2) + ๐2(๐ฆ22 + ๐ง2
2) + ๐3(๐ฆ32 + ๐ง3
2)
= 2(1 + 1) + 3(1 + 0) + 4(1 + 0)
= 4 + 3 + 4 = 11
So,
๐ผ๐ฅ๐ฅ = 11
๐ผ๐ฆ๐ฆ = โ๐๐(
๐
๐ฅ๐2 + ๐ง๐
2)
= ๐1(๐ฅ12 + ๐ง1
2) + ๐2(๐ฅ22 + ๐ง2
2) + ๐3(๐ฅ32 + ๐ง3
2)
= 2(0 + 1) + 3(1 + 0) + 4(1 + 1)
= 2 + 3 + 8 = 1
So,
๐ผ๐ฆ๐ฆ = 13
121
๐ผ๐ง๐ง = โ๐๐(๐ฅ๐2 +
๐
๐ฆ๐2)
= ๐1(๐ฆ12 + ๐ฅ1
2) + ๐2(๐ฅ22 + ๐ฆ2
2) + ๐3(๐ฅ32 + ๐ฆ3
2)
= 2(0 + 1) + 3(1 + 1) + 4(1 + 0)
= 2 + 6 + 4 = 12
So,
๐ผ๐ง๐ง = 12
Hence Showed
122
Module No. 161
Example 2 of Moment of Inertia
Problem Statement
A boy of mass ๐ = 30kg is running with a velocity of 3 m/sec on ground just tangentially
to a merry-go-round which is at rest. The boy suddenly jumps on the merry-go-round. Calculate
the angular velocity acquired by the system. The merry-go-round has a radius of ๐ = 2m and a
mass ๐ = 120kg and its moment of inertia is 120 kgmโ2.
Solution
The merry-go-round rotates about an axis which we regard as passing through its c.m.
Letโs give the following notations:
M.I of boy = ๐ผ1
M.I of merry go round = ๐ผ2
Vel. of boy = ๐ฃ1
Vel. of merry go round = ๐ฃ2
Angular vel. of boy = ๐1
Angular vel. of merry go round = ๐2
The moment of inertia ๐ผ1 of the boy about the axis of rotation can be found by
๐ผ1 = ๐๐2 = 30 ร 22 = 120๐๐๐โ2
The moment of inertia ๐ผ2 of merry go round is given to be
๐ผ2 = 120๐๐๐โ2
Since the velocity of the boy about the merry-go-round is ๐ฃ1 = 3msโ1, therefore his angular
velocity ๐1 about the axis of rotation is therefore
๐1 =๐ฃ
๐=
3
2= 1.5 radian per second
Initially the merry go round is at rest, therefore ๐ฃ2 = 0, and thus its angular velocity ๐2 = 0.
We ignore friction and therefore there is no external torque on the system.
Hence by the law of conservation of angular momentum
(๐ผ1 + ๐ผ2)๐ = ๐ผ1๐1 + ๐ผ2๐2
๐ =๐ผ1๐1 + ๐ผ2๐2
(๐ผ1 + ๐ผ2)
123
where ๐ is the angular velocity of the system when the boy jumps on the merry-go-round.
๐ =120(5) + 0
120 + 120
= 0.75 radian per sec
124
Module No. 162
Example 3 of Moment of Inertia
Problem Statement[1]
Two particles of masses ๐1 and ๐2 are connected by a rigid massless rod of length ๐ and moves
freely in a plane. Show that the M. I. of the system about an axis perpendicular to the plane and
passing through the center of mass is ๐๐2 where
๐ =๐1๐2
๐1 + ๐2
Solution
Let ๐1 be the distance of mass ๐1 from center of mass ๐ถ. Then ๐ โ ๐1 is the distance of the mass
๐2 from ๐ถ.Since ๐ถ is the center of the mass.
So,
๐1๐1 = ๐2(๐ โ ๐1)
โน ๐1๐1 = ๐2๐ โ ๐2๐1
โน (๐1 + ๐2)๐1 = ๐2๐
โน ๐1 =๐2๐
(๐1 + ๐2)
125
โน ๐ โ ๐1 = ๐ โ๐2๐
(๐1 + ๐2)
โน ๐ โ ๐1 =๐1๐ + ๐2๐ โ ๐2๐
(๐1 + ๐2)
โน ๐ โ ๐1 =๐1๐
(๐1 + ๐2)
Thus the M.I. about an axis through ๐ถ is
๐1๐12 + ๐2(๐ โ ๐1
2)2
= ๐1 (๐2๐
๐1 + ๐2)2
+ ๐2 (๐1๐
๐1 + ๐2)2
=๐1๐2๐
2
(๐1 + ๐2)2(๐1 + ๐2) =
๐1๐2
๐1 + ๐2๐2
๐ผ = ๐๐2
where
๐ =๐1๐2
๐1 + ๐2
Hence showed.
126
Module No. 163
Example 4 of Moment of Inertia
Problem (ยฉ engineering.unl.edu)
Calculate the moment of inertia of the shaded area given in figure about y-axis.
Solution
Here, we have
๐ฆ = ๐ฅ2/3 (1)
We consider an elementary strip from the shaded area whose M.I is
๐ผ๐ ๐ก๐๐๐ =1
3๐ฅ2(๐ฅ๐๐ฆ) =
1
3๐ฅ3๐๐ฆ
127
Then by integration, we obtain the moment of inertia of the whole shaded area
๐ผ๐ฆ = โ3โซ๐ฆ3
2โ ๐๐ฆ
4
1
= โ32
5|๐ฆ
52โ |
1
4
= โ32
5[(4)
52โ โ (1)
52โ ]
=2โ3
5[(4)
52โ โ (1)
52โ ]
=2โ3
5(32 โ 1)
= 21.5 inch4
128
By using (1), we get
๐ผ๐ ๐ก๐๐๐ = โ3๐ฆ3
2โ ๐๐ฆ
129
Module No. 164
Example 5 of Moment of Inertia
Problem
Determine the moment of inertia of the shaded area shown in figure with respect to each of the
coordinate axes.
Solution
Here we have
๐ฆ = ๐๐ฅ2 (1)
From fig. we have
๐ฅ = ๐, ๐ฆ = ๐, then
๐ = ๐๐2 โน ๐ =๐
๐2
Substituting the value
of k in (1), we obtain
๐ฆ =๐
๐2๐ฅ2 or ๐ฅ =
๐
โ๐โ๐ฆ
Now the moment of inertia along x-axis will be
130
๐ผ๐ฅ = โซ ๐ฆ2๐๐ด = โซ๐ฆ2(๐ โ ๐ฅ)๐๐ฆ
๐
0๐ด
= โซ๐ฆ2 (๐ โ๐
โ๐โ๐ฆ)๐๐ฆ
๐
0
= ๐ โซ๐ฆ2๐๐ฆ โ๐
โ๐โซ๐ฆ
52โ ๐๐ฆ
๐
0
๐
0
= ๐ |๐ฆ3
3|0
๐
โ๐
โ๐|2
7๐ฆ
72โ |
0
๐
=๐๐3
3โ
๐
โ๐
2
7๐
72โ
=๐๐3
3โ
2๐๐3
7
๐ผ๐ฅ =๐๐3
21
๐ผ๐ฆ = โซ ๐ฅ2๐๐ด = โซ ๐ฅ2๐ฆ๐๐ฅ
๐
0๐ด
=๐
๐2|๐ฅ5
5|0
๐
=๐
๐2
๐5
5
๐ผ๐ฆ =๐3๐
5
Hence
๐ผ๐ฅ =๐๐3
21
and
๐ผ๐ฆ =๐3๐
5
are moment of inertia about x-axis and y-axis respectively.
131
Module No. 165
Example 5 of Moment of Inertia
Theorem Statement
The moment of inertia of a rigid body about a given axis is equal to the same about a parallel
axis through the centroid plus the moment of inertia due to the total mass placed at the centroid,
(the last quantity will be referred to as moment of inertia of the centroid).
Proof
By definition, if ๐ผ denotes the M.I about the given axis, and ๐๐ denotes the distance of ๐๐กโ particle
from the given axis, then
๐ผ = โ๐๐๐๐2
๐
If ๐ is the unit vector in the direction of the axis, then we have the following result
๐๐2 = (๐ ร ๐๐)
2
Let ๐๐ denotes the position vector of the centroid and
๐๐โฒ the position vector of the ๐๐กโ particle w.r.t to the centroid, then
132
๐๐ = ๐๐ + ๐๐โฒ
On making substitutions, we obtain
๐ผ = โ๐๐ [๐ ร (๐๐ + ๐๐โฒ)]
2
๐
= โ๐๐ [๐ ร ๐๐ + ๐ ร ๐๐โฒ]
2
๐
= โ๐๐
๐
[(๐ ร ๐๐ )2 + (๐ ร ๐๐
โฒ)2 + 2(๐ ร ๐๐ ) . (๐ ร ๐๐โฒ)]
= โ๐๐(๐ ร ๐๐ )2 + โ๐๐
๐
(๐ ร ๐๐โฒ)2
๐
+2(๐ ร ๐๐ ) .โ๐๐(๐ ร ๐๐โฒ)
๐
= โ ๐๐๐๐2 + โ๐๐
๐
๐๐โฒ2 + 2(๐ ร ๐๐ ) . ๐ ร โ๐๐๐๐
โฒ
๐๐
But โ ๐๐๐๐โฒ
๐ = 0 (We studied in earlier module).
Therefore
๐ผ = (โ๐๐)๐๐2 + โ๐๐
๐
๐๐โฒ2
๐
= ๐๐๐2 + ๐ผโฒ
where
๐ = โ๐๐
๐
is the total mass of the system.
or
๐ผ = ๐ผ0 + ๐ผโฒ
where ๐ผ0 denotes the moment of inertia of the centroid, and ๐ผโฒ denotes the M.I of the system
w.r.to a parallel axis through the centroid.
133
Module No. 166
Example 1 of Parallel Axis Theorem
Problem
Use the parallel axis theorem to find the moment of inertia of a solid circular cylinder about a
line on the surface of the cylinder and parallel to axis of cylinder.
Solution
Suppose the cross section of cylinder as in figure. Then the axis of the cylinder is passing
through the point ๐ถ, while the line on the surface of cylinder is passing through ๐ด. So, we have to
find out M.I of circular cylinder about a line passing through the point ๐ด whose radius is ๐
(radius of circular cylinder) and mass is ๐.
By parallel axis theorem
๐ผ๐ด = ๐ผ๐ + ๐๐2 (1)
Since ๐ผ๐ถ which is the moment of inertia of a solid circular cylinder about an axis passing from
the center of mass is defined by
๐ผ๐ถ = 1
2 ๐๐2 (2)
where ๐ is the radius of a solid circular cylinder.
By substituting equation (2) in equation (1) we have
๐ผ๐ด = 1
2 ๐๐2 + ๐๐2
134
โน
๐ผ๐ด = (1
2+ 1) ๐๐2
๐ผ๐ด = 3
2๐๐2
135
Module No. 167
Example 2 of Parallel Axis Theorem
Problem
Prove that the moment of inertia of a uniform right circular cone using parallel axis theorem of
mass ๐, height โ and semi vertical angle ๐ผ about a diameter of its base is
๐โ2(3 tan2 ๐ผ + 2) 20โ
Solution
In the case of M.I about its diameter, we consider the elementary disc of mass ๐ฟ๐ whose
moment of inertia about a diameter will be
๐ฟ๐ผ0 =1
4๐2๐ฟ๐
We note that the diameter passes through the center (which is also the centroid) of the
elementary disc.
Hence by parallel axis theorem, the M.I. ๐ฟ๐ผ of the elementary disc about a parallel axis (parallel
diameter) at the base is given by
136
๐ฟ๐ผ = ๐ฟ๐ผ0 + (๐ฟ๐)๐ง2
=1
4๐2๐ฟ๐ + ๐ฟ๐๐ง2 = ๐ฟ๐(
1
4๐2 + ๐ง2)
= ๐๐๐2๐ฟ๐ง(1
4๐2 + ๐ง2) = ๐๐ (
1
4๐4 + ๐2๐ง2) ๐ฟ๐ง
From the similar triangles,
we have
๐
๐=
โ โ ๐ง
โ or ๐ = ๐
โ โ ๐ง
โ
Therefore = ๐๐ [๐4
4โ4 (โ โ ๐ง)4 +๐2
โ2 (โ โ ๐ง)2๐ง2] ๐ฟ๐ง
= ๐๐ [๐4
4โ4(โ โ ๐ง)4 +
๐2
โ2(โ2๐ง2 โ 2โ๐ง3 + ๐ง4)] ๐ฟ๐ง
Therefore M.I of complete right circular cone about a diameter is given by
๐ผ = ๐๐ โซ{๐4
4โ4(โ โ ๐ง)4 +
๐2
โ2(โ2๐ง2 โ 2โ๐ง3 + ๐ง4)}
โ
0
๐ฟ๐ง
๐ผ = ๐๐ (๐4
4โ4
โ5
5+
๐2
โ2
โ5
30)
Since we know that ๐ =๐
(1 3โ )๐๐2โ
๐ผ =๐
20(3๐2 + 2โ2)
137
Since the semi vertical angle of
the right circular cone is ๐ผ,
So by right triangle AOB, we have
tan๐ผ =๐ด๐
๐๐ต=
๐
โ
๐ = โ tan๐ผ
Therefore
๐ผ =๐
20[3(htan๐ผ)2 + 2โ2]
or
๐ผ =๐โ2
20[3 tan2 ๐ผ + 2]
Hence proved.
138
Module No. 168
Example 3 of Parallel Axis Theorem
Problem[1]
Find the moment of inertia of a uniform circular cylinder of length ๐ and radius ๐ about an axis
through the center and perpendicular to the central axis, namely ๐ผ๐ฅ or ๐ผ๐ฆ.
Solution
Consider the elementary disc of mass ๐๐ and thickness ๐๐ง located at a distance ๐ง from the ๐ฅ๐ฆ
plane. Then the moment of inertia about a diameter will be
๐ฟ๐ผ0 =1
4๐2๐๐
Then, using the parallel-axis theorem, moment of inertia of the thin disc about the ๐ฅ โ ๐๐ฅ๐๐ will
be
๐๐ผ๐ฅ =1
4๐2๐๐ + ๐ง2๐๐
where ๐๐ = ๐๐๐2๐๐ง.
Thus
139
๐ผ๐ฅ = ๐๐๐2 โซ (1
4๐2 + ๐ง2)
๐2โ
โ๐2โ
๐๐ง
= ๐๐๐2 (1
4๐2๐ +
1
12๐3)
but the mass of the cylinder ๐ is ๐๐๐2๐,
therefore
๐ผ๐ฅ = ๐ (1
4๐2 +
1
12๐2)
Due to symmetry we have
๐ผ๐ฅ = ๐ผ๐ฆ = ๐ (1
4๐2 +
1
12๐2)
140
Module No. 169
Example 4 of Parallel Axis Theorem
Problem
Calculate the moment of inertia ๐ผ๐๐ for a uniform rod of length ๐ and mass ๐ rotating about an
axis through the center, perpendicular to the rod.
Solution
In order to calculate the moment of inertia through the center of mass c.m., we use parallel axes
theorem.
In a transparent notation
๐ผ๐ = ๐ผ๐๐ + ๐๐2 (1)
where ๐ is the distance between the origin and the center of mass and ๐ = ๐2โ .
Also ๐ผ๐ is the moment of inertia of rod about one of its end (which we calculated earlier).
Here
๐ผ๐ =1
3๐๐2 (2)
From (1), we have
๐ผ๐๐ = ๐ผ๐ โ ๐๐2
Substituting value from (2) in (1)
๐ผ๐๐ =1
3๐๐2 โ ๐ (
๐
2)
2
=1
3๐๐2 โ
1
4๐๐2
๐ผ๐๐ =1
12๐๐2
which is required moment of inertia about its center of mass.
141
Module No. 170
Perpendicular Axis Theorem
Theorem Statement
The moment of inertia of a plane rigid body about an axis perpendicular to the body is equal to
the sum of the moment of inertia about two mutually perpendicular axis lying in the plane of the
body and meeting at the common point with the given axis.
Proof
We choose the coordinates such that ๐๐ axes lie in the plane of the rigid body and the ๐-axis is
perpendicular to it.
Then the theorem can be stated as
๐ผ33 = ๐ผ11 + ๐ผ22
๐r
๐ผ๐ง๐ง = ๐ผ๐ฅ๐ฅ + ๐ผ๐ฆ๐ฆ
where ๐ผ11 = ๐ผ๐ฅ๐ฅ etc. are M.I about the ๐ฅ-axis etc.
Since the ๐ง-axis has been
chosen to be perpendicular
to the laminar body, therefore
๐ผ33 = ๐ผ๐ง๐ง = โ ๐๐๐๐2
๐
142
where ๐๐ is the distance of the ๐๐กโ particle (lying in the ๐ฅ๐ฆ-plane) from the ๐ง-axis.
If we denote the coordinate of this particle by (๐ฅ๐, ๐ฆ๐), then
๐๐2 = ๐ฅ๐
2 + ๐ฆ๐2
and therefore
๐ผ33 = ๐ผ๐ง๐ง = โ๐๐(
๐
๐ฅ๐2 + ๐ฆ๐
2)
= โ๐๐๐ฅ๐2
๐
+ โ๐๐๐ฆ๐2
๐
= ๐ผ๐ฅ๐ฅ + ๐ผ๐ฆ๐ฆ
Hence the proof.
143
Module No. 171
Example 1 of Perpendicular Axis Theorem
Problem Statement
Find the moment of inertia of a uniform circular plate (or disc) about any diameter.
Proof
Since we have deduced that moment of inertia of a uniform circular disc is
๐ผ๐๐๐ ๐ = 1
2 M๐2
which is about a line passing through center and perpendicular to the plane.
Considering the same axis with same terms in 3-dim body (i.e. ๐ง โ ๐๐ฅ๐๐ passing through center
of mass and perpendicular to ๐ฅ๐ฆ โ ๐๐๐๐๐), we have using perpendicular axis theorem
๐ผ๐ง๐ง = ๐ผ๐ฅ๐ฅ + ๐ผ๐ฆ๐ฆ (1)
We have to find out M.I. of uniform circular plate (disc) about any of its diameters which are
along ๐ฅ โ ๐๐ฅ๐๐ and ๐ฆ โ ๐๐ฅ๐๐ (i.e. we have to find out ๐ผ๐ฅ๐ฅ ๐๐ ๐ผ๐ฆ๐ฆ both of them are equal to ๐ผ๐)
Since
๐ผ๐ฅ๐ฅ = ๐ผ๐ = ๐ผ๐ฆ๐ฆ
So equation (1) becomes
๐ผ๐ง๐ง = ๐ผ๐ + ๐ผ๐
144
or
๐ผ๐ง๐ง = 2๐ผ๐
๐ผ๐ =1
2๐ผ๐ง๐ง
๐ผ๐ = 1
2(1
2 M๐2)
(โด ๐ผ๐ง๐ง = ๐ผ๐๐๐ ๐ = 1
2 M๐2)
๐ผ๐ =1
4๐๐2
where ๐ is the mass of disc and a is its radius.
145
Module No. 172
Example 2 of Perpendicular Axis Theorem
Problem Statement
Find the M.I. of a uniform elliptical lamina with semi major axes and semi minor axes ๐, ๐
respectively about respective axes (x, y, z-axis) using perpendicular axis theorem.
Proof
We consider the elliptical plate as
๐ฅ2
๐2+
๐ฆ2
๐2= 1 (1)
with semi major axes along x-axis as shown in figure.
From (1) we have
๐ฆ = ยฑ๐
๐โ๐2 โ ๐ฅ2
let ๐ฆ1 =๐
๐โ๐2 โ ๐ฅ2
We consider a small element
of mass ๐ฟ๐ of the elliptical
plate, then we will have
๐ฟ๐ = ๐๐๐ = ๐๐๐ฅ๐๐ฆ
146
The moment of inertia of this element along x-axis will be equal to ๐ผ = ๐ฟ๐๐ฆ2.
Then the moment of inertia of whole plate will be
๐ผ๐ฅ๐ฅ = โซ(๐ฟ๐)๐ฆ2 = โซ ๐๐๐ ๐๐๐๐ก๐
๐ฆ2
= ๐ โซ ( โซ ๐ฆ2๐๐ฆ
๐ฆ1
โ๐ฆ1
)๐๐ฅ
๐
โ๐
= ๐ โซ2๐ฆ1
3
3๐๐ฅ
๐
โ๐
=2๐
3โซ
๐3
๐3(๐2 โ ๐ฅ2)
32โ ๐๐ฅ
๐
โ๐
due to symmetry, we can write it as
=4๐๐3
3๐3โซ(๐2 โ ๐ฅ2)
32โ
๐
0
๐๐ฅ
By making use of polar coordinates,
substitute ๐ฅ = ๐ sin ๐, then ๐๐ฅ = ๐ cos ๐ this integral becomes
๐ผ๐ฅ๐ฅ =4๐๐3
3๐3โซ ๐3 cos3 ๐ (๐ cos ๐
๐2โ
0
)๐๐
=4๐๐๐3
3โซ cos4 ๐
๐2โ
0
๐๐
=1
4๐๐๐3๐
for elliptical plate, we have
๐ =๐
๐๐๐
Thus
๐ผ๐ฅ๐ฅ =1
4๐๐3๐ ร
๐
๐๐๐
147
๐ผ๐ฅ๐ฅ =1
4๐๐2
Similarly, about y-axis
๐ผ๐ฆ๐ฆ = โซ(๐ฟ๐)๐ฅ2 = โซ ๐๐๐
๐๐๐๐ก๐
๐ฅ2
= ๐ โซ( โซ ๐ฅ2๐๐ฅ
๐ฅ1
โ๐ฅ1
)๐๐ฆ
๐
โ๐
= ๐ โซ2๐ฅ1
2
3๐๐ฆ
๐
โ๐
=2๐
3โซ
๐3
๐3(๐2 โ ๐ฆ2)
32โ ๐๐ฆ
๐
โ๐
due to symmetry, we can write it as
=4๐๐3
3๐3โซ(๐2 โ ๐ฆ2)
32โ
๐
0
๐๐ฆ
Using polar coordinates, substitute ๐ฆ = ๐ sin ๐, then ๐๐ฆ = ๐ cos ๐ ๐๐ this integral becomes
๐ผ๐ฆ๐ฆ =4๐๐3
3๐3โซ ๐3 cos3 ๐ (๐ cos ๐
๐2โ
0
)๐๐
=4๐๐3๐
3โซ cos4 ๐
๐2โ
0
๐๐
=4๐๐3๐
3
3
8ร
๐
2
=1
4๐๐3๐๐
=1
4๐3๐๐ ร
๐
๐๐๐
๐ผ๐ฆ๐ฆ =1
4๐๐2
now using perpendicular axis theorem, we obtain
148
๐ผ๐ง๐ง = ๐ผ๐ฅ๐ฅ + ๐ผ๐ฆ๐ฆ
=1
4๐๐2 +
1
4๐๐2
๐ผ๐ง๐ง =1
4๐(๐2 + ๐2)
which is M.I. of elliptical lamina along z-axis
149
Module No. 173
Example 3 of Perpendicular Axis Theorem
Problem
Find the moment of inertia of a rectangular plate with sides ๐ and ๐ about an axis perpendicular
to the plate and passing through a vertex using perpendicular axis theorem.
Solution
We consider a rectangular plate (lamina) of sides of length ๐ and ๐. We consider an element of
length ๐๐ฅ and ๐๐ฆ as shown in figure.
We find the M.I about ๐ฆ โ ๐๐ฅ๐๐ .
The mass of selected element will be
๐๐ = ๐๐๐ฅ๐๐ฆ
Itโs moment of inertia about the ๐ฆ โ ๐๐ฅ๐๐ is
๐ผ๐๐๐๐๐๐๐ก = ๐๐๐ฅ๐๐ฆ๐ฅ2 = ๐๐ฅ2๐๐ฅ๐๐ฆ.
where ๐ฅ is the perpendicular distance from the element to the ๐ฆ โ ๐๐ฅ๐๐ .
150
Thus the total moment of inertia about y-axis is
๐ผ๐ฆ = โซ โซ ๐๐ฅ2๐๐ฅ๐๐ฆ
๐
๐ฆ=0
๐
๐ฅ=0
= ๐๐ โซ๐ฅ2
๐
0
= ๐๐ |๐ฅ3
3|0
๐
=1
3๐๐๐3
Since the density of the rectangular plate is
๐ =๐
๐๐
the moment of inertia will be
๐ผ๐ฆ =1
3๐๐2
In the similar manner, we will calculate the moment of inertia about x-axis
The total moment of inertia about x-axis is
๐ผ๐ฅ = โซ โซ ๐๐ฆ2๐๐ฅ๐๐ฆ
๐
๐ฆ=0
๐
๐ฅ=0
= ๐๐ โซ๐ฆ2๐๐ฆ
๐
0
= ๐๐ |๐ฆ3
3|0
๐
=1
3๐๐๐3
Using the relation of the total mass of rectangular plate
๐ = ๐๐๐
Then the moment of inertia will be
๐ผ๐ฅ =1
3๐๐2
Thus by using perpendicular axes theorem, we obtain the moment of inertia along z-axis
๐ผ๐ง = ๐ผ๐ฅ + ๐ผ๐ฆ
๐ผ๐ง =1
3๐๐2 +
1
3๐๐2
151
๐ผ๐ง =1
3๐(๐2 + ๐2)
which are the required moments of inertia of rectangle along ๐ฅ, ๐ฆ, ๐ง-axis.
152
Module No. 174
Problem of Moment of Inertia
Problem
Find the moment of inertia about the line of an apparatus (as shown in figure) consisting of a sphere
of mass ๐ and radius ๐ attached to a rod of length 2๐ & mass ๐.
Solution
Since from the figure it is clear that ๐ is the radius of the sphere whose mass is ๐ and is attached
to the rod of length 2๐ whose mass is ๐.
Therefore
๐ผ๐ฟ = ๐ผ1๐ฟ + ๐ผ2๐ฟ
where ๐ผ1๐ฟ is the M.I. of a rod of length 2๐ about ๐ฟ and ๐ผ2๐ฟ is the M.I. of a sphere of radius ๐
about the line ๐ฟ, then
153
๐ผ1๐ฟ = โซ ๐๐ฅ2๐๐ฅ
2๐
0
= ๐ |๐ฅ3
3|0
2๐
= ๐8
3๐3
using relation for ๐, we get
๐ผ1๐ฟ =4
3๐๐2 (2)
Now
๐ผ2๐ฟ = ๐ผโฒ + ๐๐2
where ๐ผโฒ is the MI of sphere about its diameter
๐ผ2๐ฟ =2
5๐๐2 + ๐(2๐ + ๐)2 (3)
Substituting equation (2) & (3) in (1), we get
๐ผ๐ฟ =4
3๐๐2 +
2
5๐๐2 + ๐(2๐ + ๐)2
which is the required M.I. of the apparatus.
154
Module No. 175
Existence of Principle Axes Introduction
The axes relative to which product of inertia are zero are called the principal axes and the
moment of inertia along these axes are called principal moment of inertia.
Theorem Statement
For a rigid body, there exist a set of three mutually orthogonal axes called principal axes
relative to which the product of inertia are zero and ๏ฟฝ๏ฟฝ and ๐ฟ are considered along the
same direction.
Proof
We assume that there exists an axis through a point ๐ of the rigid body such that angular velocity
๏ฟฝ๏ฟฝ and angular momentum ๐ฟ of the body are parallel to this axis. Then we can write
๐ฟ = ๐ผ๏ฟฝ๏ฟฝ (1)
where ๐ผ is a constant of proportionality.
Form equation (1), we have
๐ฟ1 = ๐ผ๐1, ๐ฟ2 = ๐ผ๐2, ๐ฟ3 = ๐ผ๐3
Also from the general theory of angular momentum
๐ฟ๐ = โ ๐ผ๐๐๐ ๐๐ (2)
From equation (1) and (2), we have
๐ฟ1 = ๐ผ11๐1 + ๐ผ12๐2 + ๐ผ13๐3 = ๐ผ๐1
๐ฟ2 = ๐ผ21๐1 + ๐ผ22๐2 + ๐ผ23๐3 = ๐ผ๐2
๐ฟ3 = ๐ผ31๐1 + ๐ผ32๐2 + ๐ผ33๐3 = ๐ผ๐3
which can also be written as
(๐ผ11โ๐ผ)๐1 + ๐ผ12๐2 + ๐ผ13๐3 = 0
๐ผ21๐1 + (๐ผ22โ๐ผ)๐2 + ๐ผ23๐3 = 0
๐ผ31๐1 + ๐ผ32๐2 + (๐ผ33 โ ๐ผ)๐3 = 0 (3)
155
Equation (3) is a system of three homogeneous linear algebraic equation in the unknown
๐1, ๐2, ๐3.
This system will have a non-trivial solution, i.e. (๏ฟฝ๏ฟฝ โ 0), if the determinant of the matrix of
coefficients is zero. i.e.
|๐ผ11 โ ๐ผ ๐ผ12 ๐ผ13
๐ผ21 ๐ผ22 โ ๐ผ ๐ผ23
๐ผ31 ๐ผ32 ๐ผ33 โ ๐ผ| = 0 (4)
This is a cubic equation in ๐ผ and will in general have three roots. Equation (4) is called
characteristic equation of the matrix (๐ผ๐๐).
The roots of equation (4) are called eigen values of the inertia matrix (๐ผ๐๐).
The problem of finding principal moment of inertia and directions of inertia has been reduced to
that of finding the eigenvalues and eigenvectors of a symmetric 3 ร 3 matrix.
The following results from the eigenvalue theory of matrices will deduce further results about the
principal moments and directions (axes) of inertia.
Related Theorems
Theorem 1
A 3 ร 3 symmetrical matrix has three real eigenvalues, which may be distinct or repeated.
Theorem 2
The eigenvectors of a symmetrical matrix corresponding to distinct eigenvalues are orthogonal.
Theorem 3
It is always possible to find three mutually orthogonal eigenvectors for a 3 ร 3 symmetric
matrix, whether the eigenvalues are distinct or repeated.
Results
In the light of above theorems, we deduce the following results about the inertia matrix ๐ผ๐๐๐ก at
point ๐ of a rigid body.
i. The principal of moment of inertia are always real numbers. This is obviously physical
because the moment of inertia is defined as the quantity โ ๐๐๐๐2 ๐ where ๐๐ and ๐๐ are
both real.
156
ii. When all three principal moments ๐ผ1, ๐ผ2, ๐ผ3 are distinct, then by the theorem 2, three
mutually orthogonal principal axes can be found.
iii. When ๐ผ1 = ๐ผ2 but ๐ผ1 โ ๐ผ3, then by the theorem 3, we can still determine three mutually
orthogonal principal axes, because the inertia matrix is symmetric.
157
Module No. 176
Determination of Principal Axes of Other Two When One is known
In many instances a body possesses sufficient symmetry so that at least one principal axis can be
found by inspection, i.e;the axis can be chosen so as to make two of the three products of inertia
vanish.
We consider a plane rigid body i.e. a 2D body; for example a plate of uniform thickness. Such a
system can be regarded as a coplanar distribution of mass.
Since there are three mutually orthogonal principal axes, one of them must be perpendicular to
the plane of the body.
The other two axes will lie in the plane of lamina.
We choose ๐ and ๐ axes in the plane of lamina, and the ๐ axis perpendicular to its plane, as
shown in figure.
๐ผ๐ฅ๐ง = ๐ผ๐ฆ๐ง = 0 whereas ๐ผ๐ฅ๐ฆ โ 0 (1)
Since we have obtained the following equations for principal axes.
(๐ผ11 โ ๐ผ)๐1 + ๐ผ12๐2 + ๐ผ13๐3 = 0
๐ผ21๐1 + (๐ผ22 โ ๐ผ)๐2 + ๐ผ23๐3 = 0
๐ผ31๐1 + ๐ผ32๐2 + (๐ผ33 โ ๐ผ)๐3 = 0 (2)
By making use of (1), and using the first two equations of (2), we obtain
(๐ผ11 โ ๐ผ)๐1 + ๐ผ12๐2 = 0
๐ผ21๐1 + (๐ผ22 โ ๐ผ)๐2 = 0
Since the principal axes perpendicular to the plane of lamina is supposed to be known, and we
are interested in determining the principal axes in XY-plane, therefore we didnโt consider third
eq. of (2).
Let ๐๐1, ๐๐2 denote the principal axes in the XY-plane and let ๐ denote the angle between the
๐๐1 and the ๐ โ ๐๐ฅ๐๐ .
158
We define
tan๐ = ๐2 ๐1โ
which can also be written as
๐1
cos๐=
๐2
sin ๐= ๐, (3)
where ๐ is any arbitrary constant.
Substituting for ๐1, ๐2 from equation (3) in (2) we have, after simplification
(๐ผ11 โ ๐ผ) cos๐ + ๐ผ12 sin๐ = 0
๐ผ21 cos๐ + (๐ผ22 โ ๐ผ) sin๐ = 0
These equations can be put in the form
๐ผ11 โ ๐ผ = โ๐ผ12
sin๐
cos๐, ๐ผ22 โ ๐ผ = โ๐ผ12
cos๐
sin๐
159
Module No. 177
Determination of Principal Axes by Diagonalizing the Inertia Matrix
Introduction
Suppose a rigid body has no axis of symmetry. Even so, the tensor that represents the moment of
inertia of such a body, is characterized by a real, symmetric 3 ร 3 matrix that can be
diagonalized. The resulting diagonal elements are the values of the principal moments of inertia
of the rigid body.
The axes of the coordinate system, in which this matrix is diagonal, are the principal axes of the
body, because all products of inertia have vanished.
Thus, finding the principal axes and corresponding moments of inertia of any rigid body,
symmetric or not, is virtually the same as to diagonalzing its moment of inertia matrix.
Explanation
There are a number of ways to diagonalize a real, symmetric matrix. We present here a way that
is quite standard.
First, suppose that we have found the coordinate system (principal axes) in which all products of
inertia vanish and the resulting moment of inertia tensor is now represented by a diagonal matrix
whose diagonal elements are the principal moments of inertia.
Let ๐๐ be the unit vectors that represent this coordinate system, that is, they point along the
direction along the three principal axes of the rigid body.
If the moment of inertia tensor is "dotted" with one of these unit vectors, the result is equivalent
to a simple multiplication of the unit vector by a scalar quantity, i.e.
๐ผ๐๐ = ๐๐๐ (1)
The quantities ๐๐ are just the principal M.I about their respective principal axes.The problem of
finding the principal axes is one of finding those vectors ๐๐ that satisfy the condition
(๐ผ โ ๐๐ผ)๐๐ = 0 (2)
In general this condition is not satisfied for any arbitrary set of orthonormal unit vectors ๐๐. It is
satisfied only by a set of unit vectors aligned with the principal axes of the rigid body.
160
Any arbitrary ๐ฅ๐ฆ๐ง coordinate system can always be rotated such that the coordinate axes line up
with the principal axes. The unit vectors specifying these coordinate axes then satisfy the
condition in eq (2). This condition is equivalent to vanishing of the following determinant
|๐ผ โ ๐๐ผ| = 0 (3)
Explicitly, this equation reads
|๐ผ11 โ ๐ผ ๐ผ12 ๐ผ13
๐ผ21 ๐ผ22 โ ๐ผ ๐ผ23
๐ผ31 ๐ผ32 ๐ผ33 โ ๐ผ| = 0
It is a cubic in ๐, namely,
โ ๐3 + ๐ด๐2 + ๐ต๐ + ๐ถ = 0 (4)
in which ๐ด, ๐ต, and ๐ถ are functions of the ๐ผ's. The three roots ๐1, ๐2 and ๐3 are the three
principal moments of inertia.
We now have the principal moments of inertia, but the task of specifying the components of the
unit vectors representing the principal axes in terms of our initial coordinate system remains to
be solved.
Here we can make use of the fact that when the rigid body rotates about one of its principal axes;
the angular momentum vector is in the same direction as the angular velocity vector.
Let the angles of one of the principal axes relative to the initial ๐ฅ๐ฆ๐ง coordinate system be
๐ผ, ๐ฝ and ๐พ and let the body rotate about this axis. Therefore, a unit vector pointing in the
direction of this principal axis has components (cos ๐ผ, cos ๐ฝ, cos ๐พ).
Using eq (1),
๐ผ๐1 = ๐1๐1
where ๐1, the first principal moment of the three (๐1, ๐2, ๐3), is obtained by solving eq (4).
In matrix form
[
๐ผ11 โ ๐1 ๐ผ12 ๐ผ13
๐ผ21 ๐ผ22 โ ๐1 ๐ผ23
๐ผ31 ๐ผ32 ๐ผ33 โ ๐1
] [
cos ๐ผcos ๐ฝcos ๐พ
] = 0
The direction cosines may be found by solving the above equations.
The solutions are not independent. They are subject to the constraint
161
cos2 ๐ผ + cos2 ๐ฝ + cos2 ๐พ = 1
In other words the resultant vector ๐1 specified by these components is a unit vector.
162
Module No. 178
Relation of Fixed and Rotating Frames of Reference
In order to derive the relationship between fixed and rotating frames of reference, we will study
the following theorem[1,2].
Rotating Axes Theorem
Theorem Statement
If a time dependent vector function ๐ด is represented by ๐ด๐ and ๐ด๐ in fixed and rotating coordinate
system, then
(๐๐ด
๐๐ก)
๐
= (๐๐ด
๐๐ก)
๐
+ ๐ ร ๐ด๐
where it is understood that the origins of the two systems coincide at ๐ก = 0
Proof
We denote the fixed and rotating coordinate systems by ๐๐0๐0๐0 and ๐๐๐๐ and denote the
associated unit vectors by {๐0, ๐0, ๐0} and {๐, ๐, ๐}.
Consider a vector ๐ด which is changing with time. To an observer fixed relative to ๐๐๐๐ system,
the time rate of change of ๐ด = ๐ด1๐ + ๐ด2๐ + ๐ด3๏ฟฝ๏ฟฝ will be
๐
๐๐ก๐ด๐ =
๐
๐๐ก๐ด1๐ +
๐
๐๐ก๐ด2๐ +
๐
๐๐ก๐ด3๏ฟฝ๏ฟฝ
where ๐๐ด๐
๐๐ก denotes the time derivative of ๐ด relative to the rotating frame of reference.
However, the time rate of change of ๐ด relative to the fixed system ๐๐0๐0๐0 symbolized by the ๐๐ด๐
๐๐ก needs to be found.
To the fixed observer the unit vectors ๐, ๐, ๐ actually change with time.
Thus
163
๐
๐๐ก๐ด๐ =
๐
๐๐ก(๐ด1๐ + ๐ด2๐ + ๐ด3๏ฟฝ๏ฟฝ)
=๐๐ด1
๐๐ก๐ +
๐๐ด2
๐๐ก๐ +
๐๐ด3
๐๐ก๏ฟฝ๏ฟฝ + ๐ด1
๐๐
๐๐ก+ ๐ด2
๐๐
๐๐ก+ ๐ด3
๐๏ฟฝ๏ฟฝ
๐๐ก
๐๐ด๐
๐๐ก=
๐๐ด๐
๐๐ก+ ๐ด1
๐๐
๐๐ก+ ๐ด2
๐๐
๐๐ก+ ๐ด3
๐๏ฟฝ๏ฟฝ
๐๐ก (1)
Since ๐ is a unit vector, ๐๐/๐๐ก is perpendicular to ๐. Then ๐๏ฟฝ๏ฟฝ
๐๐ก must lie in the plane of ๐ and ๏ฟฝ๏ฟฝ.
Therefore
๐๐
๐๐ก= ๐ผ1๐ + ๐ผ2๏ฟฝ๏ฟฝ (2)
Similarly,
๐๐
๐๐ก= ๐ผ3๏ฟฝ๏ฟฝ + ๐ผ4๐ (3)
๐๏ฟฝ๏ฟฝ
๐๐ก= ๐ผ5๐ + ๐ผ6๐ (4)
Form ๐. ๐ = 0, differentiation yields
๐.๐๐
๐๐ก+ ๐.
๐๐
๐๐ก= 0 โน ๐.
๐๐
๐๐ก= โ๐.
๐๐
๐๐ก
But from (2), we have
๐.๐๐
๐๐ก= ๐ผ1 and ๐.
๐๐
๐๐ก= ๐ผ4
โน ๐ผ4 = โ๐ผ1
Similarly form ๐. ๏ฟฝ๏ฟฝ = 0 we obtain
๐.๐๏ฟฝ๏ฟฝ
๐๐ก+ ๏ฟฝ๏ฟฝ.
๐๐
๐๐ก= 0 โน ๐.
๐๏ฟฝ๏ฟฝ
๐๐ก= โ๏ฟฝ๏ฟฝ.
๐๐
๐๐ก
From (3), we have
๏ฟฝ๏ฟฝ.๐๐
๐๐ก= ๐ผ2 and ๏ฟฝ๏ฟฝ.
๐๐
๐๐ก= ๐ผ5
โน ๐ผ5 = โ๐ผ2
164
and from ๐. ๏ฟฝ๏ฟฝ = 0 we obtain
๐.๐๏ฟฝ๏ฟฝ
๐๐ก+ ๏ฟฝ๏ฟฝ.
๐๐
๐๐ก= 0 โน ๐.
๐๏ฟฝ๏ฟฝ
๐๐ก= โ๏ฟฝ๏ฟฝ.
๐๐
๐๐ก
From (4), we have
๏ฟฝ๏ฟฝ.๐๏ฟฝ๏ฟฝ
๐๐ก= ๐ผ2 and ๐.
๐๏ฟฝ๏ฟฝ
๐๐ก= ๐ผ5
โน ๐ผ6 = โ๐ผ3
Then
๐๐
๐๐ก= ๐ผ1๐ + ๐ผ2๏ฟฝ๏ฟฝ
๐๐
๐๐ก= ๐ผ3๏ฟฝ๏ฟฝ โ ๐ผ1๐
๐๏ฟฝ๏ฟฝ
๐๐ก= โ๐ผ2๐ โ ๐ผ3๐
follows that
๐ด1
๐๐
๐๐ก+ ๐ด2
๐๐
๐๐ก+ ๐ด3
๐๏ฟฝ๏ฟฝ
๐๐ก= (โ๐ผ1๐ด2 โ ๐ผ2๐ด3)๐ + (โ๐ผ1๐ด1 โ ๐ผ3๐ด3)๐ + (โ๐ผ2๐ด1 โ ๐ผ3๐ด2)๏ฟฝ๏ฟฝ
where
๏ฟฝ๏ฟฝ = ๐1๐ + ๐2๐ + ๐3๏ฟฝ๏ฟฝ
The vector quantity ๏ฟฝ๏ฟฝ is the angular velocity of the moving system relative to the fixed system.
Thus from (1) and (5), we obtain
(๐๐ด
๐๐ก)
๐
= (๐๐ด
๐๐ก)
๐
+ ๐ ร ๐ด
165
Module No. 179
Equation of Motion in Rotating Frame of Reference
There are two cases to be discussed in this article. Frist is when the origins of fixed and rotating
coordinate system coincide and other is when the origins of two system are distant.
Case I
In this case we consider the origins of the fixed and rotating coordinate system coincide. This
case was earlier discussed in detail, where it was found that:
To an observer fixed relative to ๐๐๐๐ system, the time rate of change of ๐ = ๐1๐ + ๐2๐ + ๐3๏ฟฝ๏ฟฝ
will be
๐
๐๐ก๐๐ =
๐
๐๐ก๐1๐ +
๐
๐๐ก๐2๐ +
๐
๐๐ก๐3๏ฟฝ๏ฟฝ (1)
where ๐๐๐
๐๐ก denotes the time derivative of ๐ relative to the rotating frame of reference.
However, the time rate of change of ๐ relative to fixed system ๐๐0๐0๐0 symbolized by ๐๐๐
๐๐ก will
be
๐๐๐
๐๐ก= (
๐๐
๐๐ก)๐
= (๐๐
๐๐ก)๐+ ๐ ร ๐ (2)
or in operator form, we can write
(๐
๐๐ก)๐
= (๐
๐๐ก)๐+ ๐ ร
166
Differentiating both sides of (2) w.r.t the fixed coordinate system, we have
(๐
๐๐ก)๐๐ฃ๐ = (
๐
๐๐ก)๐(๐ฃ๐ + ๐ ร ๐)
by applying operator, we have
(๐
๐๐ก)๐๐ฃ๐ = (
๐
๐๐ก+ ๐ ร)
๐(๐ฃ๐ + ๐ ร ๐)
= ๐๐ + 2๐ ร๐๐
๐๐ก+ ๏ฟฝ๏ฟฝ ร ๐ + ๐ ร (๐ ร ๐)
or
๐๐ = ๐๐ + 2๐ ร ๐ฃ๐ + ๏ฟฝ๏ฟฝ ร ๐ + ๐ ร (๐ ร ๐)
(3)
where
๐๐ =๐๐ฃ๐
๐๐ก, ๐๐ =
๐๐ฃ๐
๐๐ก
are the acceleration in the fixed and rotating coordinate systems. The relation (3) is referred as
Coriolis theorem. The term 2๐ ร ๐ฃ๐ is called Coriolis acceleration, whereas the term ๐ ร (๐ ร
๐) is called centripetal acceleration.
The equations of motion in fixed and moving/ rotating coordinate system are
๐น = ๐๐๐ , ๐นโฒ = ๐๐๐
where F and ๐นโฒ are the total forces in the fixed and the rotating coordinate systems.
On making substitution for ๐๐ from (3) in equation ๐น = ๐๐๐ , we obtain
These forces are called fictitious or apparent or inertial forces. They do not have physical forces
and do not arise from interactions of the particles.
Case II
In this case we consider the origins of fixed and rotating coordinate systems are not coincident.
We have
(๐)๐ = ๐ = (๐)๐ + ๐0
= ๐ฅ๐ + ๐ฆ๐ + ๐ง๏ฟฝ๏ฟฝ + ๐0
167
where ๐0 is the position vector of the origin of the rotating system w.r.t the origin of the fixed
system.
Therefore
(๐๐
๐๐ก)๐
=๐๐0๐๐ก
+๐
๐๐ก(๐ฅ๐ + ๐ฆ๐ + ๐ง๏ฟฝ๏ฟฝ)
๐ฃ๐ = ๐ฃ0 + (๏ฟฝ๏ฟฝ๐ + ๏ฟฝ๏ฟฝ๐ + ๏ฟฝ๏ฟฝ๏ฟฝ๏ฟฝ) + ๐ฅ๐๐
๐๐ก+ ๐ฆ
๐๐
๐๐ก+ ๐ง
๐๏ฟฝ๏ฟฝ
๐๐ก
๐ฃ๐ = ๐ฃ0 + ๐ฃ๐ + ๐ ร ๐
where ๐ โก (๐)๐.
Similarly the acceleration in the two systems will be related by
๐๐ = ๐0 + ๐๐ + 2๐ ร ๐ฃ๐ + ๏ฟฝ๏ฟฝ ร ๐ + ๐ ร (๐ ร ๐)
168
Module No. 180
Example 1 of Equation of Motion in Rotating Frame of Reference
Problem Statement
The angular velocity of a rotating coordinate system ๐๐๐๐ relative to a fixed coordinate system
๐๐0๐0๐0 is
๏ฟฝ๏ฟฝ = 2๐ก๐ โ ๐ก2๐ + (2๐ก + 4)๏ฟฝ๏ฟฝ
where ๐ก is the time and {๐, ๐, ๏ฟฝ๏ฟฝ} unit vectors associated with ๐๐๐๐. The position vector of a
particle at time ๐ก in the body system (๐๐๐๐ system) is given by
๐ = (๐ก2 + 1)๐ โ 6๐ก๐ + 4๐ก3๏ฟฝ๏ฟฝ
To Find
i. The apparent and true velocities at time ๐ก = 1.
ii. The apparent and true acceleration at time ๐ก = 1
Solution
The apparent velocity is given by
๐ฃ๐ = (๐๐
๐๐ก)๐
=๐
๐๐ก[(๐ก2 + 1)๐ โ 6๐ก๐ + 4๐ก3๏ฟฝ๏ฟฝ]
= ๐๐
๐๐ก(๐ก2 + 1) โ ๐
๐
๐๐ก6๐ก + ๏ฟฝ๏ฟฝ
๐
๐๐ก4๐ก3
= 2๐ก๐ โ 6๐ + 12๐ก2๏ฟฝ๏ฟฝ
Therefore the apparent velocity at time ๐ก = 1 is given by
๐ฃ๐(๐ก = 1) = 2๐ โ 6๐ + 12๏ฟฝ๏ฟฝ
The true velocity at any time t is given by
๐ฃ๐ = ๐ฃ๐ + ๏ฟฝ๏ฟฝ ร ๐
169
= 2๐ โ 6๐ + 12๏ฟฝ๏ฟฝ + |๐ ๐ ๏ฟฝ๏ฟฝ
2๐ก โ๐ก2 2๐ก + 4๐ก2 + 1 โ6๐ก 4๐ก3
|
Therefore
๐ฃ๐(๐ก = 1) = 2๐ โ 6๐ + 12๏ฟฝ๏ฟฝ + |๐ ๐ ๏ฟฝ๏ฟฝ2 โ1 62 โ6 4
|
= 34๐ โ 2๐ + 2๏ฟฝ๏ฟฝ
ii. For apparent acceleration
๐๐ =๐๐ฃ๐
๐๐ก=
๐
๐๐ก(2๐ก๐ โ 6๐ + 12๐ก2๏ฟฝ๏ฟฝ)
= 2๐ + 24๐ก๏ฟฝ๏ฟฝ
Therefore
๐๐(๐ก = 1) = 2๐ + 24๏ฟฝ๏ฟฝ
For true acceleration we have
๐๐ = ๐๐ + 2๐ ร ๐ฃ๐ + ๏ฟฝ๏ฟฝ ร ๐ + ๐ ร (๐ ร ๐)
(1)
now
๐(๐ก = 1) = 2๐ โ ๐ + 6๏ฟฝ๏ฟฝ
and
๏ฟฝ๏ฟฝ = 2๐ โ 2๐ก๐ + 2๏ฟฝ๏ฟฝ
๏ฟฝ๏ฟฝ(๐ก = 1) = 2๐ โ 2๐ + 2๏ฟฝ๏ฟฝ
Since ๐ = (๐ก2 + 1)๐ โ 6๐ก๐ + 4๐ก3๏ฟฝ๏ฟฝ and
๐ฃ๐ = 2๐ก๐ โ 6๐ + 12๐ก2๏ฟฝ๏ฟฝ, therefore
๏ฟฝ๏ฟฝ ร ๐ = |๐ ๐ ๏ฟฝ๏ฟฝ2 โ2 22 โ6 4
| = 4๐ โ 4๐ โ 8๏ฟฝ๏ฟฝ
and
170
2๐ ร ๐ฃ๐ = 2 |๐ ๐ ๏ฟฝ๏ฟฝ2 โ1 62 โ6 12
|
= 2(24๐ โ 12๐ โ 10๏ฟฝ๏ฟฝ)
Also since
๐ ร ๐ = |๐ ๐ ๏ฟฝ๏ฟฝ2 โ1 62 โ6 4
| = 32๐ + 4๐ โ 10๏ฟฝ๏ฟฝ
Therefore
๐ ร (๐ ร ๐) = |๐ ๐ ๏ฟฝ๏ฟฝ2 โ1 632 4 โ10
|
= โ14๐ + 212๐ + 40๏ฟฝ๏ฟฝ
Hence on making substitution, we have
๐๐ = (2๐ + 24๐ก๏ฟฝ๏ฟฝ) + (4๐ โ 4๐ โ 8๏ฟฝ๏ฟฝ) + 2(24๐ โ 12๐ โ 10๏ฟฝ๏ฟฝ) + (โ14๐ + 212๐ + 40๏ฟฝ๏ฟฝ)
= 40๐ + 184๐ + 36๏ฟฝ๏ฟฝ
which is required true acceleration.
171
Module No. 181
Example 2 of Equation of Motion in Rotating Frame of Reference
Problem Statement
A coordinate system OXYZ rotates with angular velocity ๏ฟฝ๏ฟฝ = 2๐ โ 3๐ + 5๏ฟฝ๏ฟฝ relative to the fixed
coordinate system O๐0๐0๐0 both systems having the same origin. If ๐ด = sin ๐ก ๐ โ cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ
where ๐, ๐, ๏ฟฝ๏ฟฝ refer to the rotating coordinate system OXYZ then find ๐๐ด
๐๐ก and
๐2๐ด
๐๐ก2 w.r.to
i. Fixed system
ii. The rotating system
Solution
i. To find ๐๐ด
๐๐ก and
๐2๐ด
๐๐ก2 in the fixed system, we have to apply the following formulas
(๐๏ฟฝ๏ฟฝ
๐๐ก)๐
= (๐๏ฟฝ๏ฟฝ
๐๐ก)๐+ ๏ฟฝ๏ฟฝ ร ๐ด (1)
(๐2๏ฟฝ๏ฟฝ
๐๐ก2)๐
= (๐2๏ฟฝ๏ฟฝ
๐๐ก2)๐+ 2๏ฟฝ๏ฟฝ ร
๐๏ฟฝ๏ฟฝ
๐๐ก+
๐๏ฟฝ๏ฟฝ
๐๐กร ๐ด + ๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐ด) (2)
(๐๐ด
๐๐ก)
๐
= [๐๐ด
๐๐ก]๐
=๐๐ด๐
๐๐ก
=๐
๐๐ก(sin ๐ก ๐ โ cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ)
= cos ๐ก ๐ + sin ๐ก ๐ โ ๐โ๐ก๏ฟฝ๏ฟฝ
๏ฟฝ๏ฟฝ ร ๐ด = |๐ ๐ ๏ฟฝ๏ฟฝ2 โ3 5
sin ๐ก โ cos ๐ก ๐โ๐ก
|
๏ฟฝ๏ฟฝ ร ๐ด = (โ3๐โ๐ก + 5 cos ๐ก)๐ โ (2๐โ๐ก โ 5 sin ๐ก)๐ + (โ2 cos ๐ก + 3 sin ๐ก)๏ฟฝ๏ฟฝ
(๐๐ด
๐๐ก)
๐
= cos ๐ก ๐ + sin ๐ก ๐ โ ๐โ๐ก๏ฟฝ๏ฟฝ + (โ3๐โ๐ก + 5 cos ๐ก)๐ โ (2๐โ๐ก โ 5 sin ๐ก)๐
+ (โ2 cos ๐ก + 3 sin ๐ก)๏ฟฝ๏ฟฝ
172
= (cos ๐ก + 5 cos ๐ก โ 3๐โ๐ก)๐ + (5 sin ๐ก + sin ๐ก โ 2๐โ๐ก)๐ + (โ๐โ๐ก โ 2 cos ๐ก + 3 sin ๐ก)๏ฟฝ๏ฟฝ
= (6 cos ๐ก โ 3๐โ๐ก)๐ + (6 sin ๐ก โ 2๐โ๐ก)๐ + (โ๐โ๐ก โ 2 cos ๐ก + 3 sin ๐ก)๏ฟฝ๏ฟฝ
Now for solving equation (2), we proceed as follows
(๐2๐ด
๐๐ก2)
๐
=๐
๐๐ก(๐๐ด
๐๐ก)
๐
=๐
๐๐ก(cos ๐ก ๐ + sin ๐ก ๐ โ ๐โ๐ก๏ฟฝ๏ฟฝ)
= (โsin ๐ก ๐ + cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ)
2๏ฟฝ๏ฟฝ ร (๐๐ด
๐๐ก)
๐
= 2(2๐ โ 3๐ + 5๏ฟฝ๏ฟฝ) ร (cos ๐ก ๐ + sin ๐ก ๐ โ ๐โ๐ก๏ฟฝ๏ฟฝ)
= (4๐ โ 6๐ + 10๏ฟฝ๏ฟฝ) ร (cos ๐ก ๐ + sin ๐ก ๐ โ ๐โ๐ก๏ฟฝ๏ฟฝ)
= |๐ ๐ ๏ฟฝ๏ฟฝ4 โ6 10
cos ๐ก sin ๐ก โ๐โ๐ก
|
2๏ฟฝ๏ฟฝ ร (๐๐ด
๐๐ก)
๐
= (6๐โ๐ก โ 10 sin ๐ก)๐ + (4๐โ๐ก + 10 cos ๐ก)๐ + (4 sin ๐ก + 6 cos ๐ก)๏ฟฝ๏ฟฝ
๏ฟฝ๏ฟฝ = 2๐ โ 3๐ + 5๏ฟฝ๏ฟฝ โน ๏ฟฝ๏ฟฝ =๐๏ฟฝ๏ฟฝ
๐๐ก= 0
So,
๐๏ฟฝ๏ฟฝ
๐๐กร ๐ด = 0 ร (sin ๐ก ๐ โ cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ) = 0
๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐ด) = (2๐ โ 3๐ + 5๏ฟฝ๏ฟฝ) ร [(2๐ โ 3๐ + 5๏ฟฝ๏ฟฝ) ร (sin ๐ก ๐ โ cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ)]
= (2๐ โ 3๐ + 5๏ฟฝ๏ฟฝ) ร [(โ3๐โ๐ก + 5 cos ๐ก)๐ โ (2๐โ๐ก โ 5 sin ๐ก)๐ + (โ2 cos ๐ก + 3 sin ๐ก)๏ฟฝ๏ฟฝ]
= |๐ ๐ ๏ฟฝ๏ฟฝ2 โ3 5
โ3๐โ๐ก + 5 cos ๐ก โ2๐โ๐ก + 5 sin ๐ก โ2 cos ๐ก + 3 sin ๐ก
|
= (6 cos ๐ก โ 9 sin ๐ก โ 25 sin ๐ก + 10๐โ๐ก)๐ + (4 cos ๐ก โ 6 sin ๐ก โ 15๐โ๐ก + 25 cos ๐ก)๐ + (4๐โ๐ก
โ 10 sin ๐ก + 9๐โ๐ก โ 15 cos ๐ก)๏ฟฝ๏ฟฝ
173
= (6 cos ๐ก โ 34 sin ๐ก + 10๐โ๐ก)๐ + (29 cos ๐ก โ 6 sin ๐ก โ 15๐โ๐ก)๐
+ (โ4๐โ๐ก + 10 sin ๐ก + 9๐โ๐ก โ 15 cos ๐ก)๏ฟฝ๏ฟฝ
= (โ sin ๐ก ๐ + cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ) + (6๐โ๐ก โ 10 sin ๐ก)๐ + (4๐โ๐ก + 10 cos ๐ก)๐ + (4 sin ๐ก + 6 cos ๐ก)๏ฟฝ๏ฟฝ
+ 0 + (6 cos ๐ก โ 34 sin ๐ก + 10๐โ๐ก)๐ + (29 cos ๐ก โ 6 sin ๐ก โ 15๐โ๐ก)๐ + (10 sin ๐ก
โ 13๐โ๐ก + 15 cos ๐ก)๏ฟฝ๏ฟฝ
(๐2๐ด
๐๐ก2)
๐
= (6 cos ๐ก โ 45 sin ๐ก + 16๐โ๐ก)๐ + (40 cos ๐ก โ 6 sin ๐ก โ 11๐โ๐ก)๐
+ (14 sin ๐ก โ 12๐โ๐ก + 21 cos ๐ก)๏ฟฝ๏ฟฝ
ii. The rotating system
(๐๐ด
๐๐ก)
๐
= [๐๐ด
๐๐ก]๐
=๐๐ด๐
๐๐ก
=๐
๐๐ก(sin ๐ก ๐ โ cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ)
= cos ๐ก ๐ + sin ๐ก ๐ โ ๐โ๐ก๏ฟฝ๏ฟฝ
(๐2๐ด
๐๐ก2)
๐
=๐
๐๐ก(๐๐ด
๐๐ก)
๐
=๐
๐๐ก(cos ๐ก ๐ + sin ๐ก ๐ โ ๐โ๐ก๏ฟฝ๏ฟฝ)
= (โsin ๐ก ๐ + cos ๐ก ๐ + ๐โ๐ก๏ฟฝ๏ฟฝ)
174
Module No. 182
Example 3 of Equation of Motion in Rotating Frame of Reference
Problem Statement
A bead slides on a smooth helix whose central axis is vertical. The helix is forced to rotate about
its central axis with constant angular speed ๐. Find the equation of motion of the bead relative to
the helix.
Solution
We choose a coordinate ๐๐๐๐ fixed in the helix such that ๐-axis is coincident with the axis of
the helix.
Then the parametric eqs. of the helix are given by
๐ฅ = ๐ cos ๐ , ๐ฆ = ๐ sin ๐,
๐ง = ๐๐
In vector form these can be written as
๐ = ๐ cos ๐ ๐ + ๐ sin ๐ ๐ + ๐๐๏ฟฝ๏ฟฝ
where ๐, ๐, ๏ฟฝ๏ฟฝ point along the coordinate axis. Here ๏ฟฝ๏ฟฝ is constant but ๐, ๐ are variable.
175
The equation of motion in the rotating coordinate system is given by
๐๐๐ = ๐น โ 2๐๐ ร ๐ฃ๐ โ ๐๏ฟฝ๏ฟฝ ร ๐ โ ๐๐ ร (๐ ร ๐) (1)
In this problem, since ๐ is constant,
๐ = ๐๏ฟฝ๏ฟฝ , ๏ฟฝ๏ฟฝ = 0 (2)
Also
๐ฃ๐ = ๏ฟฝ๏ฟฝ = (โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)๏ฟฝ๏ฟฝ (3)
and
๐๐ = ๏ฟฝ๏ฟฝ =๐
๐๐ก[(โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)๏ฟฝ๏ฟฝ]
๐
= (โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)๏ฟฝ๏ฟฝ +(โ๐ cos ๐ ๐ โ ๐ sin ๐ ๐)๏ฟฝ๏ฟฝ2 (4)
If ๐น denotes the external force in the fixed (space) coordinate system, then
๐น = โ๐๐๏ฟฝ๏ฟฝ + ๐ (5)
where ๐ is the reaction of the helix on the bead.
Before making substitution from equation (2-5) into equation (1), we obtain simplified
expression for the second, and the fourth terms of (1), (the third term being zero).
๐ ร ๐ฃ๐ = ๐๏ฟฝ๏ฟฝ ร (โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)๏ฟฝ๏ฟฝ
= ๐๐(โ sin ๐ ๐ โ cos ๐ ๐)๏ฟฝ๏ฟฝ (6)
and
๐ ร ๐ = ๐๏ฟฝ๏ฟฝ ร (๐ cos ๐ ๐ + ๐ sin ๐ ๐ + ๐๐๏ฟฝ๏ฟฝ)
= ๐๐(cos ๐ ๐ โ sin ๐ ๐)
๐ ร (๐ ร ๐) = ๐2๐๏ฟฝ๏ฟฝ ร (cos ๐ ๐ โ sin ๐ ๐)
= ๐2๐(โcos ๐ ๐ โ sin ๐ ๐) (7)
Therefore on making substitutions from (4), (5), (6) and (7) into (1), we obtain
๐(โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)๏ฟฝ๏ฟฝ โ ๐๐(cos ๐ ๐ + sin ๐ ๐) = โ๐๐๏ฟฝ๏ฟฝ + ๐ + 2๐๐๐(sin ๐ ๐ +
cos ๐ ๐) + ๐๐๐2(cos ๐ ๐ + sin ๐ ๐)
176
๐(โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)๏ฟฝ๏ฟฝ โ ๐๐(cos ๐ ๐ + sin ๐ ๐) = โ๐๐๏ฟฝ๏ฟฝ + ๐ + (2๐๐๐ +
๐๐๐2) (sin ๐ ๐ + cos ๐ ๐) (8)
Now if we write
๐ฃ๐ = (โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)๏ฟฝ๏ฟฝ = ๐ข๏ฟฝ๏ฟฝ
Then it is clear that the vector ๐ข is tangent to the helix. Since ๐ is normal to the helix,
๐ . ๐ฃ๐ = ๐ . ๐ข๏ฟฝ๏ฟฝ = 0
Rewriting (8) in terms of ๐ข (after eliminating the common factor ๐), we have
๐ข๏ฟฝ๏ฟฝ โ ๐(cos ๐ ๐ + sin ๐ ๐)๏ฟฝ๏ฟฝ2 = โ๐๐ + ๐ + (2๐๐ + ๐๐2)(cos๐ ๐ + sin ๐ ๐) (9)
Taking dot product of both sides of (9) with u, we have
๐ข. ๐ข๏ฟฝ๏ฟฝ โ ๐๐ข. (cos ๐ ๐ + sin ๐ ๐)๏ฟฝ๏ฟฝ2 = โ๐๐. ๐ข + ๐ . ๐ข + (2๐๐ + ๐๐2)(cos๐ ๐ + sin ๐ ๐). ๐ข (10)
Now
๐ข. ๐ข = ๐2 sin2 ๐ + ๐2 cos2 ๐ + ๐2 = ๐2 + ๐2
๏ฟฝ๏ฟฝ. ๐ข = ๏ฟฝ๏ฟฝ. (โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ) = ๐
๐ . ๐ข = 0
(sin ๐ ๐ + cos ๐ ๐). ๐ข = (sin ๐ ๐ + cos ๐ ๐). (โ๐ sin ๐ ๐ + ๐ cos ๐ ๐ + ๐๏ฟฝ๏ฟฝ)
= ๐ cos ๐ sin ๐ โ ๐ cos ๐ sin ๐
= 0
Therefore (10) reduces to
(๐2 + ๐2)๏ฟฝ๏ฟฝ = โ๐๐
or
๏ฟฝ๏ฟฝ = โ๐๐
(๐2 + ๐2)
177
Module No. 183
Example 4 of Equation of Motion in Rotating Frame of Reference
Problem Statement
Prove that the centrifugal force acting on a particle of mass ๐ on the earthโs surface is the vector
directed away from the earthโs center and perpendicular to the angular velocity vector ๏ฟฝ๏ฟฝ.
i. Show that its magnitude is ๐๐2๐๐ cos ๐.
ii. Determine the places on the surface of the earth where centrifugal force will be maximum
or minimum.
Solution
i. Since centrifugal force is โ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐).
We have to show that itโs vector is directed away from center of the earth and perpendicular to
the angular velocity vector ๏ฟฝ๏ฟฝ and its magnitude is ๐๐2๐๐ cos ๐
where ๐ is the latitude as shown in figure and ๐๐ is the radius of the earth.
It is quite clear that the particle is moving away from the center of earth and centrifugal force is
produced in the opposite direction of the path of the particle.
178
It can be observed from the given figure that the movement of the particle (i.e. ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)) is
perpendicular to the angular velocity ๏ฟฝ๏ฟฝ and vector ๏ฟฝ๏ฟฝ ร ๐๐ , becomes centrifugal force in the
opposite direction of the particle, so its centrifugal force is also perpendicular to ๏ฟฝ๏ฟฝ and ๏ฟฝ๏ฟฝ ร ๐๐ .
Now we move onto the proof of magnitude of centrifugal force, since
๏ฟฝ๏ฟฝ๐๐ = โ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)
We need to show that
|๏ฟฝ๏ฟฝ๐๐| = |โ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)| = ๐๐2๐๐ cos ๐
Now we assume that the motion takes place in the ๐๐-plane.
Then,
๏ฟฝ๏ฟฝ = ๏ฟฝ๏ฟฝ cos ๐ + ๐ sin ๐
= ๏ฟฝ๏ฟฝ cos(๐ 2โ โ ๐) + ๐ sin(๐ 2โ โ ๐)
๏ฟฝ๏ฟฝ = ๏ฟฝ๏ฟฝ sin ๐ + ๐ cos ๐
Consider the centrifugal force
= โ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)
If ๏ฟฝ๏ฟฝ denotes a unit vector along the NS-axis, then we can write ๏ฟฝ๏ฟฝ = ๐๏ฟฝ๏ฟฝ and the expression
becomes ๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐) = ๐2๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)
โ๐2๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐) = โ๐2[(๏ฟฝ๏ฟฝ. ๐๐)๏ฟฝ๏ฟฝ โ (๏ฟฝ๏ฟฝ. ๏ฟฝ๏ฟฝ)๐๐]
= โ๐2[(๏ฟฝ๏ฟฝ. ๐๐)๏ฟฝ๏ฟฝ โ ๐๐]
Now
๏ฟฝ๏ฟฝ. ๐๐ = (๏ฟฝ๏ฟฝ sin ๐ + ๐ cos ๐). ๐๐๏ฟฝ๏ฟฝ
= ๐๐ cos ๐ = cos(๐ 2โ โ ๐) = ๐๐ sin ๐
So expression (1) becomes
โ๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐) = โ๐2๐๐ sin ๐ (๏ฟฝ๏ฟฝ sin ๐ + ๐ cos ๐) + ๐2๐๐๏ฟฝ๏ฟฝ
= (โ๐2๐๐ sin2 ๐ + ๐2๐๐)๏ฟฝ๏ฟฝ โ ๐2๐๐ sin ๐ cos ๐ ๐
= (โ๐2๐๐(1 โ cos2 ๐) + ๐2๐๐)๏ฟฝ๏ฟฝ โ ๐2๐๐ sin ๐ cos ๐ ๐
179
|๏ฟฝ๏ฟฝ๐๐| = |โ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)| = |โ๐((โ๐2๐๐(1 โ cos2 ๐) + ๐2๐๐)๏ฟฝ๏ฟฝ โ ๐2๐๐ sin ๐ cos ๐ ๐)|
= |โ๐(๐2๐๐ cos2 ๐)๏ฟฝ๏ฟฝ + ๐๐2๐๐ sin ๐ cos ๐ ๐)|
= ๐โ(๐2๐๐ cos2 ๐)2 + (๐2๐๐ sin ๐ cos ๐)2
= ๐โ๐4๐๐2 cos4 ๐ + ๐4๐๐2 cos2 ๐ sin2 ๐
= ๐โ๐4๐๐2 cos4 ๐ + ๐4๐๐2 cos2 ๐ (1 โ cos2 ๐)
= ๐โ๐4๐๐2 cos4 ๐ + ๐4๐๐2 cos2 ๐ โ ๐4๐๐2 cos4 ๐
= ๐โ๐4๐๐2 cos2 ๐
|๏ฟฝ๏ฟฝ๐๐| = ๐๐2๐๐ cos ๐
Hence the required result.
The centrifugal force will be maximum at ๐ = 0, where cos ๐ = 1 (maximum) i.e. at equator
and minimum will be at NS-poles i.e. at ๐ = ๐2โ where cos ๐ = 0 (minimum).
180
181
Module No. 184
Example 5 of Equation of Motion in Rotating Frame of Reference
Problem Statement
A coordinate system ๐๐๐๐ is rotating with angular velocity ๏ฟฝ๏ฟฝ = 5๐ โ 4๐ โ 10๏ฟฝ๏ฟฝ relative to fixed
coordinate system ๐๐0๐0๐0 both systems having the same origin. Find the velocity of the
particle at rest in the ๐๐๐๐ system at the point (3,1, โ2) as seen by an observer in the fixed
system.
Solution
Since the given angular velocity is
๏ฟฝ๏ฟฝ = 5๐ โ 4๐ โ 10๏ฟฝ๏ฟฝ
and the point P(3,1, โ2) at rest in ๐๐๐๐ system.
Then
๐๐ = ๐ = 3๐ + ๐ โ 2๏ฟฝ๏ฟฝ
Also, we know the equation of motion in case when the origins of the coordinate systems
coincide each other
๐ฃ๐ = ๐ฃ๐ + ๏ฟฝ๏ฟฝ ร ๐
But we have to find out the velocity of the particle at rest in ๐๐๐๐ system at P(3,1, โ2).
i.e.
๐ฃ๐ = ๐ฃ0 + ๏ฟฝ๏ฟฝ ร ๐
๐ฃ๐ = (0,0,0) + (5๐ โ 4๐ โ 10๏ฟฝ๏ฟฝ) ร (3๐ + 1๐ โ 2๏ฟฝ๏ฟฝ)
= (0๐ + 0๐ + 0๏ฟฝ๏ฟฝ) + |๐ ๐ ๏ฟฝ๏ฟฝ5 โ4 โ103 1 โ2
|
= (0๐ + 0๐ + 0๏ฟฝ๏ฟฝ) + ((8 + 10)๐ โ (โ10 + 30)๐ + (5 + 12)๏ฟฝ๏ฟฝ)
182
= (0๐ + 0๐ + 0๏ฟฝ๏ฟฝ) + (18๐ โ 20๐ + 17๏ฟฝ๏ฟฝ)
Hence
๐ฃ๐ = 18๐ โ 20๐ + 17๏ฟฝ๏ฟฝ
is the required velocity of the system.
183
Module No. 185
General Motion of a Rigid Body
In general motion of the body, (i.e. no point of the body is fixed in space), let ๐ญ๐๐ฅ๐ก be the total
external force on the rigid body and ๐บ๐๐๐ฅ๐ก the total external torque about its center of mass (i.e.
centroid). Then the equations of motion are
๐๐๐ =๐ญ๐๐ฅ๐ก (1)
and
๐ณ๏ฟฝ๏ฟฝ = ๐ฎ๐๐๐ฅ๐ก (2)
where ๐๐ is the acceleration of the c.m. and ๐ณ๐ is the total angular momentum about it.
Now we resolve the vectors ๐๐,๐ญ๐๐ฅ๐ก,๐ฎ๐๐๐ฅ๐กand ๐ณ along the unit vector ๐, ๐, ๐ taken along the
principal axes at the mass center.
The triad of vectors ๐, ๐, ๐ may be inferred to as a principal triad. It will be assumed to be
permanently a principal triad.
Let ฮฉ be its angular velocity. If the triad is fixed in the body then ฮฉ = ฯ, the angular velocity of
the body.
Now using the operator: (๐
๐๐ก)๐
= (๐
๐๐ก)๐+ ๐ ร
(๐๏ฟฝ๏ฟฝ
๐๐ก)
๐
= (๐๏ฟฝ๏ฟฝ
๐๐ก)
๐
+ ฯ ร ๏ฟฝ๏ฟฝ
which relates the rate of change of a vector in a fixed (i.e. inertial) frame and a rotating frame,
we have (on dropping the suffix r)
๐๐ โก (๐๏ฟฝ๏ฟฝ
๐๐ก)
๐
=๐๏ฟฝ๏ฟฝ
๐๐ก+ ฮฉ ร ๏ฟฝ๏ฟฝ (โด ๐๐ = ๐)
or ๐๐ = ๐๐ฃ
๐๐ก + ฮฉ x ๏ฟฝ๏ฟฝ (3)
where ๐ฃ = ๐ฃ1๐ + ๐ฃ2๐ + ๐ฃ3๐ is the velocity of the mass center (in the rotating coordinate system).
Substituting for ๐๐ = ๐๐ from equation (3) into (1), we obtain
184
๐(๐๐
๐๐ก+ ฮฉ ร ๐) = ๐ญ๐๐ฅ๐ก
which is equivalent to
๐(๐ฃ1 + ฮฉ2๐ฃ3 + ฮฉ3๐ฃ2) = ๐น1
๐(๐ฃ2 + ฮฉ3๐ฃ1 + ฮฉ1๐ฃ3) = ๐น2
๐(๐ฃ3 + ฮฉ1๐ฃ2 + ฮฉ2๐ฃ1) = ๐น3
} (4)
From (2), on using
(๐๐ฟ
๐๐ก)๐ =
๐๐ฟ
๐๐ก+ ๐บ x ๐ฟ
and the relation ๐ฟ = ๐ผ1๐1๐ + ๐ผ2๐2๐ + ๐ผ3๐3๏ฟฝ๏ฟฝ, we obtain the equations
๐ผ1ฯ1 ๐ + ๐ผ2ฯ2 ๐ + ๐ผ3ฯ3 ๏ฟฝ๏ฟฝ + (ฮฉ2๐ฟ3 โ ฮฉ3๐ฟ2)๐ + (ฮฉ2๐ฟ1 โ ฮฉ1๐ฟ3)๐ + (ฮฉ1๐ฟ2 โ ฮฉ2๐ฟ1)๏ฟฝ๏ฟฝ = ๏ฟฝ๏ฟฝ
From this vector equation we obtain the following three scalar equations
๐ผ1ฯ1 + ฮฉ2๐ฟ3 โ ฮฉ3๐ฟ2 = ๐บ1
๐ผ2ฯ2 + ฮฉ3๐ฟ1 โ ฮฉ1๐ฟ3 = ๐บ2
and
๐ผ3ฯ3 + ฮฉ1๐ฟ2 โ ฮฉ2๐ฟ1 = ๐บ3
where on using the results
๐ฟ1 = ๐ผ1ฯ1, ๐ฟ2 = ๐ผ2ฯ2, ๐ฟ3 = ๐ผ3ฯ3
we have
๐ผ1ฯ1 + ฯ3ฮฉ2I3 โ ฯ2ฮฉ3I2 = ๐บ1
๐ผ2ฯ2 + ฯ1ฮฉ3I1 โ ฯ3ฮฉ1I3 = ๐บ2
๐ผ3ฯ3 + ฯ2ฮฉ1I2 โ ฯ1ฮฉ2I1 = ๐บ3
} (5)
where ๐ผ1, ๐ผ2, ๐ผ3 denote principal M.I at the centroid of the body.
The set of eqs (4) and (5) constitute six equations for the components of velocity of centroid and
the components of angular velocity of the body.
185
Module No. 186
Equation of Motion Relative to Coordinate System Fixed on Earth
We derived the equation of motion when the origins of fixed and rotating coordinate systems are
distant.
๏ฟฝ๏ฟฝ๐ = ๏ฟฝ๏ฟฝ0 + ๏ฟฝ๏ฟฝ๐ + 2๏ฟฝ๏ฟฝ ร ๏ฟฝ๏ฟฝ๐ + ๏ฟฝ๏ฟฝ ร ๐ + ๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐) (1)
From (1) the most general from of the equation of motion in a moving frame can be written as
๐ (๐2๐
๐๐ก2)๐
= ๏ฟฝ๏ฟฝ โ ๐๏ฟฝ๏ฟฝ ร ๐ โ 2๐๏ฟฝ๏ฟฝ ร ๐ฃ๐ โ ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐) โ ๐๏ฟฝ๏ฟฝ0 (2)
where the subscript ๐ refers to the rotating frame, ๏ฟฝ๏ฟฝ is the total external force in the fixed
coordinate system and ๏ฟฝ๏ฟฝ0 is the acceleration of the origin ๐ in moving (rotating) coordinate
system, ๐๐๐๐. In the case of the earth, which is a rotating coordinate system, we choose the
origin as a point fixed on the earth.
We choose the fixed coordinate system at the center ๐ถ of the earth and denote it by ๐ถ๐0๐0๐0 .
For a particle near the surface of the earth
๏ฟฝ๏ฟฝ = ๐๏ฟฝ๏ฟฝ (3)
where ๏ฟฝ๏ฟฝ is the acceleration due to gravity.
๏ฟฝ๏ฟฝ0, the acceleration of the origin ๐ is the centripetal acceleration of ๐ due to the rotation of the
earth. It may be represented as
๏ฟฝ๏ฟฝ0 = ๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐) (4)
Therefore on substitution from (3) and (4) into (2), we obtain
๐ (๐2๐
๐๐ก2)๐
= ๐๏ฟฝ๏ฟฝ โ ๐๏ฟฝ๏ฟฝ ร ๐ โ 2๐๏ฟฝ๏ฟฝ ร ๐ฃ๐ โ ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐) โ ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐) (5)
which gives the full equation of motion for a particle of mass ๐ w.r.t. a coordinate system fixed
on the earth.
Next we obtain a simple form of (5) taking into account the fact that the angular velocity ๏ฟฝ๏ฟฝ of
the earth is nearly constant both in magnitude and direction, (which is along NS i.e. North-South
polar line), and of small magnitude. In fact
186
๐ = |๏ฟฝ๏ฟฝ| =2๐ radian
24 hours
=2๐
86400 radian per second
= 7.27 ร 10โ5rad/sec
Since ๏ฟฝ๏ฟฝ may be taken as constant, ๏ฟฝ๏ฟฝ = 0. Since the fourth term on R.H.S. of (5) has the
magnitude ๐๐2r (where r is the distance of the particle from the NS-axis), is negligibly small
because of ๐2. However the fifth term ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐), the last term of equation (5) is not
negligible because |๐๐| (the magnitude of the radius of the earth) is very large.
Hence equation (5) can be written as
๐ (๐2๐
๐๐ก2)
๐
= ๐๏ฟฝ๏ฟฝ โ 2๐๏ฟฝ๏ฟฝ ร ๐ฃ๐ โ ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)
which is a second order differential equation in ๐ and may also written as
๐๐2๐
๐๐ก2 + 2๐๏ฟฝ๏ฟฝ ร๐๐
๐๐ก= ๐๏ฟฝ๏ฟฝ โ ๐๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐) (6)
where we have dropped the subscript ๐, it being understood that the terms on L.H.S of (6) refer
to the rotating coordinate system.
If ๏ฟฝ๏ฟฝ denotes a unit vector along the NS-axis, then we can write ๏ฟฝ๏ฟฝ = ๐๏ฟฝ๏ฟฝ, and equation of motion
takes the form
๏ฟฝ๏ฟฝ + 2๐๏ฟฝ๏ฟฝ ร ๐ = ๏ฟฝ๏ฟฝ โ ๐2๏ฟฝ๏ฟฝ ร (๏ฟฝ๏ฟฝ ร ๐๐)