Force Final

8/12/2019 Force Final

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Gazi Moyin Uddin M. Sc. (Nuclear Physics)

Force

Forceis the effect which acting on objects and changes (or tries to change):

Its state of rest (Acceleration) Its state of motion (Acceleration or deceleration) Its direction of motion. Its shape or size. All the above effects.

Force is a vector quantity (that is it has magnitude/numerical value and

direction). The unit of force is newton.

Types of Forces: there are three types of forces existing in nature. They

are:

Gravitational force. Electromagnetic force ( Electrostatic force + Magnetic force) Nuclear forces.

These forces are also known as Non-Contact forces. All other forces are

generated by the effect of these forces. If these forces are classified by their

action they can be classified by two ways:

Non-Contact force. Contact force.

Non-Contact Force

Non-contact forces exist naturally. These forces can act from a distance. The

space through these forces act is called Field or Force Field. Non contact

forces are already mentioned.

i. Gravitational Force.ii. Electromagnetic force (Electro static force+ Magnetic force)iii. Nuclear force.


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Gravitational Force:

This is the force between masses. All masses apply a force of attraction to

the other masses by these forces. This force is always attractive,and its

magnitude depends on two factors:

a) Product of the masses involved.

.. (1)

If the masses are larger, then gravitational force between them willbe greater.

If the masses are small the gravitational force between them will benegligible.

b) Distance between the masses from the center of gravity.

.. (2)The force of gravity is inversely proportional to the square of the

displacement between the masses. (This displacement must be measured

from the center of gravity, not from the contact surface). So if the

displacement between the masses is halved the force of attraction due to

gravity will be four times greater than before.

Load and weight is another names of the gravitational forces.

m1m2

r

F


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As the weight is a force it will produce acceleration to a body moving

(attracted) to a planet (as the Earth), this is falling. This acceleration is

called the gravitational acceleration or the acceleration due to gravity,

and it is denoted by a letter g. And as we know:

Thus Force of gravity = mass gravitational acceleration.

.(a)

When the object is at rest then g is called the gravitational field strength

and when the object is falling then it is called the gravitational acceleration.

Mass: Mass means the total amount of substance of an object. Mass

remains same everywhere, it is a scalar quantity.

- The SI unit of mass is kilogram.- A mass of 1 kg sugar will be always 1 kg, bur the force on it can be

different in different parts of the universe.


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Nuclear force: The force between the nucleons inside the nucleus which

holds the nucleons together is the Nuclear force. This force acts between the

nucleons. This is a short ranged force.

Contact Forces:

Contact force cannot act or generate without direct contact of objects.

Contact forces are:

i) Solid- Solid friction forces:This force generates when a solidslides past another solid. Example: The force which holds a car

in an inclined plane.

This force acts between two solid (or more than two) surfaces Pressing each

other in contact and opposes the relative sliding motion on one surface over

the other. These frictional forces are two types.

a) Static frictionb)Kinetic or sliding friction.

When an object (a box) is at rest in an inclined plane then the downward

force is cancelled by the static friction.

To measure the static friction a newton meter is attached to an object

placed on a horizontal surface and the newton meter is pulled horizontally,

the pulling force is gradually increased and in one point the object will just to

move.

++++

Proton & Proton Proton & Neutron

++++

Force applied on the block

Spring balance/Newton meter

Static friction


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The applied force is noted from the newton meter, will be the static frictional

force. For a more result the experiment is repeated carefully and an average

is calculated.

Factors affecting the frictional forces:

i) Frictional forces depend on the force pressing the surfacestogether as the weight or normal reaction forces. More the

pressing forces more the frictional forces.

ii) Types of surfaces in contact: If the surfaces are smooth frictionwill be less, if the surface is rough the friction will be more.

If the lubricants (oil or grease) are used between the sliding parts the

friction will be less. Lubricants fill up the creases and make the surfaces

smooth, and friction reduces. By using ball bearing friction can be reduced.

ii) Solid- Fluid Drag force: This force generates when a solidmoves through a fluid. Example: the force which opposes the

motion of a submarine through water.

As the liquid and gas can flow, they are called fluid and when a solid moves

through a fluid if faces friction, this force is called Drag force or simply

Fluid friction. Fluids can be considered as layers of particles stack together

and there are intermolecular forces. When a solid moves it collides with the

particles of the fluid and faces friction: example submarine, fishes boat, air

Drag forces

Velocity of the submarine

Drag forces on a submarine


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Normal contact and Reaction Force:

i) Normal contact and Reaction forces: This force generateswhen an object is placed on a solid surface. This force is

perpendicular to the solid surface and the reaction force is also

perpendicular to the solid surface these force are in the same

line of action and they are Newtons 3rdlaw pair.

All forces appear in nature in pair (Action- reaction, Newtons 3rdlaw). Thus

all reaction forces are produced due to the force of action, which is due to

normal contact force.

Normal contact forcer

Reaction force


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Tension:

Tension is the pulling force through a string, rope or spring. It is also

produced due to action reaction pair.

Tension The box is being pulled with a force

F= mg

Ruler

Spring

Clamped Stand

Tension


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NEWTONS LAWS OF MOTION

The 1stlaw of motion:

When the net force acting on an object is zero (F =0) then:

i) The object at rest (v=0) will be at rest.ii) The object at motion (v 0 ), will move with a constant

velocity (a =0)

Inertia is a property of mass. The tendency of a rest mass to remain at

rest or moving mass to keep on moving at a constant velocity is called

Inertia.

When cars start moving suddenly, passengers are moved (pushed) backward

due to inertia and when the fast moving cars suddenly break to stop, the

passengers are thrown forward due to inertia. Thus seat-belts are used so

that these seat-belts can supply necessary force in the opposite direction to

stop the passengers safely.

The seat-belts supply an opposite force to cancel the forward force and

make the passengers safe.

V =Constant

F = 0V =0 (at rest)

F = 0

Thus when the net force acting on

an object is zero the object willbe at rest. V =0

Thus when the net force acting

on a moving object is zero theobject will be moving with a

constant velocity. a= 0


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Newtons 2ndLaw of Motion:

When the net force acting on an object is not zero (F 0) then:

i) The rate of change of momentum is directly proportional to thenet applied force.

ii) The momentum change and the applied force have the samedirection.

The initial momentum and the final momentum of the car are p1and p2. This

change take within time t.

P1= m u . (a)

p2= m v ..(b)

The momentum change in time t is p = p2 p1..(c)

In time t the momentum change is p

In time 1 the momentum change is

Mass = m

Velocity = u

Mass = m

Velocity = v


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Thus according to the law:

.(d)

.(e)

When the proportional constant k = 1,

.. (f) as

The acceleration of an object is directly proportional to the

net/resultant force (When the mass is constant)

1)F a ( mass is constant)When the net force is doubled acceleration will be doubled or when the net

force is halved the corresponding acceleration will be halved.

a/ ms-2

F/N


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2)m

1( Force is constant)

When the mass is doubled the acceleration will be halved and when mass is

halved the acceleration will be doubled.

3)F m( acceleration is constant)To produce same acceleration when force is doubled the corresponding mass

should be doubled and when the force is halved the mass will be halved as

well.

The graph shows the mass-

acceleration graph of an

object. The acceleration is

inversely proportional to the

acceleration, thus a curve is

formed

The graph shows the force-

mass graph of an object.

The graph shows the

acceleration (Gradient) is

constant

m/ (kg)

am/sec

2

m/kg

F/(N)


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..(f)

(g)and (h)

- So force is directly proportional to the velocity change.- Thus if a huge velocity drops to a little velocity then a large force is

acted on that object,

- And force is inversely proportional to the time needed for the velocitychange. If a moving object changes its velocity in a short time then a

large force is acted on that object.

This graph shows that when impact

time increases the correspondingimpact force decreases.

Time/s

F

/N


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Situation-1: Landing on thick mattress:

When a person lands on a thick mattress from a height the mattress

compresses and takes a longer time to make the person completely stopped

i.e. it increases the stopping time of the person, and as

and as the stopping time increases the impact force decreases and thus the

reaction force on the person due to impact is also less and the person is not

injured.

Situation -2: If a hammer head is made out of rubber then it cannot

protrude effective force when it is used to protruding a nail by hitting. This is

because when a nail is hammered the hammer head compresses as it is

made from rubber thus taking a longer time to stop and as . Force is

inversely proportional to the impact time, as the impact time increases theimpact force decreases, thus it cannot effectively protrudes the nail.

( )t

uvmF

=

tF

1

Impact force ishigh

Hard FloorCompression Height

Impact force is less

Mattress


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Newtons 3rdLaw of Motion:

According to Newtons 3rd law Whenever a force acts in a body, an

equal, opposite and same type of force acts on another body. The

action force and the reaction force have the following properties:

i. . . . .ii. . . . .iii. . . . .iv. . . . .v. . . . .vi. . . . .

Newtons 3rdlaw explains how a force appears in nature.

Situation-1:When a canon is fired: When a cannon is fired the barrel of the

cannon recoils in the opposite direction. Explosion in the gun power causes a

force on the cannon ball, and equal amount of force acts on the cannon

barrel in the opposite direction. So standing just right behind the cannon

barrel when it is fired is same dangerous as to face the cannon shell,

because both the case the momentum change or force acting by them are

the same.

mVM v Mv = mVF1= F2

F1F2


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2. When the net force (or the component of the net force) is applied in

perpendicular to the direction of the velocity then the force change the

direction of the moving body.

A net force is applied in an angle with the velocity, then the vertical

component will change the direction and the parallel component will keep

the car going.

Velocity

Force perpendicular to velocity

A net force is applied in the

perpendicular to the velocity to change

the direction of motion

Force

Force perpendicular to velocity

Force = F

Velocity

F cos

F sin


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If two 10N forces are acting perpendicularly each other then the resultant

force will be the diagonal of the parallelogram drawn by the forces as shown.

= 90.10.10.21010 22 Cos++

= 14.142N

If two forces F1 and F2 are acting at any angle in any point O then the

resultant force F can be calculated by completing the parallelogram drawn by

these two forces F1and F2, the diagonal will be the resultant of these two

forces.

When the two forcesF1and F

2are acting in the same direction then the angle

between them = 0 and the net force F = F1+F2.When the forces are acting

in the opposite direction then = 180othen the net force F = F1~F2. And

when the forces are acting in perpendicular to one another, then = 90o,

and the net force F= 222

1 FF + which is Pythagoras theorem.

The net/resultant/ force on this box is 14.142N

NETforce = )2( 22 PQCosQP ++

10N

10N

14.14N

F2

F1 F

F= )CosFFFF( ++ 212

22

1 2


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Center of gravity: Center of gravity is the point where all the forces of

gravity of an object seem to be acting. The object can be balanced in this

point.

Notes:

i. All forces must act in a point, or they will produce turning effect.ii. If all the forces are balanced then they form a polygon.iii. To lift something upward force and downward force must be same.iv. Turning and lifting are not the same.v. Turning effect will only produce if the force is not passing through the

turning point (pivot/fulcrum).

Centre of gravity of the ruler


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ii. A car is moving with a constant velocity on a road: When acar is moving with a

Constant velocity on a road then the net force acting on the car is zero.

There are at least four

forces acting on it, and they also cancels out each other.

Here the magnitude of the gravitational force is equal to the magnitude of

the reaction force and as they are acing in the opposite direction thus their

effect is cancelled out. The magnitude of the thrust of the engine is equal to

the magnitude of the total (Ground and air frictional force ) frictional force in

the opposite direction, thus they cancels out and the total net force acting

on the car is zero, and it is moving with a constant velocity. If the

accelerator of the car is pressed then the engine will produce more thrust

and net force will be acting on the car and the car will accelerate in the

direction of the force, at the same time the air friction will also increase in

the opposite direction and soon the forward force and the backward force

will be equal and the car will move with a greater constant velocity. The

gravitational force will cancel the reaction force as long as the car remains in

the road.

Gravitational force

Reaction force

Ground+ air friction

Thrust by the engine


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i) A speed boat is moving through the water:When a boat floats in the water and if it is stopped and still then the net

force acting on it zero, and then at least two forces acting on it- the

gravitational force and the up-thrust by the water. These two forces are

acting in the opposite direction with the same magnitude, and cancelling

each other.

In the diagram the gravitational force F2is equals to the up Thrust F1in

magnitude and as they are in the opposite direction they are cancelled out.

Then the forward force F initially will be the net force acting on the boat and

producing acceleration. And this acceleration can be calculated by the

equation a = F/m (m/s2). Soon the fluid friction on the boat will increase

and becomes equal then the boat will move with a constant velocity.

F2

F1

Forward thrust

Fluid friction


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Forces in moving references:

i) This lift is at rest on the surface of the Earth.ii) The man stands on a scale. It is showing his weight (The force of

gravity).

iii) Suppose this force is F (mg).

i) Now the lift is moving upward with a constantvelocity, the net force on the lift is zero

ii) The scale shows the same reading asweight F (mg).

iii) When the lift moves with a constant velocityThe scale shows the weight F (mg) as there is

No extra force

Constant Velocity

Force F = mg

Force F = mg


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The total force acting downward is: F + F1

The scale shows a higher reading (F + F1)

When the lift is moving upward and decelerating then reaction force on the

man will act upward, which must be subtracted from the gravity, thus the

scale will show less reading. The net force = F - F1

When the lift is moving downward and accelerating the person feels an

upward force thus the reading on the scale will be less. If the lift accelerates

at the same rate as gravity, then the upward net force will be same as

gravity and the person will fell no force (weightless), and he will fly inside

the lift.

Now there is net force acting on the lift upward and

the lift is accelerating upward. The man is also

accelerating upward, by a net force and, the man

applies a reaction force in the opposite direction

(Downward) on the scale. This force will add up with

the gravity thus the scale will show higher reading.

The man will feel massive or a pressing force

downward.

Force F = mg

Reaction force F1

Acceleration Applied force


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Filename: Force-FinalDirectory: C:\As-Class-pro\Unit-1Template:

C:\Users\ASUS\AppData\Roaming\Microsoft\Templates\Normal.dotm

Title: ForceSubject:Author: MoyinKeywords:Comments:Creation Date: 29-Nov-12 9:54:00 PMChange Number: 87Last Saved On: 19-Mar-13 11:49:00 AMLast Saved By: Moyin

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Force Final

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