Post on 05-Jan-2016
Applied Physics Chap 4 Energy 1
Chapter 8: Energy The universe is composed of two substances called matter and energy which are interrelated on some fundamental level (Einstein’s law E = mc2 )
But: we really don’t know what Energy is.
Applied Physics Chap 4 Energy 2
Video: Introduction to energy and work
Applied Physics Chap 4 Energy 3
matter possesses Inertia and Gravity
matter forms all the stuff that makes up our world.
Matter is measured by its mass in kg.
Energy appears in a variety of different forms.
Energy can be changed from one form into another
Energy causes changes in matter
What we do know about matter:
What we do know about energy:
Applied Physics Chap 4 Energy 4
Energy: The ability to cause a change in matter.
Different types of Energy:
Thermal Energy: Energy from heat and fire
Mechanical Energy: energy from motion or position
Chemical Energy: Burning fuel, food, batteries
Nuclear Energy: Energy from the atom
Electrical Energy: Energy from electrical interactions
Radiant Energy: Energy in the form of light, etc.Work: the transfer of energy from one kind to another through motion
Applied Physics Chap 4 Energy 5
Video: energy exchanges
Applied Physics Chap 4 Energy 6
Video: Physical energy exchanges
Applied Physics Chap 4 Energy 7
Mechanical energy: is energy that results from the position of something (called Potential Energy) or from its motion (called Kinetic Energy).
Potential Energy PE stored energy
Gravitational PE: is energy stored in an objects height above the ground
Chemical PE: is energy stored in the position of atoms in a molecule
Elastic PE: is energy stored in a spring or rubber band
Kinetic Energy: KE, energy in motion.
Applied Physics Chap 4 Energy 8
Video: Kinetic and Potential Energy
Applied Physics Chap 4 Energy 9
Gravitational Potential Energy depends upon:
the mass of the object being lifted (in kg).
the height the object is lifted to (in meters).
PEg = Gravitational Force (weight) x Change in height.
PE = Fg h
Since Fg = mg then PEg also = mg h
hm
Fm
Applied Physics Chap 4 Energy 10
Units of Energy: Joule 1 N 1 m = 1 Joule
1 Newton pushing something a distance of 1 meter performs 1 Joule of work
James Joule: 1818 - 1889
The English physicist James Prescott Joule (1818-1889) proved that mechanical and thermal energies are inter-convertible on a fixed basis, and thus he established the great principle of conservation of energy.
Applied Physics Chap 4 Energy 11
The product of the force on an object and how far the object is moved by that force.
Wk = Force distance
Work measures how much energy is converted from one form into another
Since work is equal to the quantity of energy transferred, then the units for work would also be JOULES.
WORK: The transfer of energy through motion.
Applied Physics Chap 4 Energy 12
Video: energy and work
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WORK DONE AGAINST FRICTION:
Fm
Fm
d
Calculating work done against friction:
Work = Force of Friction x distance moved.
Wk = Ff d
Work is done by changing chemical energy to kinetic energy to thermal energy which is lost to the surrounding air.
Applied Physics Chap 4 Energy 14
Work done lifting a mass to a height.
h
F
As the object is lifted, work is done converting chemical energy (in muscles) into gravitational potential Energy which is stored in its position.
on the ground, the box has 0 J of PEg
After it is lifted the box has PEg = mgh
Since mg = Weight then:
PEg = W h
Applied Physics Chap 4 Energy 15
The faster something is moving, the more kinetic energy it contains. KE = ½ mv2
KE depends on:
The square of the object’s velocity in m/s
The objects mass in kg.
KINETIC ENERGY Mechanical energy that comes from the motion of an object.
Applied Physics Chap 4 Energy 16
Recall: work is done when energy is changed from one form to another through motion.
M = 1500 kg
V = 15 m/s
Work is done accelerating an automobile because, energy stored in gasoline is changed to KE by burning it in the engine.
The amount of work done is equal to the KE of the car after it reaches a top speed.
Applied Physics Chap 4 Energy 17
Kinetic energy and the automobile
M = 1500 kg
V = 15 m/s
J168750/151500 221 smkgKE
M = 1500 kg
V = 30 m/s
J675000/301500 221 smkgKE
Since all the KE comes from gasoline, it takes 4 times as much gasoline to travel at 30 m/s as at 15 m/s.
Double the speed
Increase KE by 4 times as much.
Applied Physics Chap 4 Energy 18
Power: is the rate that work is done
To increase power, just do the same amount of work in a shorter time.
Similarly, taking longer to do work requires less power.
time
workPower
power is given the units of Watts. Named after James Watt the inventor of the steam engine.
Applied Physics Chap 4 Energy 19
Law of conservation of energy.
Energy cannot be created or destroyed.
It can only be transformed from one form into another, leaving the total amount of energy unchanged.
Law of conservation of Mechanical energy: The sum total of KE + PE in a system does not change. but at any point energy is either PE KE
ME = Constant
KEi + PEi = KEf + PEf
Applied Physics Chap 4 Energy 20
Conservation of energy in a Pendulum.
B
As the weight falls toward point B. PE is converted into KE until at Point B all the energy is Kinetic.
When Pendulum is released from point A, it has PE but no KE.
A
As the weight rises toward point C. KE is converted back into PE until it stops moving at point C.
C
Applied Physics Chap 4 Energy 21
Height = 10 m
At the top KE = 0 J
PE = mgh = (5 kg)(9.8)(10m) = 490J
ME = KE + PE = 0 + 490J = 490 J
At the bottom PE = 0 J
KE = 490J
ME = KE + PE = 0 + 490J = 490 J
Conservation of Mechanical energy
Consider a heavy rock at the top of a cliff
Applied Physics Chap 4 Energy 22
Conservation of Energy and the Roller coaster
No PE
No KE
Work is done increasing the car’s PE
PE changes to KE
Applied Physics Chap 4 Energy 23
Conservation of energy: The Pile Driver.
A.) Work is done lifting the weight, converting chemical energy into potential energy.
B.) When the weight falls it PE is converted into KE
C.) When the weight hits the piling it does work driving the piling into the ground. Energy is converted from KE to thermal energy through friction with the ground.
Applied Physics Chap 4 Energy 24
Work, Machines and Mechanical Advantage
For example, to accomplish work Wk = F d we could
use a large force to push an object a short F d
or a small force times a long distance F d as long as the product of force and distance equal the same amount of work
We use machines because they permit us to do the same amount of work while using a smaller amount of force at the expense of a longer effort distance.
Wk = F d = F d
Applied Physics Chap 4 Energy 25
We need to lift a 50N box to a height of 2.0 m. The work we need to do is:
Wk = Fd = 50N 2m = 100 J
F2 m
50N
However its easier to use a ramp
Using a ramp does the same work with less force.
2 m
50 N
50 N
We could lift it straight up:
Jm
J
d
WkF 10
10
100
The force we would need:
Applied Physics Chap 4 Energy 26
Mechanical Advantage: MA: The ratio of the output force of a machine to the input force you have to exert.
1510
50to
N
N
F
R
forceEffort
forceResistanceMA
Effort Force F is the force a worker has to apply to lift an object
Resistance force R is either the weight of the object or the force needed to move the object.
Mechanical advantage is a multiplier. You multiply the effort force by the MA to find how much weight can be lifted.
Applied Physics Chap 4 Energy 27
Simple Machines:
Simple machines devices that use the principle of mechanical advantage to permit accomplishing a certain amount of mechanical work with less force.
There are six basic kinds of simple machines: Levers,
pulleys, wheel and axel,
screw, wedge and
inclined plane. In combination, many other types of compound machines can be constructed.
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Levers: a device composed of a rigid arm called the lever arm, and a rest point called a fulcrum.
Fulcrum
Input Force F
Resistance force R
Effort Length E
Resistance Length L
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Fulcrum: point around which the lever rotates.
Effort Length E : the distance from the point where you apply force to the fulcrum.
Resistance Length L. The distance from the point where the output force is applied to the fulcrum.
Effort force: F The amount of force that the worker or the machine has to apply to do the work
Resistance force: R Either the weight of the object or the force that must actually be used
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F
R
L
EMA
forceeffort
forceResistance
lengthresistance
lengtheffortMA
Mechanical advantage for a lever.
Mechanical advantage is equal to either:
The ratio of effort length to resistance length
OR
The ratio of resistance force to effort force.
Applied Physics Chap 4 Energy 31
Screw: An inclined plane wrapped around a cylinder.A screw is simple a long inclined plane that is rotated to continue moving an object upward. When coupled with a wheel and axel like a screw driver, it allows a large quantity of force to be applied in lifting an object up or down.
Wedge: A double inclined plane that is used to turn a downward force sideways. Example: a log splitting wedge takes the downward force from the maul and redirects it at a 900 angle in each direction to force the log apart.
Applied Physics Chap 4 Energy 32
Resistance Length L
Effort Length E
Wheel and Axel: a special kind of lever.
Effort Length
Resistance Length
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Effortlength
Resistance length
Example: Resistance Length = 10 cm and Effort Length = 20 cm.
12 tocm10
cm20
ArmResistance
ArmEffortMA
Wheel and Pulley
Applied Physics Chap 4 Energy 34
Gears are determined like unequal diameter pulley’s as a ratio either of the radius of the gears or as a ratio of the number of teeth on each gear. A
14 :20
80
A Teeth
B TeethMA
Applied Physics Chap 4 Energy 35
Pulley systems: a special type of wheel and axel
Applied Physics Chap 4 Energy 36
300 N
Effort force
= 300 NMA = 1
300 N
Effort force
= 150 NMA = 2
300 N
Effort force
= 100 NMA = 3
Mechanical advantage in pulleys.