Energy can change from one form to another without a net loss
or gain. LAW OF CONSERVATION OF ENERGY!!! (You will learn to
identify these transformations)
Slide 3
think! Suppose that you apply a 60-N horizontal force to a 32
kg package, which pushes it 4 meters across a mailroom floor. How
much work do you do on the package? 9.1 Work
Slide 4
think! Suppose that you apply a 60-N horizontal force to a
32-kg package, which pushes it 4 meters across a mailroom floor.
How much work do you do on the package? Answer: W = Fd = 60 N 4 m =
240 J 9.1 Work
Slide 5
9.7Conservation of Energy Same energy transformation applies
The 2 J of heat can be called non- useful work (work that is not
part of the objects total mechanical energy). 10 J of PE does 8 J
useful work on the arrow and 2 J of non-useful work on the
molecules that compose the bow and string and arrow. The arrow has
8 J of KE as a result. Dissipated energy (DE) is amount of energy
transferred away from the total mechanical energy. More DE means
less KE, which reduces TME, which means less speed! Total
Mechanical Energy Non-mechanical Energy (dissipated) Total
Mechanical Energy
Slide 6
9.7Conservation of Energy The 2 J of heat can be called non-
useful work (work that is not part of the objects total mechanical
energy). Dissipated energy (DE) is amount of energy transferred
away from the total mechanical energy. More DE means less KE, which
reduces TME, which means less speed! Total Mechanical Energy
Non-mechanical Energy (dissipated) Total Mechanical Energy
Slide 7
Energy The ability to do work or cause change Can be
transferred into other forms Is conserved (can neither be created
nor destroyed) SI Unit is Joules I can define energy
Slide 8
Work Force times distance the force is applied (W = Fd) When
work is done, energy is transferred, stored or used SI Unit is
Joules Positive work is work done in the direction of motion.
Negative work does work against the object (in a direction opposite
of motion) I can define work. Positive work? Negative work?
Slide 9
Watch the transfer of KE and PE. What happens to the PE when
the skier moves down the hill? What happens to the KE and TME when
the skier travels over the unpacked snow? What work is done?
Slide 10
a) Did the weightlifter do work on the barbell and weights? b)
Is the weightlifter currently doing work on the barbell and
weights? c) Explain two ways that the work done by the weightlifter
be increased. 1. 2. 6.1 Work = force distance
Slide 11
Did the weightlifter do work on the barbell and weights? Yes,
when he first lifted them above his head. Is the weightlifter
currently doing work on the barbell and weights? No, the barbell
and weights are not moving. Explain two ways that the work done by
the weightlifter be increased. 9.1 Work = force distance 1)Increase
the weight on the ends of the barbell 2)Increase the distance over
which the weightlifter pushes the barbell and weights.
Slide 12
While the weight lifter is holding a barbell over his head, he
may get really tired, but he does no work on the barbell. Work may
be done on the muscles by stretching and squeezing them, but this
work is not done on the barbell. When the weight lifter raises the
barbell, he is doing work on it. 9.1 Work
Slide 13
When is work done on an object? When is work not done on an
object? 9.1 Work When the object moves. When the object does not
move.
Slide 14
Ramp Work: Force and Distance To test the relationship between
force and distance when using a ramp of different lengths Use the
equation W = Fd
Slide 15
Work has the same units as energy Joules Newton x meter JN x m
9.1 Work One joule (J) of work is done when a force of 1 N is
exerted over a distance of 1 m (lifting an apple over your
head).
Slide 16
Kinetic Energy The energy of motion KE = m x v 2 Different
forms of KE (mechanical, electrical, thermal, electromagnetic or
light) What is kinetic energy? What are the forms of KE?
Slide 17
Kinetic Energy KE increases with speed KE increases with
mass
Slide 18
WIND ENERGY Atmospheric pressure differences cause air
particles to move.
Slide 19
SOUND ENERGY Energy caused by compression of air
particles.
Slide 20
ELECTRICAL ENERGY Energy of moving charged particles.
Slide 21
THERMAL ENERGY The energy of moving and vibrating molecules
Sometimes called heat.
Slide 22
LIGHT or RADIANT ENERGY Energy that travels in waves as
electromagnetic radiation and/or as photons.
Slide 23
When you throw a ball, you do work on it to give it speed as it
leaves your hand. The moving ball can then hit something and push
it, doing work on what it hits. 9.5 Kinetic Energy WORK
Slide 24
If the speed of an object is doubled, its kinetic energy is
quadrupled (2 2 = 4). It takes four times the work to double the
speed. An object moving twice as fast takes four times as much work
to stop and will take four times as much distance to stop. 9.5
Kinetic Energy
Slide 25
Kinetic Energy How does KE increase or decrease? Increase or
decrease the velocity or the mass!!!! Double the velocity,
Quadruple the KE!!!!! Prove it: Calculate the KE of a 2500 kg car
traveling at 20 m/s and at 40 m/s KE at 20 m/sKE at 40 m/s (500,000
J)(2,000,000 J)
Slide 26
Kinetic Energy More mass, same speed, more KE. Double the mass,
double the KE Prove it: Calculate the KE of a 100 kg cart and a 200
kg cart, each traveling at 15 m/s 100 kg cart at 15 m/s 200 kg cart
at 15 m/s (11,250 J)(22,500 J)
Slide 27
Potential Energy Stored energy or the energy of position
Gravitational PE is based on height and mass Gravitational PE is
mass x gravity x height (GPE = mgh) Increases in height cause
increases in stored energy What is potential energy? How does GPE
change?
Slide 28
Gravitational Potential Energy Energy is stored in an object as
the result of increasing its height. Work is required to elevate
objects against Earths gravity. Example: Water in an elevated
reservoir and the raised ram of a pile driver have gravitational
potential energy. 9.4 Potential Energy
Slide 29
The amount of gravitational potential energy possessed by an
elevated object is equal to the work done against gravity to lift
it. PE = mgh What is the gravitational PE of a 10.0 kg object at
4.00 m above the ground? mg is weight (in newtons) [mass (kg) x
gravity (m/s 2 )] 10 kg x 9.8 m/s 2 x 4 m = 392 J 9.4 Potential
Energy
Slide 30
The potential energy of the 100-N boulder with respect to the
ground below is 200 J in each case. a.The boulder is lifted with
100 N of force. 9.4 Potential Energy
Slide 31
The potential energy of the 100-N boulder with respect to the
ground below is 200 J in each case. a.The boulder is lifted with
100 N of force. b.The boulder is pushed up the 4-m incline with 50
N of force. 9.4 Potential Energy
Slide 32
The potential energy of the 100-N boulder with respect to the
ground below is 200 J in each case. a.The boulder is lifted with
100 N of force. b.The boulder is pushed up the 4-m incline with 50
N of force. c.The boulder is lifted with 100 N of force up each
0.5-m stair. 9.4 Potential Energy
Slide 33
think! You lift a 100-N boulder 1 m. a. How much work is done
on the boulder? b. What power is expended if you lift the boulder
in a time of 2 s? c. What is the gravitational potential energy of
the boulder in the lifted position? 9.4 Potential Energy
Slide 34
Other forms of PE Other forms of PE (Chemical PE, Elastic PE,
Electric PE, Magnetic PE, Nuclear PE) Changes in position in a
force field changes the PE (gravitational fields, magnetic fields
and electric fields) What are the forms of potential energy?
Slide 35
Elastic Potential Energypotential to do work Energy stored in a
stretched or compressed spring or material. When a bow is drawn
back, energy is stored and the bow can do work on the arrow. These
types of potential energy are elastic potential energy. 9.4
Potential Energy
Slide 36
CHEMICAL POTENTIAL ENERGY Energy due to the bond position
between molecules (stored during bonding). Potential chemical
energy is released from chemical reactions (burning, for example).
Fuels, Food, Batteries, for example.
Slide 37
Name three examples of potential energy. 9.4 Potential
Energy
Slide 38
Difference between kinetic energy and potential energy Kinetic
energy The energy of motion Potential energy The energy of position
or stored energy
Slide 39
Mechanical Energy The sum of the KE and PE in a system: (total
ME = KE + PE) Describes energy associated with the motion of
objects The KE and GPE are conserved for moving objects (neglecting
friction, drag, vibrations and sound) What is mechanical
energy?
Slide 40
Mechanical Energy = PE + KE The total mechanical energy = 100 J
100 J = 100 J PE + 0 J KE 100 J = 50 J PE + 50 J KE 100 J = 0 J PE
+ 100 J KE
Slide 41
Non-Mechanical Energy Energy not associated with the motion of
objects Typical examples are vibrations, sound and heat Referred to
as dissipated energy or waste energy Can be observed at the
molecular level Path of energy transfer that reduces the KE of the
object What is non- mechanical energy?
Slide 42
Starter 1-10 Energy transfers Energy transfers into different
forms PE transfers to KE When you fall, your PE decreases and your
KE increases Half way down PE = KE As the person falls, the PE and
KE flip Energy is not destroyed (start and end with 10,000 J)
Slide 43
Starter 1-10 Gravity is involved (GPE-- gravitational PE) Max
KE can never exceed the Max PE As PE decreases, KE increases PE
stored is used when object is in motion No energy is lost from PE
to KE The farther the diver falls, the greater the KE Energy
transforms from PE to KE from point A to point B
Slide 44
Starter 1-10 As the KE increases, the PE decreases As the diver
hits the bucket, all PE has been transferred to KE Work produces
energy Energy changes throughout the dive At the top of the
platform, all GPE and no KE The diver always possesses 10,000 J of
energy (energy is conserved) Inverse relationship between PE and
KE
Slide 45
Starter 1-10 Energy is constantly changing as the diver falls
No energy is lost during the dive; it transfers from PE to KE All
PE has been transferred to KE The PE decreases 2500 J change in PE
or KE As the PE decreases, the KE increase (gravitational PE) PE is
inversely related to KE
Slide 46
Total Mechanical Energy Total mechanical energy = kinetic
energy + potential energy TME = KE + PE 100 J = 0 J + 100 J At
rest, no KE, no motion 100 J = 50 J + 50 J In motion, 50 J of PE
transferred, object now has 50 J of KE. 100 J = 100 J + 0 J In
motion, no potential energy (100 J transferred to KE) In each case,
the total mechanical energy is the same. As the PE decreases, the
KE increases.
Slide 47
Indicate where: KE is at a minimum and maximum GPE is at a
minimum and maximum The speed is greatest The speed is least Energy
is being stored and released Positions 1 and 5 are at the same
height 1. Explain how energy transforms and is conserved as the
pendulum swings back and forth 2. What happens as the KE increases?
3. What happens as the GPE increases?
Slide 48
KE min PE max PE min KE min KE max PE max transformation of PE
to KE (release) V = 0 m/s V = maximum transformation of KE to PE
(storage)
Slide 49
Analyzing KE and PE Distance (from motion detector) time
closest farthest
Slide 50
Slide 51
Power The rate at which energy is transferred or work is done
(work per second) SI Unit is Watts (Joules/second) The faster the
energy is used, the greater the power More powerful if more work is
done in same time same work is done in less time What is
power?
Slide 52
Jet engine vs. lawn mower engine Both receive gallon of fuel
(same energy, same work) A high-power jet engine does work rapidly,
uses gallon in 1 second. The low-powered lawn mower engine does
work slowly, using gallon in 30 minutes. 9.2 Power vs.
Slide 53
Power is the rate at which work is done. 9.2 Power The unit of
power is the joule per second, also known as the watt. One watt (W)
of power is expended when one joule of work is done in one second.
One kilowatt (kW) equals 1000 watts. One megawatt (MW) equals one
million watts. P = w/t
Slide 54
Power When you run 3 km rather than walk, you use the energy
more quickly because your body demands more energy per unit time.
When you compare the amount of energy required to operate an
electric dryer vs. a laptop computer, the electric dryer demands
more energy per unit time. More energy per unit time means more
power is required! Needs 5500 J/s Needs 50 J/s
Slide 55
Power 100 W incandescent light bulb How much electrical energy
per second? 100 joules per second.
Slide 56
Power vs. Work When carrying a load up some stairs, you do the
same amount of work whether you walk or run up the stairs. Whether
you walk 3 km or run 3 km, you do the same amount of work (your
weight x distance), burn roughly the same amount of calories, and
use the same amount of energy. So what is power?
Slide 57
Power Consider a person climbing stairs. Name two ways that the
person can double their power when moving. Do twice the work in the
same amount of time (climb a second flight of stairs in the same
time) Do the same amount of work in half the time (climb one flight
of stairs in half the time).
Slide 58
The three main engines of the space shuttle can develop 33,000
MW of power when fuel is burned at the enormous rate of 3400 kg/s.
9.2 Power
Slide 59
think! If a forklift is replaced with a new forklift that has
twice the power, how much greater a load can it lift in the same
amount of time? If it lifts the same load, how much faster can it
operate? 9.2 Power
Slide 60
think! If a forklift is replaced with a new forklift that has
twice the power, how much greater a load can it lift in the same
amount of time? If it lifts the same load, how much faster can it
operate? Answer: The forklift that delivers twice the power will
lift twice the load in the same time, or the same load in half the
time. 9.2 Power
Slide 61
Does work against the moving object (negative work in opposite
direction of motion) Or it takes work (energy) to overcome friction
or drag Role of friction and drag in work
Slide 62
Work Energy Theorem Work done changes the energy. If a car has
34,000 J of KE, 34,000 J of work was done on the car to speed it
up, and braking will require 34,000 J of negative work due to
friction to bring the car to rest What is the relationship between
work and kinetic energy (work-energy theorem)?
Slide 63
Due to friction, energy is transferred both into the floor and
into the tire when the bicycle skids to a stop. a.An infrared
camera reveals the heated tire track on the floor. 9.6 Work-Energy
Theorem http://www.batesville.k12.in.us/physics/phy
net/mechanics/energy/braking_distance.ht m
Slide 64
Due to friction, energy is transferred both into the floor and
into the tire when the bicycle skids to a stop. a.An infrared
camera reveals the heated tire track on the floor. b.The warmth of
the tire is also revealed. 9.6 Work-Energy Theorem kinetic energy
is transformed into thermal energy, sound and vibrations, which
represent work done to slow the bike (Fd)
Slide 65
Work Energy Theorem What is the relationship between work and
kinetic energy (work-energy theorem)?
http://www.youtube.com/watch?v=u_89NXUio9w
http://www.youtube.com/watch?v=0mR89s T0YfA
http://www.youtube.com/watch?v=WjvVbX Dy20w
http://www.youtube.com/watch?v=Z_n- HIBnfts
Slide 66
The work-energy theorem states that whenever work is done,
energy changes. 9.6 Work-Energy Theorem Work = KE Work equals the
change in kinetic energy.
Slide 67
Calculating Stopping Distance Fd = mv 2 What is the stopping
distance for a 650 kg car that is traveling 5 m/s if 4,500 N of
braking force is applied? d = mv 2 F d = 1.8 m Calculate the
stopping distance for the same car that travels at 10 m/s. 7.2
m.
Slide 68
Calculating Stopping Distance Calculate the stopping distance
for the same car that travels at 10 m/s. 7.2 m. How does this
stopping distance compare with the stopping distance at 5 m/s? It
is four times greater! Double the speed, quadruple the stopping
distance.
Slide 69
Calculate Stopping Distance Fd = mv 2 - Calculate the
difference in stopping distance for a car that travels at 30 km/h
and the same car that travels 60 km/h. Assume that the mass of the
car is 800 kg and the braking force is 5000 N. Show your work and
analyze your results. (Note: you must first convert km/h to m/s)
How does speed influence stopping distance?
Slide 70
A car moving at twice the speed of another has four times as
much kinetic energy, and will require four times as much work to
stop. The frictional force is nearly the same for both cars, so the
faster one takes four times as much distance to stop. Kinetic
energy depends on speed squared. 9.6 Work-Energy Theorem
Slide 71
Typical stopping distances for cars equipped with antilock
brakes traveling at various speeds. The work done to stop the car
is friction force distance of slide. 9.6 Work-Energy Theorem
Slide 72
Typical stopping distances for cars equipped with antilock
brakes traveling at various speeds. The work done to stop the car
is friction force distance of slide. 9.6 Work-Energy Theorem
Slide 73
Typical stopping distances for cars equipped with antilock
brakes traveling at various speeds. The work done to stop the car
is friction force distance of slide. 9.6 Work-Energy Theorem
Slide 74
think! When the brakes of a car are locked, the car skids to a
stop. How much farther will the car skid if its moving 3 times as
fast? 9.6 Work-Energy Theorem
Slide 75
think! When the brakes of a car are locked, the car skids to a
stop. How much farther will the car skid if its moving 3 times as
fast? Answer: Nine times farther. The car has nine times as much
kinetic energy when it travels three times as fast: 9.6 Work-Energy
Theorem
Slide 76
For moving objects such as cars: The more kinetic energy it
has, the more work is required to stop it. Twice as much kinetic
energy means twice as much work. Brakes do work on wheels (you do
work by pushing the brake pedal). When a car brakes, the work is
the friction force (supplied by the brakes) multiplied by the
distance over which the friction force acts. KE is transformed by
work (friction) into thermal energy, sound energy and larger-scale
vibrations. 9.6 Work-Energy Theorem
Slide 77
The law of conservation of energy states that energy cannot be
created or destroyed. It can be transformed from one form into
another, but the total amount of energy never changes. 9.7
Conservation of Energy For any system in its entiretyas simple as a
swinging pendulum or as complex as an exploding galaxythere is one
quantity that does not change: energy. Energy may change form, but
the total energy stays the same.
Slide 78
When energy is transformed, it is conserved, meaning that it
will change form without losing its original amount of energy. 9.7
Conservation of Energy
Slide 79
When the woman leaps from the burning building, the sum of her
PE and KE remains constant at each successive position all the way
down to the ground. 9.7 Conservation of Energy
Slide 80
Elastic potential energy will become the kinetic energy of the
arrow when the bow does work on the arrow. 9.7 Conservation of
Energy As you draw back the arrow in a bow, you do work stretching
the bow. The bow then has potential energy. When released, the
arrow has kinetic energy equal to this potential energy. It
delivers this energy to its target.
Slide 81
Everywhere along the path of the pendulum bob, the sum of PE
and KE is the same. Because of the work done against friction, this
energy will eventually be transformed into heat. 9.7 Conservation
of Energy Non-useful work can also be called non-useful
energy!
Slide 82
9.7Conservation of Energy Why does a tennis ball eventually
stop bouncing? Eventually, all of the total mechanical energy is
transformed into non-useful energy (heat, sound, movement of
fibers) 50 J PE 50 JKE New height less than before means less PE
stored 35 J PE Bounce! 35 J KE Bounce! 20 J PE (bounce and so on!)
20 J KE
Slide 83
Slides showing transformation of KE and PE Source:
http://www.physicsclassroom.com/mmedia /index.cfm
http://www.physicsclassroom.com/mmedia /index.cfm
Slide 84
Watch how KE and gravitational PE transform Where is the KE at
the maximum? Where is the PE at the maximum? How is PE stored?
Slide 85
Watch the change in height vs. the change in speed! How does
the change in height affect KE and PE?
Slide 86
What happens to KE and TME when the brakes are applied? What
work is being done?
Slide 87
Watch the transfer of KE and PE. What happens to the PE when
the skier moves down the hill? What happens to the KE and TME when
the skier travels over the unpacked snow? What work is done?
Slide 88
Same work, more force, less displacement (from left to
right)