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Chapter 3 outline: Main Ideas
 Work and Power
 Energy




Momentum



Kinetic Energy
Potential Energy
Conservation of Energy
Linear Momentum
Angular Momentum
Energy and Civilization
3-1
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Work
 We
have seen that FORCE is the
physical phenomenon behind motion
and changes in motion
 WORK is a way to determine the
measure of CHANGE caused by a
force
3-2
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Not the usual definition

Definition of WORK:
Work=Force x Distance





Energy: same as work


This means that something has to move
The force must be the force along the direction of motion
UNIT: Joule = Nm
Other Unit: foot-pound (English System)
UNIT: Joule
Power: Energy / time

UNIT: WATT
3-3
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Mathematically
 Work
is expressed in a simple
equation:
W  Fd
 Where
the Force and the distance
through which the object is moved
are along the same direction
3-4
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Special Case: Work against gravity



Whenever any object is lifted, work must be done
against the gravitational force in order to move the
object
To determine the magnitude of this work, we begin
with the definition of the force due to gravity on an
object of mass m, also called the WEIGHT of the
object:
F  mg
And represent the distance moved (upward or
downward) with the letter h in the WORK
equation:
W  F d  (mg )h
3-5
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Reminder: Newton 2
 According to Newton’s second law:


FORCE is defined as mass x acceleration
Because of the relationship between
WORK and ENERGY,
Whenever energy is
being expended,
a force is being exerted.
3-6
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POWER
 Power is a measure of the RATE at
which work is accomplished
 Defined by
W
P
t

The unit of power: WATT (W)

1 W = 1 J/s
3-7
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Definition of ENERGY
ENERGY
CAN BE THOUGHT OF
AS
“THE ABILITY TO DO
WORK”
3-8
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Two Main Kinds of Energy


The energy of motion
ONLY MOVING OBJECTS have kinetic
energy
The energy of position
Many different types of potential energy
3-9
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KINETIC ENERGY (KE)


The energy of MOTION
SIMPLE definition
1
KE   mass  (velocity)
2
2
NO VELOCITY
 NO KE
LARGER MASS

LARGER KE
LARGER VELOCITY  LARGER KE
3-10
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Discussion Example
Which object has the greatest amount of
kinetic energy?
A. 200 g baseball traveling at 50 m/s
B. 4 kg bowling ball traveling at 7 m/s
C. 2000 kg truck sitting in the parking
lot
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Do the Math, using our definition of KE
C. 2000 kg truck sitting in the parking lot
KE = (1/2) x 2000 (kg) x [0 (m/s)]2
= (1/2) x (4) x (0) JOULES
= 0 Joules
B. 4 kg bowling ball traveling at 7 m/s
KE = (1/2) x 4 (kg) x [7 (m/s)]2
= (1/2) x (4) x (49) JOULES = 98 Joules
A. 200 g baseball traveling at 50 m/s
KE = (1/2) x 0.200 (kg) x [50 (m/s)]2
= (1/2) x (0.200) x (2500) JOULES
= 250 Joules
3-12
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Potential Energy due to Gravity

An object possesses Potential energy
due to its position in the gravitational field.

On Earth
we define gravitational potential energy in
terms of the distance of the object from
the ground.

Potential energy of an object due to gravity is the
gravitational potential energy = m x g x h
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Discussion Example
Your old car has stalled again.
Your friends push it to the top of a hill and it rolls
down the other side.
 Half way down the other side the engine starts. At
what point did your car have greatest potential
energy?

Gravitational PE  mass  height off the ground
So, the car has the greatest gravitational PE when
it is at the TOP OF THE HILL
3-14
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Discussion Example
Two cars of equal mass are on the same street. The
street is level. One car is driving with a velocity of
20 km/hr due north while the other car is parked.
Which car has more gravitational potential energy?
A. Both cars have the same gravitational potential energy
B. The parked car has more gravitational potential
energy
C. The moving car has more gravitational potential
energy
D. The heavier car has the most gravitational potential
They are the same!
Gravitational Potential energy
depends only on the distance from
the ground, not on the object’s
motion.
3-15
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Transformation of Energy
CONSERVATION OF
ENERGY
Energy may change form from one
type to another, BUT
THE TOTAL AMOUNT OF
ENERGY IN THE UNIVERSE
DOES NOT CHANGE
3-16
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SIMPLE PENDULUM
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The simple PENDULUM

During the motion of a swinging
pendulum, the energy is constantly
changing from
KE (kinetic energy) to PE (potential energy)



KE is MAX at lowest point
KE is MIN at the top of the path (v - 0)
PE is MAX at top of path
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EXAMPLE: Energy Transfer
A roller coaster (unmotorized) car
starts coasting at the top of one hill,
 rolls down into a valley and then
 comes to a stop at the top of a neighboring hill.

The top of the neighboring hill is at a lower
elevation than the starting point. The transfer of
energy in this case is potential to kinetic and
heat and back to potential and heat.
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Other FORMS of ENERGY





Thermal Energy
Elastic Potential ENERGY (Springs)
Chemical Potential Energy
Wave Energy
Mass Energy
3-20
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Thermal Energy




James P. JOULE discovered that
HEAT is a form of ENERGY
Two objects with the same temperature have the
same amount of thermal energy.
Common unit of HEAT ENERGY
calorie
OR
CALORIE
HEAT energy is EVER PRESENT!!

Whenever energy is being expended, heat is always
produced.
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The Heat Unit: Calorie or Calorie
 CALORIE
(with a big C):
amount of energy needed to raise the
temperature of one kilogram of water
by one degree Celsius.
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Other types of potential energy
 Elastic
potential energy
Springs
 rubber bands

 CHEMICAL
Potential Energy
Energy in a battery
Energy in any combustible
material

3-23
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But Wait there’s more…

Wave Energy


Energy can take the forms of waves, including
sound waves and light waves.
Sound Waves
Sound wave energy causes kinetic energy of
molecules in the air or some other medium.
 Sound waves are longitudinal waves.
 Sound waves are transmitted by molecular
motion.
 Sound waves are created by a vibrating object.

3-24
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What is HEAT??
Heat is the transfer of energy
NOTE:
we cannot say that matter “contains”
heat
 rather, matter contains energy

 The

amount of energy matter contains
can be change by transfer of heat
3-25
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


POWER = ENERGY PER TIME
ENERGY
OR
POWER 
TIME
Power Unit?



unit for energy = Joule
Unit for time = second
So, unit for power = Joule / second
Joule/second = WATT
3-26
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ENERGY and POWER


New Definition:
Power = Energy / Time
This means
Energy = Power x Time

so, another valid unit for ENERGY is
This is the
Watt x second
OR
UNIT OF ENERGY
 Watt x hour
on your
OR SOMETHING EVEN MORE
FAMILIAR
ELECTRICITY BILL
 Kilowatt x hour

(kWh)
3-27
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Watt about Horsepower???



Horsepower is a unit invented by
JAMES WATT
so that he could impress people about the
strength of his steam engine
one horsepower = 550 ft-lb per second
= 746 watts
3-28
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More about describing motion
MOMENTUM
A quantity that helps to
describe the effect of
an object’s motion
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Momentum


Momentum is a quantity which
tells us more about the motion
than speed or position
Usually, linear momentum is denoted by the
letter “p”
p=mxv
MOMENTUM
3-30
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Discussion/ Thought

A kid rolling down the street on a
skateboard...
or

a large truck at rest?
3-31
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Discussion Question
A student on a bike is traveling down a street with a velocity of 10
m/s toward the south. She passes a delivery van parked on the side
of the road. Compare the momentum of the biker with that of the
van.
A) The van has more momentum than the biker due to its much larger
mass.
B) The biker has more momentum than the van because the biker has
a non-zero velocity.v->
c) The van and the biker have about the same momentum
D) None of the above
v=0
3-32
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Collisions-a way to use momentum


Collisions provide a way to better understand
motion.
Two main classes of collisions


Elastic collision
Inelastic collision
Elastic collision: objects not deformed
bouncing balls, bumper cars...
Inelastic collision: objects become
permanently deformed
auto accidents, throwing pie in someone’s
face
3-33
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Conservation of Momentum

When outside forces are not present, the
combined total momentum of objects in
motion does not change



This is true for all motion
Momentum can be EXCHANGED during
the interaction of particles in motion, but
the system as a whole can not gain or lose
momentum
So, for any interaction:
MOMENTUM BEFORE = MOMENTUM AFTER
3-34
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HUMAN ENERGY USE



Humans convert CHEMICAL ENERGY,
stored in food substances, into KINETIC
ENERGY, present whenever our muscles
are at work.
If we take in more calories than we expend,
our body stores energy as fat to make up the
difference.
Estimate the amount of power given off
each day by a person consuming 1500
KCal per day on a diet which stores no
food as fat (all energy is expended)
3-35
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FIGURE IT OUT!!

POWER = ENERGY / TIME
Find the energy (Joules)
1500 CALORIES = 1500 kilocalories
# Joules(J) = 4.184 x Calories
Energy (J) = 4.184 x 1500 x 103 calories
Energy = 6,276,000 Joules

TIME = # SECONDS IN ONE DAY
24 hours x 3600 seconds/hour = 86,400 s

POWER = ENERGY / TIME
= 6,276,000 Joules / 86,400 s
= 73 WATTS
3-36
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Energy and Civilization



The source of all fuels used for energy on
our planet is the SUN
All food sources and fuel sources were at
one time given energy via the radiation that
falls on our planet from the sun
Today, the majority of our energy is
provided by fossil fuel sources

These sources are ‘finite’
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Fossil Fuels



PETROLEUM PRODUCTS
COAL
NATURAL GAS
All are carbon based
 All are rich in chemical potential energy.


The are classified as nonrenewable
resources.

The natural processes that form fossil fuels still
happen on Earth- but at a much slower rate
than previously
3-38
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Fossil Fuels USE


For industrial nations fossil fuels account
for Ninety percent of the total fuel
consumption
Where did the energy stored in fossil fuels
come from originally?
The energy stored in fossil fuels had to come
from somewhere….
 originally in The Sun (think about it...)

3-39
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SOLAR ENERGY


An example of a RENEWABLE energy
resource (sort of)
BIG problem: STORAGE


Battery technology still has a long way to go
With poor means of storage, Solar Energy is
available ONLY during times when the sun
is shining
3-40
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Energy: Summary

Work is defined as a measure of the change caused
by a force


Power is the rate of work done


The unit of power is WATT (W)
Energy is the property of something to do work



The unit of work is JOULE (N m)
The unit of energy is JOULE
Energy occurs in several forms: kinetic, potential,
chemical and rest energy
Linear momentum is a measure of the tendency of
a moving object to stay in motion: p = mv
3-41
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Energy: Summary(2)
 Angular momentum is measure of the
tendency of a rotating body to keep rotating
 All momentum is conserved
 The special theory of relativity defines the
rules for measurements when there is
relative motion between the observer and
the object to be measured
 General relativity relates gravity to the
structure of spacetime
3-42
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In-Lecture Quiz Chapter 3
If you touch the desktop, and then touch the
metal legs of the desk, one will “feel” colder.
Which object has a higher temperature?
A. Metal Leg
B. Desktop
C. They are the same
D. Not enough information to tell
3-43
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In-Lecture Quiz Chapter 3
Name a unit that could be used to measure
HEAT.
A. Newton
B. Joule
C. Calorie
D. B and C
E. A, B and C
3-44
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In-Lecture Quiz Chapter 3
In which of the following cases is
WORK DONE?
A. You try to pull your car out of a ditch, but it
does not move.
B. Your room-mate kicks his dirty clothes into
the corner of his room.
C. Your mother holds a sleeping baby.
D. You sit at your desk and study physics all
night.
3-45
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In-Lecture Quiz Chapter 3
 Suppose you climb a flight of
stairs to an elevation of 4 meters.
How much work did you do?
A. No Work (0 Joules)
B. It depends on how much you weigh
C. It depends on how wide the stairs are
D. It depends on whether you are wearing
your favorite shoes
3-46
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In-Lecture Quiz Chapter 3
A driver increases his velocity from 50 km/hr
to 150 km/hr. How much does he increase
the kinetic energy of the car with this
increase in speed?




A. Kinetic energy is 3 times greater
B. Kinetic energy is 6 times greater
C. Kinetic energy is 9 times greater
D. Kinetic energy is 12 times greater
3-47
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In-Lecture Quiz Chapter 3
You realize that you have left your shoes in
your car. You must walk outside with bare
feet. It is a HOT sunny afternoon. You have
the choice of
(A) walking on the grass or
(B) walking on the sidewalk.
Think about which one you choose?

3-48
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In-Lecture Quiz Chapter 3
In a simple pendulum system, we know that
energy is continuously converted from potential
to kinetic. At which point in the path of the
pendulum is the kinetic energy greatest?
A. Top of path (highest point)
B. Bottom of path (lowest point)
C. Somewhere in between A & B,
A
like point C
B
C
D. It depends on the mass
3-49
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In-Lecture Quiz Chapter 3
 An object which is AT REST can
have:
A.
B.
C.
D.
E.
A non-zero Kinetic energy
A non-zero Gravitational Potential Energy
A non-zero Momentum
non-zero Mass
non-zero Veloctiy
NOTE: There may be more than one choice…
3-50
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In-Lecture Quiz Chapter 3
When two objects collide in a perfectly elastic
collision, which of the following is true?

A. A. Only momentum is conserved
B. Only energy is conserved
C. Both Energy and Momentum are conserved
Energy and What are equivalent in
Eisenstein’s` famous equation?




A. The speed of light
B. Mass
C. Momentum
3-51