REVIEW
Teaching Resources
• TeacherEXPRESS
• Virtual Physics Lab 12
• Conceptual Physics Alive!
   DVDs Energy
9REVIEW
Concept Summary
••••••
For: Self-Assessment
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think! Answers
9.1
9.2
W
Fd
60 N
4m
240 J
The forklift that delivers twice the power
will lift twice the load in the same time, or
the same load in half the time.
                100 N m100 J
             100 J
                       50 Wb. Power
               2s
c. It depends. Relative to its starting posi-
tion, the boulder’s PE is 100 J. Relative to
some other reference level, its PE would be
some other value.
Careful. Although you do 100 J of work on
the cart, this may not mean the cart gains
100 J of KE. How much KE the cart gains
depends on the net work done on it.
Nine times farther. The car has nine times
as much kinetic energy when it travels
three times as fast:
122 m(3v)
•
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Work is done when a force acts on an ob-
ject and the object moves in the direction
of the force.
Power equals work divided by the time.
The two forms of mechanical energy are
kinetic energy and potential energy.
Three examples of potential energy are
elastic potential energy, chemical energy,
and gravitational potential energy.
The kinetic energy of a moving object is
equal to the work done on it.
The work-energy theorem states that
whenever work is done, energy changes.
Energy cannot be created or destroyed.
A machine transfers energy from one
place to another or transforms it from
one form to another.
In a machine, some energy is trans-
formed into atomic kinetic energy.
There is more energy stored in the mol-
ecules in food than there is in the reac-
tion products after the food is metabo-
lized. The energy difference sustains life.
The sun supplies most of Earth’s energy.
••••••
9.4
a. W
Fd
9.6.1
9.6.2
9( 2 mv 2)
1
9.9
The ideal, or theoretical, mechanical
advantage is
 input distance
output distance
10 m
 1m
10
Key Terms
work (p. 145)
joule (p. 146)
power (p. 146)
watt (p. 146)
energy (p. 147)
164
mechanical
  energy (p. 147)
potential energy
  (p. 148)
kinetic energy
  (p. 150)
work-energy
   theorem (p. 151)
law of conservation
   of energy (p. 153)
machine (p. 155)
lever (p. 155)
fulcrum (p. 155)
mechanical
   advantage (p. 155)
pulley (p. 156)
efficiency (p. 158)
fuel cell (p. 162)
164

ASSESS
9 ASSESS
Check Concepts
Section 9.1
••••••
Check Concepts
Section 9.4
1. A force sets an object in motion. When the
   force is multiplied by the time of its appli-
   cation, we call the quantity impulse, which
   changes the momentum of that object. What
   do we call the quantity force distance, and
   what quantity can this change?
2. Work is required to lift a barbell. How many
   times more work is required to lift the bar-
   bell three times as high?
7. a. If you do 100 J of work to elevate a bucket
      of water, what is its gravitational potential
      energy relative to its starting position?
   b. What would the gravitational potential
      energy be if the bucket were raised twice
      as high?
Section 9.5
8. A boulder is raised above the
   ground so that its potential
   energy relative to the ground
   is 200 J. Then it is dropped.
   What is its kinetic energy just
   before it hits the ground?
Section 9.6
3. Which requires more work, lifting a 10-kg
   load a vertical distance of 2 m or lifting a
   5-kg load a vertical distance of 4 m?
4. How many joules of work are done on an
   object when a force of 10 N pushes it a dis-
   tance of 10 m?
Section 9.2
9. Suppose you know the amount of work the
   brakes of a car must do to stop a car at a
   given speed. How much work must they do
   to stop a car that is moving four times
   as fast? How will the stopping distances
   compare?
10. How does speed affect the friction between
    a road and a skidding tire?
Section 9.7
5. How much power is required to do 100 J of
   work on an object in a time of 0.5 s? How
   much power is required if the same work is
   done in 1 s?
Section 9.3
11. What will be the kinetic energy of an arrow
    having a potential energy of 50 J after it is
    shot from a bow?
12. What does it mean to say that in any system
    the total energy score stays the same?
13. In what sense is energy from coal actually
    solar energy?
6. What are the two main forms of mechanical
   energy?
CHAPTER 9CHAPTER 9
 ENERGYENERGY
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14
    ASSESS
9ASSESS
(continued)
14. How does Summary ••••••Concept the amount of work done on an
automobile by its engine relate to the energy
content of the gasoline?
25. Can we correctly say that a new source of
    energy is hydrogen? Why or why not?
Section 9.8
Think and Rank
••••••
15. In what two ways can a machine alter an
    input force?
16. In what way is a machine subject to the law
    of energy conservation? Is it possible for a
    machine to multiply energy or work input?
17. What does it mean to say that a machine has
    a certain mechanical advantage?
18. In which type of lever is the output force
    smaller than the input force?
Section 9.9
Rank each of the following sets of scenarios in
order of the quantity or property involved. List
them from left to right. If scenarios have equal
rankings, then separate them with an equal sign.
(e.g., A = B)
26. The mass and speed of three vehicles are
    shown below.
19. What is the efficiency of a machine that
    requires 100 J of energy to do 35 J of work?
20. Distinguish between theoretical mechanical
    advantage and actual mechanical advantage.
    How would these compare if a machine
    were 100% efficient?
21. What is the efficiency of her body when a
    cyclist expends 1000 W of power to deliver
    mechanical energy to the bicycle at the rate
    of 100 W?
Section 9.10
Think and Rank
a. Rank the vehicles by momentum from
   greatest to least.
b. Rank the vehicles by kinetic energy from
   greatest to least.
27. Consider these four situations.
(A) a 3-kg ball at rest atop a 5-m-tall hill
(B) a 4-kg ball at rest atop a 5-m-tall hill
(C) a 3-kg ball moving at 2 m/s atop a
     5-m-tall hill
(D) a 4-kg ball moving at 2 m/s at ground
     level
a. Rank from greatest to least the potential
   energy of each ball.
b. Rank from greatest to least the kinetic
   energy of each ball.
c. Rank from greatest to least the total
   energy of each ball.
22. In what sense are our bodies machines?
Section 9.11
23. What is the ultimate source of the energy
    derived from the burning of fossil fuels,
    from dams, and from windmills?
24. What is the ultimate source of geothermal
    energy?
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28. 
ASSESS
28. A ball is released at the left end of the metalConcept Summary ••••••
track shown below. Assume it has only
enough friction to roll, but not to lessen its
speed.
31. Carts moving along the lab floor run up
    short inclines. Friction effects are negligible.
a. Rank from greatest to least the ball’s
   momentum at each point.
b. Rank from greatest to least the ball’s
   kinetic energy at each point.
c. Rank from greatest to least the ball’s
   potential energy at each point.
29. The roller coaster ride starts with the car at
    rest at point A.
a. Rank the carts by kinetic energy before
   they meet the incline.
b. Rank the carts by how high they go up
   the incline.
c. Rank the carts by potential energy when
   they reach the highest point on the incline.
d. Why are your answers different for b
   and c?
32. Rank the scale readings from greatest to
    least. (Ignore friction.)
a. Rank from greatest to least the car’s
   speed at each point.
b. Rank from greatest to least the car’s
   kinetic energy at each point.
c. Rank from greatest to least the car’s
   potential energy at each point.
30. Rank the efficiency of these machines from
    highest to lowest.
    (A) energy in 100 J; energy out 60 J
    (B) energy in 100 J; energy out 50 J
    (C) energy in 200 J; energy out 80 J
    (D) energy in 200 J; energy out 120 J
CHAPTER 9CHAPTER 9
 ENERGYENERGY
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Plug and Chug
     ASSESS
9 ASSESS
(continued)
Plug and Chug ••••••Concept Summary ••••••
The key equations of the chapter are shown below
in bold type.
Work
  W
force
Fd
distance
Kinetic energy
KE
1
2 mass
122 mv
speed2
40. Calculate the number of joules of kinetic
    energy a 1-kg book has when tossed across
    the room at a speed of 2 m/s.
Work
KE
Think and Explain
33. Calculate the work done when a force of 1 N
    moves a book 2 m.
34 Calculate the work done when a 20-N force
   pushes a cart 3.5 m.
35. Calculate the work done in lifting a 500-N
    barbell 2.2 m above the floor. (What is the
    potential energy of the barbell when it is
    lifted to this height?)
Power
 work done
time interval
41. How much work is required to increase the
    kinetic energy of a car by 5000 J?
42. What change in kinetic energy does an air-
    plane experience on takeoff if it is moved a
    distance of 500 m by a sustained net force
    of 5000 N?
Think and Explain
••••••
36. Calculate the watts of power expended
    when a force of 1 N moves a book 2 m in a
    time interval of 1 s.
37. Calculate the power expended when a 20-N
    force pushes a cart 3.5 m in a time of 0.5 s.
Gravitational potential
                energy
43. Which requires more work: stretching a
    strong spring a certain distance or stretch-
    ing a weak spring the same distance? Defend
    your answer.
44. Two people who weigh the same amount
    climb a flight of stairs. The first person
    climbs the stairs in 30 s, while the second
    person climbs them in 40 s. Which person
    does more work? Which uses more power?
45. A physics teacher demonstrates energy
    conservation by releasing a heavy pendu-
    lum bob, as shown in the sketch,
    allowing it to swing to and fro.
    What would happen if, in his
    exuberance, he gave the bob a
    slight shove as it left his nose? Explain.
weight
mgh
height
PE
38. How many joules of potential energy does
    a 1-kg book gain when it is elevated 4 m?
    When it is elevated 8 m? (Let g = 10 N/kg.)
39. Calculate the increase in potential energy
    when a 20-kg block of ice is lifted a vertical
    distance of 2 m.
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Think and Solve
ASSESS
46. Consider Summary ••••••Conceptthe kinetic energy of a fly in the
cabin of a fast-moving train. Does it have
the same or different kinetic energies rela-
tive to the train? Relative to the ground
outside?
Think and Solve
••••••
50. A stuntman on a cliff has a PE of 10,000 J.
    Show that when his potential energy is
    2000 J, his kinetic energy is 8000 J.
51. Relative to the ground below, how many
    joules of PE does a 1000-N boulder have at
    the top of a 5-m ledge? If it falls, with how
    much KE will it strike the ground? What
    will be its speed on impact?
52. A hammer falls off a rooftop and strikes the
    ground with a certain KE. If it fell from a
    roof that was four times higher, how would
    its KE of impact compare? Its speed of im-
    pact? (Neglect air resistance.)
53. A car can go from 0 to 100 km/h in 10 s. If
    the engine delivered twice the power, how
    many seconds would it take?
47. When a driver applies brakes to keep a
    car going downhill at constant speed and
    constant kinetic energy, the potential energy
    of the car decreases. Where does this energy
    go? Where does most of it go in a hybrid
    vehicle?
48. What is the theoretical mechanical ad-
    vantage for each of the three lever systems
    shown?
49. Dry-rock geothermal power can be a major
    contributor to power with no pollution.
    The bottom of a hole drilled down into
    Earth’s interior is fractured, making a large-
    surfaced hot cavity. Water is introduced
    from the top by a second hole. Superheated
    water rising to the surface then drives a
    conventional turbine to produce electricity.
    What is the source of this energy?
54. If a car traveling at 60 km/h will skid 20 m
    when its brakes lock, how far will it skid if
    it is traveling at 120 km/h when its brakes
    lock? (This question is typical on some
    driver’s license exams.)
Activity
Activity
••••••
55. Place a small rubber ball on top of a basket-
    ball and drop them together. How high does
    the smaller ball bounce? (Perhaps this is best
    done in the gym, or outdoors.) Can you rec-
    oncile this result with energy conservation?
Teaching Resources
• Computer Test Bank
More Problem-Solving Practice
Appendix F
CHAPTER 9CHAPTER 9
 ENERGYENERGY
• Chapter and Unit Tests
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