REVIEW
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3 REVIEW
Concept Summary
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think! Answers
3.3
Aristotle would probably say that the ball
stops because it seeks its natural state of
rest. Galileo would probably say that the
friction between the ball and the table
overcomes the ball’s natural tendency to
continue rolling—overcomes the ball’s
inertia—and brings it to a stop. Only you
can answer the last question!
Each planet would move in straight lines at
constant speed.
In a strict sense, no. We don’t know the
reason why objects persist in their motion
when nothing acts on them, but we know
that they do, and we call this property iner-
tia. We understand many things, and we
have labels for these things. There are also
many things we do not understand, and we
have labels for these things too. Education
consists not so much in acquiring new
labels, but in learning what is understood,
what is not, and why.
Two kilograms of anything has twice the
inertia and twice the mass of one kilogram
of anything else. In the same location,
where mass and weight are proportional,
two kilograms of anything will weigh twice
as much as one kilogram of anything.
Except for volume, the answer to all the
questions is yes. Bananas are much more
dense than bread, so two kilograms of
bananas must occupy less volume than one
kilogram of bread.
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Aristotle, the foremost Greek scientist,
studied motion and divided it into
two types: natural motion and violent
motion.
Copernicus reasoned that the simplest
way to interpret astronomical observa-
tions was to assume that Earth and the
other planets move around the sun.
Galileo argued that only when friction
is present—as it usually is—is a force
needed to keep an object moving.
Newton’s first law states that every object
continues in a state of rest, or of uniform
speed in a straight line, unless acted on by
a nonzero net force.
The more mass an object has, the greater
its inertia and the more force it takes to
change its state of motion.
The law of inertia states that objects in
motion remain in motion if no unbal-
anced forces act on them.
••••••
3.4.1
3.4.2
3.5
Key Terms
friction (p. 30)
inertia (p. 32)
Newton’s first
   law (p. 33)
law of inertia (p. 33)
kilograms (p. 36)
mass (p. 37)
weight (p. 37)
newton (p. 38)
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3 ASSESS
Check Concepts
Section 3.1
••••••
ASSESS
Check Concepts
Section 3.4
1. What were the two classifications of motion,
   according to Aristotle?
2. According to Aristotle, what kinds of
   motion required no forces?
9. Who was the first to consider the role of
   inertia, Galileo or Newton?
10. What is the tendency of an object at rest
    when no forces act on it?
11. What is the tendency of a moving object
    when no forces act on it?
Section 3.5
12. What relationship does mass have with
    inertia?
13. What does it mean to say mass and
    weight are proportional to each other?
14. When does an object with twice the
    mass of another weigh twice as much?
15. What do you feel when you shake some-
    thing to and fro? What do you feel when
    you hold it against the pull of gravity?
16. What is the standard (or SI) unit of
    measurement for mass?
17. What is the standard (or SI) unit of
    measurement for weight?
18. What is the weight of a 1-kg brick?
Section 3.6
Section 3.2
3. What simple way of interpreting astronomi-
   cal observations did Copernicus advocate?
Section 3.3
4. What were the consequences to Galileo for
   supporting the ideas of Copernicus?
5. Who relied on experiment, Aristotle or
   Galileo?
6. How did Galileo discredit Aristotle’s asser-
   tion that a force is needed to keep objects
   moving?
7. Galileo let a ball roll down one incline and
   then up another. Compared with its initial
   height, how high did the ball roll up the
   second incline?
8. What name is given to the property of an
   object to resist changes in motion?
19. How fast are you moving relative to
    Earth when you are standing still? How fast
    are you moving relative to the sun?
20. If you’re in a smooth-riding bus that is
    going at 40 km/h and you flip a coin verti-
    cally, how fast does the coin move horizon-
    tally while in midair?
CHAPTER 3
NEWTON’S FIRST LAW OF MOTION—INERTIA
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3 ASSESS
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Think and Rank ••••••Concept Summary ••••••
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)
21. Different materials rest on a table.
Plug and Chug
••••••
23. If a woman has a mass of 50 kg, calculate
    her weight in newtons.
24. Calculate in newtons the weight of a
    2000-kg elephant.
25. Calculate in newtons the weight of a 2.5-kg
    melon. What is its weight in pounds?
26. An apple weighs about 1 N. What is its mass
    in kilograms? What is its weight in pounds?
27. Susie Small finds she weighs 300 N.
    Calculate her mass.
Think and Explain
a. From greatest to least, rank them by how
   much they resist being set into motion.
b. From greatest to least, rank them by
   weight.
c. From greatest to least, rank them by the
   support (normal) force the table exerts on
   them.
22. The three pucks are sliding across ice at the
    noted speeds. Air resistance and ice friction
    are negligible.
Think and Explain
••••••
28. A bowling ball rolling along a lane gradually
    slows as it rolls. How would Aristotle inter-
    pret this observation? How would Galileo
    interpret it?
29. When a ball rolls down an inclined plane, it
    gains speed because of gravity. When roll-
    ing up it loses speed because of gravity. Why
    doesn’t gravity play a role when it rolls on a
    horizontal surface?
30. Jacob gives his skateboard a push and it rolls
    across the classroom floor. Emily says that
    after it leaves Jacob’s hand, his force remains
    with it, keeping it going. Sophia disagrees
    and says that Jacob’s push gives the skate-
    board speed, not force, and that when his
    hand is no longer in contact the force is no
    more. Who do you agree with? Discuss this
    with your classmates.
a. Rank them, from greatest to least, by the
   force needed to keep them going.
b. Rank them, from greatest to least, by the
   force needed to stop them in the same
   time interval.
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3 ASSESS
31. A space probe can be carried by a rocketConcept Summary ••••••
    into outer space. Your friend asks what kind
    of force keeps the probe moving after it is
    released from the rocket and on its own.
    What is your answer?
32. In an orbiting spacecraft, you are handed
    two identical closed boxes, one filled with
    sand and the other filled with feathers. How
    can you tell which is which without opening
    the boxes?
33. Many automobile passengers suffer neck
    injuries when struck by cars from behind.
    How does Newton’s law of inertia apply
    here? How do headrests help to guard
    against this type of injury?
34. Tim practices a demonstration before doing
    it for Sunday dinner. What concept is he
    illustrating, and why is he careful not to pull
    the tablecloth slightly upward?
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36. To pull a wagon across a lawn with constant
    velocity, you have to exert a steady force.
    Does this fact contradict Newton’s first law,
    which tells us that motion with constant
    velocity indicates no force?
37. When a junked car is crushed into a com-
    pact cube, does its mass change? Its volume?
    Its weight?
38. If an elephant were chasing you, its enor-
    mous mass would be very threatening. But
    if you zigzagged, the elephant’s mass would
    be to your advantage. Why?
39. When you compress a sponge, which
    quantity changes: mass, inertia, volume, or
    weight?
40. Which has more mass, a 2-kg fluffy pillow
    or a 3-kg small piece of iron? More volume?
    Why are your answers different?
41. Is it more accurate to say that a dieting per-
    son loses mass or loses weight?
42. A massive ball is suspended on a string and
    slowly pulled by another string attached to
    it from below, as shown.
    a. Is the string tension greater in
       the upper or the lower string?
       Which string is more likely to
       break? Which property, mass
       or weight, is more important
       here?
    b. If the string is instead snapped
       downward, which string is
       more likely to break? Is mass or
       weight more important this time?
35. Suppose you place a ball in the middle of a
    wagon that is at rest and then abruptly pull
    the wagon forward. Describe the motion
    of the ball relative to the ground and the
    wagon.
CHAPTER 3
NEWTON’S FIRST LAW OF MOTION—INERTIA
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3 ASSESS
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43. The head ofSummary ••••••wishConcept a hammer is loose and you
    to tighten it by banging it against the top of
    a workbench. Why is it better to hold the
    hammer with the handle down, as shown
    below, rather than with the head down?
    Explain in terms of inertia.
46. A stone is shown at rest on the ground.
    a. The vector shows the weight of the stone.
       Complete the vector diagram showing an-
       other vector that results in zero net force
       on the stone.
    b. What is the conventional name of the
       vector you have drawn?
44. As Earth rotates about its axis, it takes three
    hours for the United States to pass beneath a
    point above Earth that is stationary relative
    to the sun. What is wrong with the following
    scheme? To travel from Washington, D.C. to
    San Francisco using very little fuel, simply
    ascend in a helicopter high over Washing-
    ton, D.C., and wait three hours until San
    Francisco passes below.
45. In which position is the compression the
    least in the arms of the weightlifters shown?
    The most?
47. Here a stone is suspended at rest by a string.
    a. Draw force vectors for all the forces that
       act on the stone.
    b. Should your vectors have a zero resultant?
    c. Why, or why not?
48. Here a stone is being accelerated vertically
    upward.
    a. Draw force vectors to some suitable scale
       showing relative forces acting on the
       stone.
    b. Which is the longer vector, and why?
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3 ASSESS
49. Suppose the string in the figure inConcept Summary ••••••
Question 48 breaks and the stone slows
in its upward motion.
a. Draw a force vector diagram of the stone
   when it reaches the top of its path.
b. Is the net force on the stone zero at
   the top?
50. Here is a stone sliding down a friction-free
    incline.
    a. Identify the forces that act on it and draw
       appropriate force vectors.
    b. By the parallelogram rule, construct the
       resultant force on the stone (carefully
       showing it has a direction parallel to the
       incline—the same direction as the stone’s
       acceleration).
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54. If a woman weighed 500 N on Earth, what
    would she weigh on Jupiter, where the
    acceleration of gravity is 26 m/s2?
55. Gravitational force on the moon is only 1/6
    that on Earth. What is the weight of a 10-kg
    object on the moon and on Earth? What is
    its mass on the moon and on Earth?
Activity
Activity
Think and Solve
••••••
••••••
51. Calculate your own mass in kilograms and
    your weight in newtons.
52. A medium-size American automobile has
    a weight of about 3000 pounds. What is its
    mass in kilograms?
53. What is the weight in newtons of an auto-
    mobile with a mass of 1800 kg?
56. Grandparents are interested in the educa-
    tional progress of their grandchildren. Many
    have little science background. It’s possible
    that relatives far back in your family tree be-
    lieved that Earth was stationary in the center
    of the universe. Write a letter to Grandma or
    Grandpa. Tell what you’ve learned about a
    moving Earth and how a person who might
    be unsure of it could be convinced.
Teaching Resources
More Problem-Solving Practice
Appendix F
CHAPTER 3
NEWTON’S FIRST LAW OF MOTION—INERTIA
• Computer Test Bank
• Chapter and Unit Tests
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