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Unformatted text preview: 4...? mp. a. 5 (“way Conceptual Questions 53 The weight of an object close to Earth’s surface is given byW = mg, where g is
the acceleration due to gravity, about 10 (meters per second) per second down— ward. Weight, a force, should not be confused with mass. There is no such thing as an isolated force. All forces occur in pairs that are
equal in size and opposite in direction. As you stand on the floor, you exert a dorm
ward force on theﬂoor. According to Newton’s third law, the floor must exert an up—
ward force on you of the same size. These two forces do not cancel as they act on
different objects—one on you and one on the floor. The other force acting on you is the force of gravity. force: A push or a pull. Measured by the acceleration it produces
On a standard, isolated object, F 2 ma. Measured in newtons.
inertia: An object’s resistance to a change in its velocity. inversely proportional: A relationship in which two quantities
have a constant product. If one quantity increases by a certain
factor, the other decreases by the same factor. kilogram: The standard international (St) unit of mass. A kilo
gram of material weighs about 2.2 pounds on Earth. kinetic friction: The frictional force between two surfaces in
relative motion. This force does not depend very much on the
relative speed. law of inertia: Newton‘s ﬁrst law ol'inotion. mass: A measure of the quantity of matter in an object. The
mass determines an object’s inertia. Measured in kilograms. ' newton: The $1 unit of force. A net force ofl newton acceler ates a mass of] kiio’gram at a rate of 1 (meter per second) per
second. Newton's ﬁrst law of motion: The velocity of an object re—
mains constant unless an unbalanced force acts on the object. l. The room you are sitting in is currently moving at about
400 meters per second as a result of Earth spinning about
its axis. The walls of the room are attached to Earth but, if
you jump up into the air, you are not. Why does the west
wall not move across and strike you? 2. The room you are sitting in is currently moving at about
400 meters per second as a resuit of Earth spinning about
iLs axis. The walls of the room are attached to Earth but, if
your keys fall out of your pocket, they are not. Why do the
keys not appear to fly back toward the west wall? KEY TERMS CONCﬁPTUAL QUESTIONS Newton’s second law of motion: F,,,, = ma. The net force on
an object is equal to its mass times its acceleration. The net force
and the acceleration are vectors that always point in the same
direction. Newton’s third law of motion: If an object exerts a force on a
second object, the second object exerts an equal force back on
the ﬁrst object. proportional: A relationship in which two quantities have a con
stant ratio. If one quantity increases by acertain factor, the other
increases by the same factor. static friction: The frictional force between two surfaces at rest
relative to each other. This force is equal and opposite to the net
applied force if the force is not large enough to make the object
accelerate. terminal speed: The speed obtained in free fall when the up
ward force of air resistance is equal to the downward force of
gravity. weight: W = mg. The force of gravitational attraction of Earth
for an object. This deﬁnition is modiﬁed in Chapter 9 for accel‘
crating systems such as elevators and spacecraft. 3. Assume that you are pushing a car across a level parking
lot. When you stop pushing, the car comes to a stop. Does
this violate Newton’s ﬁrst law? Why? If you give this book a shove so that it moves across a table
' top, it slows and comes to a stop. How can you reconcile
this observation with Newton’s ﬁrst law? 5. How does the net force on the ﬁrst subway car compare
with that on the last car if the subway train has a constant
velocity? 54 Chapter 3 Explaining Motion 6. What can you say about the forces acting on a motorcycle that is traveling at a constant speed down a straight stretch
of highway? @‘ﬂiy does a tassel hanging from the rearview mirror appear to swing forward as you apply the brakes? 8. \Nhen dogs finish swimming. they often shake themselves to dry off. What is the physics behind this? 9. Modern automobiles are required to have headrests to protect your neck during collisions. For what type of colli
sion are these headrests most effective? IO. Assume that you‘re not wearing your seat belt and the car A. Copieyfvisuals Unlimited stops suddenly. Why would your head hit the wind
shield? Why does a blacksmith use an anvil when hammering a
horseshoe? i2. You ﬁnd that every time you pound a steak on your kitchen counter, the bottles fall out of the spice rack hanging on
your wall. To solve the problem, you buy a large oak cutting board, which you place on the counter under the steak.
Why does this help? I 3. ln everyday use, inertia means that something is hard to get moving. Is this the only meaning it has in physics? If not,
what other meaning does it have? 14. How would you determine if two objects have the same inertia? l5. When a number of different forces act on an object, is the net force necessarily in the same direction as one of the in
dividual forces? Why? I i6. You are analyzing a problem in which two forces act on an object. A 200newton force pulls to the right, and a
40newton force pulls to the left. Your classmate asserts that
the net force is 200 newtons because that is the dominant
force that is acting. What is wrong with this assertion? i7. Forces of 4 newtons and 6 newtons act on an object. What is the minimum value for the sum of these two forces? Two ropes are being used to pull a car out of a ditch. Each I9. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 3. 32. 33. 34. rope exerts a force of 700 newtons on the car. Is it pos
sible for the sum of these two forces to have a magnitude of
1000 newtons? Explain your reasoning. You apply a 75newton force to pull a child’s wagon across
the floor at constant speed. If you increase your pull to
80 newtons, will the wagon speed up to some new constant
speed, or will it continue to speed up indeﬁnitely? Explain
your reasoning. You push a crate full of books across the floor at a constant
speed of 0.5 meter per second. You then remove some of
the books and push exactly the same as you did before.
How does the crate’s motion differ, if at all, from before? If the net force on a boat is directed due east, what is the
direction of the acceleration of the boat? Would your an
swer change ifthc boat had a velocity due north but the net
force still acted to the east? If the net force on a hotair balloon is directed vertically
upward, what is the direction ofthe acceieration ofthe bal
loon? What would be the direction of the acceleration if
the balloon were being blown westward (with the net force
still acting vertically upward)? You are riding an elevator from your tenthfloor apartment
to the parking garage in the basement. As you approach
the garage, the elevator begins to slow. What is the direc
tion of the net force on you? ‘ You are riding an elevator from the parking garage in the
basement to the tenth floor of an apartment building. As
you approach your floor, the elevator begins to slow. What
is the direction of the net force on you? ' ifyou double the net horizontal force applied to a wagon,
what happens to the wagon’s acceleration? What happens to the acceleration of a rocket if the net
force on it is cut in half? A car can accelerate at 2 (meters per second) per second
when towing an iderltical car. What will its acceleration be
if the towrope breaks? What happens to the acceleration of an object when the
mass is tripled but the force remains the same? When an astronaut walks on the Moon, is either her mass
or her weight the same as on Earth? Explain. If you buy a bag of oranges labeled 5 kilograms, are you
buying the oranges by mass or by weight? What happens to the weight of an object if you triple its
mass? What happens to the weight of an object ifyou take it from
Earth to the Moon, where the acceleration due to gravity is
only onesixth as large? A skier is speeding up as she skis down a constant slope.
Draw a freebody diagram for the skier. A car on an uphill section of the highway is speeding up to
pass a truck. Draw a freebody diagram for the car. 35. Under what conditions will a golf ball and a PingPong ball
that are dropped simultaneously from the same height
reach the ground at the same time? 36. If a golf ball and a PingPong ball are simultaneously
dropped from the same height, they do not reach the ground at the same time. How would Aristotle explain this?
How would Galileo? 37. A marble dropped into a bottle of liquid soap quickly
reaches a terminal speed. Draw a freebody diagram for the marblejust before it hits the bottom of the bottle. What is
the acceleration of the marble at this time? 38. Draw a freebody diagram for a parachutist who has
reached terminal speed. What is his acceleration? 39. Alex is taking the highspeed elevator, which travels at a
constant speed of5 meters per second, to the 43rd floor of
a highrise building. Sam is stuck making the same trip in
the freight elevator, which only travels at a constant speed of 1.5 meters per second. Compare the net forces on Alex
and Sam. Pat and Chris are pushing identical crates across a rough floor. Pat's crate is moving at a constant 1 meter per second
while Chris's crate is moving at a constant 2 meters per sec
ond. Compare the net forces on the two crates. 4  . A friend falsely claims, “Newton’s ﬁrst law doesn’t work if
there is any friction.” How would you correct this claim? 42. One ofyour classmates falsely asserts, “Newton's second law
only works when there are no Frictional forces.“ How would
you correct this assertion? 43. You are applying a 400newton force to a freezer full of
., chocolate chip ice cream in an attempt to move it across
the basement. It will not budge. Is the frictional force ex—
erted by the lloor on the freezer greater than, equal to, or less than 400 newtons? 44. You ﬁnd that you must push with a force of 12 newtons to
keep a textbook sliding at constant speed across your desk.
With the book at rest, you apply a force of 13 newtons. Is it
possible that the book will stay at rest? Explain. 45. What force is required to pull a dog in a wagon along a level sidewalk, as in the ﬁgure, at a constant speed if the fric
tional force is 250 newtons? Conceptual Questions 55 46. A skier is skiing down a steep slope, traveling at constant
speed (that is, the skier has reached terminal velocity).
What are the size and direction of the net force on the
skier? 47. You are driving along the freeway at 75 mph when a bug
splats on your windshield. Compare the force of the
bug on the windshield to the force of the windshield on
the bug. 48. You leap from a bridge with a bungee cord tied around
your ankles. As you approach the river below, the bungee
cord begins to stretch and you begin to slow down. Which
is greater (if either), the force of the cord on Your ankles
to slow you or the force of your ankles on the cord to
stretch it? Explain. 49. Is the force that the Sun exerts on Earth bigger, smaller,
or the same size as the force that Earth exerts on the Sun?
Explain your reasoning. 50. Earth exerts a force of 500,000 newtons on an orbiting
communications satellite. Is the force that the satellite
exerts on Earth greater than, equal to, or less than
500,000 newtons? Explain. 5 I. What is the net force on an apple that weighs 4 newtons
when you hold it at rest? 52. Suppose you are holding an apple that weighs 4 newtons.
What is the net force on the apple just after you drop it? 53. Why do the cannons aboard pirates’ ships roll backward
when they are fired? 54. Why does a tennis racket slow down when it hits a ball? Jason SzeneSJCDRBIS SVGMA 55. Describe the force(s) that allow you to walk across a room. *56. We often say that the engine supplies the forces that pro—
pel a car. This is an oversimplification. What are the forces
that actually move the car? 57. A ball with a weight of 40 newtons is falling freely toward
the surface of the Moon. What force does this ball exert
on the Moon? 56 58. Chapler3 Explaining Motion The ﬁgure shows a ball hanging by a string from the ceil ' ing. Identify the action—reaction pairs in this drawing. Ar 7 gue that the sizes of all six forces are the same. Ceiling @ 60. 6. 62. i f the force exerted by a horse on a cart is equal and op—
posite to the force exerted by the cart on the horse, as re
quired by Newton’s third law, how does the horse manage
to move the cart? Gary reads about Newton’s third law while sitting in a
room with a single closed door. 'He reasons that if he ap
plies a force to the door there will be an equal and oppo—
site force that will cancel his pull and he will never be able
to escape. He bemoans: “Why did I ever take physics?"
What is wrong with Gary's reasoning? A softdrink can sits at rest on a table. Which of Newton's
laws explains why the upward force of the table acting on
the can is equai and oppOsite to Earth’s gravitational Force
pulling down on the can? A book sits at rest on a table. Which force does Newton’s
third law tell us is equal and opposite to the gravitational
force acting on the book? EXERCISES @Find the size of the net force produced by a 6N and an 8N force in each of the following arrangements:
a. The forces act in the same direction. b. The forces act in opposite directions. c. The forces act at right angles to each other. Find the size of the net force produced by a 5—N and a
i2vN force in each of the following arrangements: a. The forces act in the same direction. b. The forces act in oppOsite directions. c. The forces act at right angics to each other. . Two horizontal forces act on a wagon, 550 N forward and 300 N backward. What force is needed to produce a net
force of zero? Three forces act on an object. A 3N force acts due west and
a 4N force acts due south. If the net force on the object is
zero, what is the magnitude of the third force? What is the acceleration ofa SODkg buffalo if the net force
on the buffalo is 1800 N? . What is the acceleration ofa 2000kg car if the net force on the car is 4000 N? A 3006 bullet has a mass of 0.010 kg. If the average force
on the bullet is 9000 N, what is the bullet‘s average
acceleration? he net horizontal force on a 60,000kg railroad boxcar is
6 9. I0. 000 N. What is the acceleration of the boxcar? What net force is needed to accelerate a (SOkg ice skater at
2 m/s2? [fa sled with a mass of20 kg is to accelerate at 4 m/sz, what
net force is needed? If a 30kg instrument has a weight of 50 N on the Moon,
what is its acceieratiou when it is dropped? @A salesperson claims a 1200wkg car has an average accel ' I3." l4.
l5.
l6.
I7.
I8. *19. eration oi'4 m/sg from a standing start to 100 km/ h. What
average net force is required to do this? If a skydivcr has a net force of 300 N and an arceicration __
of 4 111/82, what is the mass of the skydiver? A child on roller skates undergoes an acceleration of
0.6 m/s2 due to a horizontal net force of 24 N. What is the
mass of the chiid? A 0.5kg ball has been thrown vertically upward. If we ig
nore the air resistance, what are the direction and size of
each force acting on the ball while it is traveling upward? A 1~kg bail is thrown straight up in the air. What is the net
force acting on the ball when it reaches its maximum
height? What is the bali's acceleration at this point? If a space explorer with a mass of 80 kg has a weight of
400 N on a newly discovered planet, what is the accelera
tion due to gravity on this planet? A fully equipped astronaut has a mass of 150 kg. If the as
tronaut has a weight of 555 N standing on the surface of
Mars, what is the acceleration due to gravity on Mars? A crate has a mass of 24 kg. What applied force is required
to produce an acceieration ofS m/s2 if the frictional force
is known to be 90 N? A rope is used to pull a lOkg block across the floor with
an acceleration of 3 m/s‘q. If the frictional force acting on
the block is 50 N, what is the tension in the rope? *Zl. *22. 23. If :1 pull of 210 N accelerates a 40kg child on ice skates at
a rate of5 m/sz. what is the frictional force acting on the
skates? If you stand on it sprint,r scale in your bathroom at home,
it reads 600 N, which means your mass is 60 kg. If instead
you stand on the scale while accelerating 211.2 ni/S2 upward
in an elevator, how many newtons would it read? Terry and Chris pull handoverhand on Opposite ends
of a rope while standing on a frictionless frozen pond. cilnfo Trac® College Edition Exercises 57 Terry's mass is 75 kg, and Chris's mass is 50 kg. If Terry’s acceleration is 2 m/s2, what is Chris's acceleration? A mother of mass 50 kg and her daughter of mass 25 kg are ice~skating. They face each other, and the mother
pushes on the daughter such that the daughter’s acceler
ation is 2 m/sﬂ. What is the force exerted by the mother on
the daughter? What is the force exerted by the daughter
on the mother? What is the mother’s acceleration? Fer additional readings, explore lnfoTrac College Edition, your online library. Go to http://mwv.infotraccollege.com/wadsworth and use the
passcode that came on the card with your book. Try these search terms:
Aristotle, body mass, Golden Knights, Isaac Newton and motion,]05eph Kittinger, mass and weight, weightlessness. ...
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 Fall '08
 GRIFFIN
 Physics

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