Download Section Review Answers Chapter 12 Section 1 1. Answers may vary

Section Review Answers
Chapter 12
Section 1
1. Answers may vary. Students should say in their
own words that an object at rest remains at rest
and an object in motion maintains its velocity
unless it experiences an unbalanced force. They
should then give an example of this law.
2. Answers may vary. Sample answer: a student
pushing a piano across a floor, an apple falling
from a tree
3. Answers may vary. Sample answer: When you
have on a seatbelt and the vehicle stops suddenly, the seatbelt applies a force that stops
you and keeps you from continuing forward,
as inertia keeps you in motion after the car has
stopped.
4. a. Newton’s second law
b. Newton’s first law
c. Newton’s first law
5. a. The car may be unable to turn. Newton’s first
law states that the object (car) will continue to
travel in a straight line unless an unbalanced
force acts on the object. Since the road is icy, the
friction between the tires and the ice may not be
large enough to turn the car.
b. The car will slide for the same reasons in (a).
Also, Newton’s second law states that acceleration is proportional to force. Since the friction
force is much smaller on an icy road, the negative
acceleration (“deceleration”) is much smaller.
6. a = F/m = 15 N/58 kg = 0.26 m/s2 forward
7. m = F/a = 34 N/4 m/s2 = 8.5 kg
Section 2
1. The moon’s mass is much smaller than Earth’s
mass. Since the force of gravitational attraction
between two objects depends on the mass of
both objects, the force of gravitational attraction
between you and the moon would therefore be
smaller than the force between you and Earth.
2. Sample answer: The law of universal gravitation says that the force of gravitational attraction
is proportional to the attracting masses and
inversely proportional to the square of the distance between the masses. If you make the mass
of one or both of the attracting masses larger,
then the force will be larger. (Weight on Jupiter is
larger than weight on Earth because the mass of
Jupiter is so much larger than the mass of Earth.)
The gravitational attraction between Earth and a
satellite gets smaller as the satellite moves farther
away from Earth.
3. Near Earth’s surface, the distance between the
object that is falling and the center of Earth does
not change very much. Also, the mass of Earth
is constant. Thus, the force given by the law of
universal gravitation depends only on the mass of
the object that is falling.
4. Orbital motion has two components—horizontal
and vertical. Horizontal motion propels the
object forward, and vertical free fall pulls the
object downward toward the center of gravity of
the larger mass.
5. Answers may vary, but they should state
something similar to the following: Newton’s
second law shows that acceleration depends
on both force and mass. A heavier object
experiences a greater gravitational force
than a lighter object (as you can see from the
law of universal gravitation). But a heavier
object is also harder to accelerate because it
has more mass. The extra mass of the heavy
object exactly compensates for the additional gravitational force. Since F = ma (or
a = F/m), if F is increased at the same rate as
m, then a remains the same.
6. The force of gravity is inversely proportional
to the square of distance. Since the distance
is made twice as close, the force of gravity
will be four times as great (the square of 2),
or 4 million N.
Section 3
1. Newton’s third law states that anytime one object
applies a force to a second object, the second
object applies a force on the first object that
is equal in size and opposite in direction. For
example, when you sit in a chair, you push down
on the chair (action force), and the chair pushes
up on you (reaction force).
2. Momentum is mass _ velocity.
3. The law of conservation of momentum says that
in any system or group of objects, the momentum will not change if a net outside force does
not act on the system or group of objects.
4. b
5. Answers may vary. They can include any actionreaction force pairs, such as fingers and keyboard
(on computer or piano, etc.), cheese and cheese
cutter, person and floor, etc.
6. The forces exerted by Earth and a skier are an
action-reaction force pair because Earth exerts
an equal and opposite force on the skier as the
skier had exerted on Earth. Since F = ma and F
is equal for both, the smaller mass of the skier
(when compared to the mass of Earth) will mean
the skier’s acceleration away from Earth will be
very much greater than Earth’s unnoticeable
acceleration away from the skier.
7. p = mv = (1.0 kg)(12 m/s) = 12 kg•m/s eastward
End of chapter Review Answers
Using Key Terms
2. Inertia is the behavior of matter
that defines mass in the laws of
motion. It is important because
inertia resists the effects of a net
force. The more mass or inertia
an object has, the smaller the
acceleration for a given net force.
3. The wrestler will weigh less on
the moon than he does on Earth,
because the force exerted on
him will be different at these
locations. Since he has the same
mass in both places, his weight
will depend on the acceleration
due to gravity at each location.
The moon causes a much smaller
acceleration due to gravity, so
the wrestler will weigh less there.
4. As a skydiver jumps from a plane,
gravity pulls her downward. Air
resistance pushes upward against
the downward motion. The
skydiver accelerates downward
until the force of air resistance
equals the downward force of
gravity. Then the skydiver stops
accelerating and falls downward
at a constant speed. This is called
terminal velocity.
5. The ball on the far right will
undergo a collision with the four
stationary balls. Conservation of
momentum requires that the ball
on the far left will rise to a height
equal to that at which the ball on
the far right started.
Understanding Key Ideas
6. c
7. d
8. a
9. b
10. b
Math Skills
21. F = ma = 12.5 kg _ 3.5 m/s2 = 44 N
22. w = mg/6 = (2.26 kg _ 9.8 m/s2)/6 = 3.7 N
23. p = mv
(65 kg)(3.0 m/s forward) = 195 kg•m/s forward
(20.0 kg)(22 m/s west) = 440 kg•m/s west
(16 kg)(0 m/s) = 0 kg•m/s