Chapter 3
Newton’s First Law of
Motion
Think Questions
Pages 42-44 (31-41, 43)
Pages 182,186 (8,42)
Pages 228-229 (20-31)
31. A space probe can be carried by a rocket 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?
 Nothing keeps the probe moving. With no
propelling force it continues moving in a
straight line—moving of its own inertia.
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?
 The one that is easier to shake back and forth
is the one with less mass (less inertia, less
resistance to changes in motion)
33.
Many auto passengers suffer neck injuries when
struck by cars from behind. How does NL of I apply? Why
headrests?
 The body is accelerated forward with the
seat, but the head remains (behind) at its
current velocity until an unbalanced force
pulls it forward (which is the neck, which can
cause whiplash if forceful enough).
 Headrests provide the unbalanced force
needed to accelerate the head with the body.
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?
 He is demonstrating the law of inertia (objects
at rest stay at rest unless receiving an
unbalanced force). Pulling upward would lift
the plates and cause them to be unbalanced.
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.
 Except for some change in motion due to
friction between the wagon and ball, the will
be no motion of the ball relative to the
ground; but relative to the wagon, the ball will
appear to move toward the back.
36. To pull a wagon across a lawn at a constant velocity, you
have to exert a steady force. Does this contradict Newton’s
first law?
 No. The key is net force. Your steady force
is balancing the rolling friction, which means
that the net force = 0, which means dynamic
equilibrium, which means constant velocity,
which means no acceleration. Removing
friction would allow the wagon to continue
moving at the same velocity without any more
pull force.
37.
When a junked car is crushed into a compact cube,
does its mass change? Its volume? Its weight?
 Mass remains unchanged.
 Volume is reduced.
 Weight remains unchanged (no change in
position within the gravitational field so no
change in gravitational force)
38. If an elephant was chasing you, its enormous mass would
be very threatening. But if you zig-zagged, the elephant’s
mass would be to your advantage. Why?
 Because the elephant’s mass is greater than
yours, so is its inertia. Therefore, the
elephant would have more difficulty switching
directions than you because its inertia is
greater.
39.
When you compress a sponge, which quantity
changes, mass, inertia, volume or weight?
 Only volume changes.
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?
 The iron has more mass (greater) and the
pillow has more volume (takes up more
space. The answers differ because mass
and volume are completely different
concepts.
41.
Is it more accurate to say that a dieting person loses
mass or loses weight?
 It is more accurate to say that a dieting
person is losing mass (the amount of matter
that composes the person). A reduction in
mass leads to a reduction in weight.
 It is possible to change weight (only) if the
person moves further from the Earth’s surface
or to another place (such as the moon).
43.
The head of a hammer is loose and you wish 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 rather than with the head
down?
 The handle stops when it hits the bench, but
the relatively massive head tends to keep
moving towards the handle and tightens.
8.
When you whirl a can at the end of a string in a
circular path, what is the direction of the force that acts on
the can?
 The force is directed inward towards the axis
of rotation. This inward-directed force is
called centripetal force.
42.
Can an object move along a curved path if no force
acts on it?
 No. An object moving along a curved path is
accelerating and requires an unbalanced
force to cause it to turn. This type of force is
referred to as centripetal force.
20.
Beginning from rest, a solid disk, a solid ball and a
hollow disk race down an incline. What happens?
 The solid ball moves fastest, followed by the
solid ring and the hollow disk. The solid ball
has the least amount of rotational inertia
because its mass is distributed closest to the
center or axis of rotation.
31.
Consider two rotating bicycle wheels, one filled with
air and the other with water. Which would be more difficult
to stop rotating? Explain.
 The bicycle wheel filled with water would be
the more difficult wheel to stop rotating
because it has the greater amount of
rotational inertia or resistance to change its
rotational motion.