Chapter 3 Newton’s First Law of Motion

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Chapter 3Newtons First Law of Motion

• Think Questions
• Pages 42-44 (31-41, 43)
• Pages 182,186 (8,42)
• Pages 228-229 (20-31)

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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 linemoving of its own inertia.

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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)

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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.

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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.

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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.

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36. To pull a wagon across a lawn at a constant
velocity, you have to exert a steady force. Does
this contradict Newtons 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.

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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)

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38. If an elephant was chasing you, its enormous
mass would be very threatening. But if you
zig-zagged, the elephants mass would be to your
advantage. Why?

• Because the elephants 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.

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39. When you compress a sponge, which quantity
changes, mass, inertia, volume or weight?

• Only volume changes.

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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.

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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 Earths surface or
  to another place (such as the moon).

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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.

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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.

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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.

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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.

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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.