CHAPTER 4: Dynamics: Newton’s Laws of Motion
Why does a child in a wagon seem to fall backward when you give the wagon a sharp pull forward?
A box rests on the (frictionless) bed of a truck. The truck driver starts the truck and accelerates
forward. The box immediately starts to slide toward the rear of the truck bed. Discuss the motion of
the box, in terms of Newton’s laws, as seen (
) by Mary standing on the ground beside the truck,
) by Chris who is riding on the truck (Fig. 4–35).
If the acceleration of an object is zero, are no forces acting on it? Explain.
Only one force acts on an object. Can the object have zero acceleration? Can it have zero velocity?
When a golf ball is dropped to the pavement, it bounces back up. (
) Is a force needed to make it
bounce back up? (
) If so, what exerts the force?
If you walk along a log floating on a lake, why does the log move in the opposite direction?
Why might your foot hurt if you kick a heavy desk or a wall?
When you are running and want to stop quickly, you must decelerate quickly. (
) What is the origin
of the force that causes you to stop? (
) Estimate (using your own experience) the maximum rate of
deceleration of a person running at top speed to come to rest.
A stone hangs by a fine thread from the ceiling, and a section of the same thread dangles from the
bottom of the stone (Fig. 4–36). If a person gives a sharp pull on the dangling thread, where is the
thread likely to break: below the stone or above it? What if the person gives a slow and steady pull?
Explain your answers.
The force of gravity on a 2-kg rock is twice as great as that on a 1-kg rock. Why then doesn’t the
heavier rock fall faster?
Would a spring scale carried to the Moon give accurate results if the scale had been calibrated (
in pounds, or (
) in kilograms?
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