A person can survive a feet-first impact at a speed of about
12 m/s (27 mi/h) on concrete; 15 m/s (34 mi/h) on soil;
and 34 m/s (76 mi/h) on water. Why the different values
for different surfaces?

Why does a baseball catcher’s mitt have more padding
than a conventional glove?
A billiard ball will stop short when it collides head-on
with a ball at rest. The ball cannot stop short, however, if
the collision is not exactly head-on—that is, if the second
ball moves at an angle to the path of the first. Do you
know why? (Hint: Consider momentum before and after
the collision along the initial direction of the first ball and
also in a direction perpendicular to this initial direction.)
You have a friend who says that after a golf ball collides
with a bowling ball at rest, although the speed gained by
the bowling ball is very small, its momentum exceeds the
initial momentum of the golf ball. Your friend further
asserts this is related to the “negative” momentum of the
golf ball after collision. Another friend says this is hogwash—
that momentum conservation would be violated.
Which friend do you agree with?
A friend says that, since Earth’s gravity is so much
stronger than the Moon’s gravity, rocks on the Moon
could be dropped to Earth. What is wrong with this
assumption?

An apple falls because of the gravitational attraction to
Earth. How does the gravitational attraction of Earth to
the apple compare? (Does force change when you interchange
and in the equation for gravity.M2m1 instead of M1M2

The planet Jupiter is more than 300 times as massive as
Earth, so it might seem that a body on the surface of
Jupiter would weigh 300 times as much as on Earth. But
it so happens that a body would scarcely weigh 3 times as
much on the surface of Jupiter as it would on the surface
of Earth. Can you think of an explanation for why this is
so? (Hint: Let the terms in the equation for gravitational
force guide your thinking.)
Explain how “elastic potential energy” dramatically
changed the sport of pole vaulting when flexible fiberglass
poles replaced stiffer wooden poles.
When the girl in Figure 7.16 jacks up a car, how can
applying so little force produce sufficient force to raise
the car?
You’re on a rooftop and you throw one ball downward to
the ground below and another upward. The second ball,
after rising, falls and also strikes the ground below. If air
resistance can be neglected, and if your downward and
upward initial speeds are the same, how will the speeds of
the balls compare upon striking the ground? (Use the idea
of energy conservation to arrive at your answer.)

Consider the swinging-balls apparatus. If two balls are
lifted and released, momentum is conserved as two balls
pop out the other side with the same speed as the released
balls at impact. But momentum would also be conserved
if one ball popped out at twice the speed. Can you explain
why this never happens? (And can you explain why this
exercise is in Chapter 7 rather than in Chapter 6?)

An inefficient machine is said to “waste energy.” Does this
mean that energy is actually lost? Explain

When you are in the front passenger seat of a car turning
to the left, you may find yourself pressed against the
right-side door. Why do you press against the door? Why
does the door press on you? Does your explanation
involve a centrifugal force, or Newton’s laws?

A heavy crate accidentally falls from a high-flying airplane
just as it flies directly above a shiny red Porsche smartly
parked in a car lot. Relative to the Porsche, where will the
crate crash?
When the space shuttle coasts in a circular orbit at constant
speed about Earth, is it accelerating? If so, in what
direction? If not, why not?
As part of their training before going into orbit, astronauts
experience weightlessness when riding in an airplane
that is flown along the same parabolic trajectory as a
freely falling projectile. A classmate says that gravitational
forces on everything inside the plane during this maneuver
cancel to zero. Another classmate looks to you for
confirmation. What is your response?