Prepare a written response to the following exercises from Conceptual Physics: Ch. 6: Exercises 8, 20, 58 & 60
8. 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?
20. Why does a baseball catcher’s mitt have more padding than a conventional
glove?
58. 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.)
60. 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? Ch. 7: Exercises 14, 36, 42, 56 & 57 14. Explain how “elastic potential energy” dramatically changed the sport of pole
vaulting when flexible fiberglass poles replaced stiffer wooden poles.
36. When the girl in Figure 7.16 jacks up a car, how can applying so little force
produce sufficient force to raise the car?
42. 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.) 56. 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?) 57. An inefficient machine is said to “waste energy.” Does this mean that energy is
actually lost? Explain. Ch. 8: Exercises 42 42. 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? Ch. 9: Exercises 8, 12 & 22 8. 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? 12. 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
inter- 22. 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.) Ch. 10: Exercises 5, 20 & 57 5. 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? 20. 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? 57. 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?
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