Physics 2

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?