Page 155 - Fisika Terapan for Engineers and Scientists

Problems 355

QUESTIONS FOR DISCUSSION

1. According to the data given in Example 1, what percentage of why is it more tiring to use the small ax? (Hint: Think about
the initial kinetic energy does the automobile have after the the kinetic energy of your arms.)
collision? 11. If you throw an (elastic) baseball at an approaching train, the
2. A (foolish) stuntman wants to jump out of an airplane at high ball will bounce back at you with an increased speed. Explain.
altitude without a parachute. He plans to jump while tightly 12. You are investigating the collision of two automobiles at an
encased in a strong safe which can withstand the impact on intersection. The automobiles remained joined together after
the ground. How would you convince the stuntman to aban- this collision, and their wheels made measurable skid marks
don this project? on the pavement before they came to rest. Assume that during
3. In the crash test shown in the photographs of Fig. 11.1, skidding all the wheels remained locked so that the decelera-
anthropomorphic dummies were riding in the automobile. tion was entirely due to sliding friction. You know the direc-
These dummies were (partially) restrained by seat belts, which tion of motion of the automobiles before the collision (drivers
limited their motion relative to the automobile. How would are likely to be honest about this), but you do not know the
the motion of the dummies have differed from that shown in speeds (drivers are likely to be dishonest about this). What do
these photographs if they had not been restrained by seat belts? you have to measure at the scene of the accident to calculate
4. For the sake of safety, would it be desirable to design automo- the speeds of both the automobiles before the collision?
biles so that their collisions are elastic or inelastic? 13. You are sitting in your car, stopped at an intersection. You
5. Two automobiles have collided at a north–south east–west inter- notice another car approaching from behind, and you notice
section.The skid marks their tires made after the collision point this car is not slowing down and is going to ram you. Because
roughly northwest. One driver claims he was traveling west; the the time to impact is short, you have only two choices: push
other driver claims he was traveling south. Who is lying? hard on your brake, or take your foot off the brake and give
your car freedom to roll. Which of these tactics will minimize
6. Statistics show that, on the average, the occupants of a heavy
damage to yourself? Which will minimize damage to your
(“full-size”) automobile are more likely to survive a crash than
car? Which will minimize damage to the other car?
those of a light (“compact”) automobile. Why would you
expect this to be true?
7. In Joseph Conrad’s tale “Gaspar Ruiz”, the hero ties a cannon to
his back and, hugging the ground on all fours, fires several shots
at the gate of a fort. How does the momentum absorbed by Ruiz
compare with that absorbed by the gate? How does the energy
absorbed by Ruiz compare with that absorbed by the gate?
8. Give an example of a collision between two bodies in which
all of the kinetic energy is lost to inelastic processes.
9. Explain the operation of the five-pendulum toy, called
Newton’s cradle, shown in Fig. 11.11.
10. In order to split a log with a small ax, you need a greater
impact speed than you would need with a large ax. Why? If
the energy required to split the log is the same in both cases, FIGURE 11.11 Newton’s cradle.

PROBLEMS

11.1 Impulsive Forces † 2. A large ship of 7.0 10 metric tons steaming at 20 km h
5
runs aground on a reef, which brings it to a halt in 5.0 s. What
1. A stuntman of mass 77 kg “belly-flops” on a shallow pool of
is the impulse delivered to the ship? What is the average force
water from a height of 11 m. When he hits the pool, he comes
on the ship? What is the average deceleration?
to rest in about 0.050 s. What is the impulse that the water
and the bottom of the pool deliver to his body during this
time interval? What is the time-average force? † For help, see Online Concept Tutorial 13 at www.wwnorton.com/physics