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330 CHAPTER 10 Systems of Particles

7. A fountain shoots a stream of water up into the air (Fig. 10.25). 13. An elephant jumps off a cliff. Does the Earth move upward
Roughly, where is the center of mass of the water that is in the while the elephant falls?
air at one instant? Is the center of mass higher or lower than 14. A juggler stands on a balance, juggling five balls (Fig. 10.26).
the middle height? On the average, will the balance register the weight of the jug-
gler plus the weight of the five balls? More than that? Less?

FIGURE 10.25 Stream of water from a fountain.

8. Consider the moving wrench shown in Fig. 10.6. If the center
of mass on this wrench had not been marked, how could you
have found it by inspection of this photograph?
9. Is it possible to propel a sailboat by mounting a fan on the FIGURE 10.26 Juggler on a balance.
deck and blowing air on the sail? Is it better to mount the fan
on the stern and blow air toward the rear?
10. Cyrano de Bergerac’s sixth method for propelling himself to 15. Suppose you fill a rubber balloon with air and then release it
the Moon was as follows: “Seated on an iron plate, to hurl a so that the air spurts out of the nozzle. The balloon will fly
magnet in the air—the iron follows—I catch the magnet— across the room. Explain.
throw again—and so proceed indefinitely.” What is wrong 16. The combustion chamber of a rocket engine is closed at the
with this method (other than the magnet’s insufficient pull)? front and at the sides, but it is open at the rear (Fig. 10.27).
11. Within the Mexican jumping bean, a small insect larva jumps Explain how the pressure of the gas on the walls of this com-
up and down. How does this lift the bean off the table? bustion chamber gives a net forward force that propels the
rocket.
12. Answer the following question, sent by a reader to the New
York Times:

A state trooper pulls a truck driver into the weigh station to see if
he’s overloaded. As the vehicle rolls onto the scales, the driver jumps
out and starts beating on the truck box with a club. A bystander combustion throat nozzle
chamber
asks what he’s doing.The trucker says:“I’ve got five tons of canaries
in here. I know I’m overloaded. But if I can keep them flying I’ll be
OK.” If the canaries are flying in that enclosed box, will the truck
really weigh any less than if they’re on the perch? FIGURE 10.27 Combustion chamber of a rocket engine.

PROBLEMS

10.1 Momentum
3. Using the entries listed in Tables 1.7 and 2.1, find the magni-
1. What is the momentum of a rifle bullet of mass 15 g and tude of the momentum for each of the following: Earth moving
speed 600 m/s? An arrow of mass 40 g and speed 80 m/s? around the Sun, jet airliner at maximum airspeed, automobile
2. What is the momentum of an automobile of mass 900 kg at 55 mi/h, man walking, electron moving around a nucleus.
moving at 65 km/h? If a truck of mass 7200 kg is to have the 4. The push that a bullet exerts during impact on a target
same momentum as the automobile, what must be its speed? depends on the momentum of the bullet. A Remington .244