Physics 101: How Things Work

These are the answers to the even-numbered exercises and problems, and all of the cases assigned in Chapter 3. The answers to the others appear in the back of the textbook.

Exercises

10. The scale will read less than your actual weight. The upward force that the table exerts on your hand contributes to the force supporting you against gravity and allows the scale to exert a smaller upward force on you. The scale only reports how hard it pushes perpendicular to its surface. When you weigh yourself on a ramp, your weight doesn't push directly toward the scale's surface and so the scale doesn't push back directly perpendicular to its surface. The perpendicular component of its force on you is less than you full weight.

14. To bounce higher than 1 m, the ball would need more total energy than it started with. Since there is nothing available to do work on the ball, it can't obtain this extra energy. In fact, it wastes some of the energy it has by producing thermal energy, which is why it can't even bounce back to 1 m high.

34. When the track makes a sharp left-hand turn, the car needs a strong leftward force to follow it. The car obtains that leftward force by riding up on the right-hand wall of the tube.

38. The police can use a long period of time and a modest forward force to give the battering ram forward momentum. The battering ram can then transfer this momentum to a door with a huge force exerted for a short period of time.

46. The pilot steers the "vomit comet" in the path of a falling object: a parabolic arc through the air. The net force on the plane is then exactly equal to its weight and it is truly falling. The objects inside it fall as well and everyone inside feels weightless.

54. The huge forward force on the bottom of the rear wheel produces a torque on the motorcycle and rider about their combined center of mass. If this torque is large enough, it overcomes the motorcycle's natural forward-back stability and the motorcycle rotates so that its front wheel leaves the pavement.

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Problems

2. The chair bends downward 6.7 cm.

6. The satellite must be moving horizontally at about 7,900 m/s. It will complete its trip in about 5100 seconds or 84 minutes.

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Cases

1a. It reports more than your weight, because it is providing the upward force to decelerate you.
1b. It still reports your entire weight. 1c. The scale reports more than your weight, because it is providing the force to accelerate you upward.
1d. The sum of the weights on the two scales equals your weight. If you are balanced between them, each will report half your weight.
1e. The top scale reports your weight. The bottom one reports your weight plus the weight of the upper scale (10 N).

3a. The force on you is upward.
3b. By Hooke's Law, it would distort 20 cm.
3c. The trampoline must exert more than your weight on you to accelerate you upward. This causes the trampoline to distort more than your weight would.
3d. You feel weightless, because your acceleration is the same as the acceleration due to gravity.
3e. You feel particularly heavy, because the trampoline is providing more force than your weight to make you jump up.

7a. The ball accelerates inward, toward you.
7b. The net force is inward, toward you.
7c. The ball is traveling counterclockwise, from right to left.
7d. The ball will keep traveling to the left if you let go then.
7e. The string pulls the ball inward so it accelerates toward you and goes in a circle instead of a straight line.