1. 47. A space probe may be carried by a rocket into outer space. What keeps the probe moving after the rocket no longer pushes it?
2. 53. Consider a pair of forces, one having a magnitude of 20 N and the other a magnitude of 12 N. What is the strongest possible net force for these two forces? What is the weakest possible net force?
3. 56. A hockey puck at rest is in equilibrium. Is it in equilibrium if it slides across ice at constant velocity? Defend your answer.
4. 62. Your friend sits at rest on a chair. Can you say that no force acts on her? Or is it correct to say that no net force acts on her? Defend your answer.
5. 70. If you pull horizontally on a crate with a force of 200 N, it slides across the floor in dynamic equilibrium. How much friction is acting on the crate?
6. 72. Consider a crate at rest on a factory floor. As a pair of workmen begin lifting it, does the support force on the crate provided by the floor increase, decrease, or remain unchanged? What happens to the support force on the workmen's feet?
7. 79. Consider a ball at rest in the middle of a toy wagon. When the wagon is pulled forward, the ball rolls against the back of the wagon. Discuss and interpret this observation in terms of Newton's first law.
8. 81. When you push a cart, it moves. When you stop pushing, it comes to rest. Does this violate Newton's law of inertia? Discuss and defend your answer.
9. 84. Consider the normal force on a book at rest on a tabletop. If the table is tilted so that the surface forms an inclined plane, will the magnitude of the normal force change? If so, how?
10. 88. If you toss a coin straight upward while riding in a train, where does the coin land when the motion of the train is uniform along a straight-line track? When the train slows while the coin is in the air?
11. 53. Three balls of different masses are thrown straight upward with initial speeds as indicated.3.6-30 Full Alternative Text
    1. a. Rank the speeds of the balls 1 s after being thrown from fastest to slowest.
    2. b. Rank the accelerations of the balls 1 s after being thrown from greatest to least. (Or are the accelerations the same?)
12. 55. One airplane travels due north at 300 km/h, while another travels due south at 300 km/h. Are their speeds the same? Are their velocities the same? Explain.
13. 57. Is a fine for speeding based on one's average speed or instantaneous speed? Explain.
14. 60. Starting from rest, one car accelerates to a speed of 50 km/h, and another car accelerates to a speed of 60 km/h. Can you say which car underwent the greater acceleration? Why or why not?
15. 67. For a freely falling object dropped from rest, what is the acceleration at the end of the fifth second of fall? Tenth second of fall? Defend your answers.
16. 69. Compare the acceleration of a ball tossed straight upward with a ball simply dropped, neglecting air resistance.
17. 74. When you drive forward and apply your brakes, you have a positive velocity and negative acceleration. Cite a driving situation where you experience a negative velocity and a positive acceleration.
18. 80. For straight-line motion, explain to your classmates how a speedometer indicates whether or not acceleration is occurring.
19. 83. Give an example of something that undergoes acceleration while moving at constant speed. Can something undergo acceleration while traveling at constant velocity? Discuss with your classmates.
20. 84. On which of these hills does the ball roll down with increasing speed and decreasing acceleration along the path? (Use this example if you wish to explain the difference between speed and acceleration.)3.8-31 Full Alternative Text
21. 53. Boxes of various masses are on a friction-free level table. Rank each of the following from greatest to least.
    1. a. net forces on the boxes.
    2. b. accelerations of the boxes.4.6-23 Full Alternative Text
22. 55. A 100-kg tool box is in locations A, B, and C. Rank from greatest to least the
    1. a. masses of the 100-kg box.
    2. b. weights of the 100-kg box.4.6-25 Full Alternative Text
23. 57. A race car travels along a race track at a constant velocity of 200 km/h. What horizontal net force acts on the car?
24. 59. You exert a force on a ball when you toss it upward. How long does that force last after the ball leaves your hand?
25. 67. Which undergoes a change when an astronaut visits the International Space Station, mass, weight, or both? Defend your answer.
26. 73. You push a heavy crate on a factory floor with a horizontal force F and the crate doesn't move. How much, if any, friction acts on the crate?
27. 75. A 400-kg bear grasping a vertical tree slides down at constant velocity. What is the friction force that acts on the bear? Discuss how "constant velocity" is the key to your answer.
28. 85. A friend says that, as long as a car is at rest, no forces act on it. What do you say if you're in the mood to correct the statement of your friend?
29. 89. A parachutist, after opening her parachute, finds herself gently floating downward, no longer gaining speed. The harness pulls upward while gravity pulls downward. Which of these two forces is greater, or are they equal in magnitude?
30. 106. Is it possible to move in a curved path in the absence of a force? Discuss why.
31. 41. Identify the two pairs of action-reaction forces on an apple that you hold overhead. (Yes, two pairs of forces).
32. 42. When you drop an apple, identify all action-reaction pair of forces that act on it as it falls. Neglect air drag.
33. 44. A baseball player bats a ball. Identify the action-reaction pairs (a) while the ball is being hit, and (b) while the ball is in flight.
34. 53. Why does a rock climber pull downward on a supporting rope to move upward?
35. 68. A stone is shown at rest on the ground. The vector shows the weight of the stone.
    1. a. Complete the vector diagram showing another vector that results in zero net force on the stone.
    2. b. What is the conventional name of the vector you have drawn?
36. 82. What two forces act on a monkey hanging stationary by a vertical vine? Which force, if either, is greater?
37. 92. A rocket becomes progressively easier to accelerate as it travels through space. Discuss why is this so? (Hint: About 90% of the mass of a newly launched rocket is fuel.)
38. 94. Is it true that while you drop from a branch to the ground below, you pull upward on Earth? If so, then why is the acceleration of Earth not noticed?
39. 95. Two 100-N weights are attached to a spring scale as shown. Does the scale read 0, 100, or 200 N, or register some other reading? (Hint: Would it read any differently if one of the ropes were tied to the wall instead of to the hanging 100-N weight?)
40. 96. Why is it difficult to accelerate your car when it is on sand, gravel, or freshly fallen snow? Why does sand, gravel, or snow fly backward from the tires?