Page 80 - Schaum's Outline of Theory and Problems of Applied Physics

CHAP. 5] LAWS OF MOTION 65

5.22. The cable supporting a 2000-kg elevator can safely withstand a tension of 25 kN. What is the maximum upward
acceleration the elevator can have if the tension in the cable is not to exceed this figure?

5.23. An 800-N man stands on a scale in an elevator. What does the scale read when the elevator is (a) ascending at a
2
constant velocity of 3 m/s, (b) ascending at a constant acceleration of 0.8 m/s , (c) descending at a constant velocity
2
of 3 m/s, (d) descending at a constant acceleration of 0.8 m/s , and (e) in free fall because the cable has broken?
5.24. An 80-kg woman stands on a scale in an elevator. When it starts to move, the scale reads 700 N. (a) Is the elevator
moving upward or downward? (b) Is its velocity constant? If so, what is it? If not, what is the elevator’s acceleration?

5.25. Two boxes, one of mass 20 kg and the other of mass 30 kg, are sliding down a frictionless inclined plane that makes
◦
an angle of 25 with the horizontal. Find their respective accelerations.
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5.26. A force of 50 lb is used to pull a 50-lb crate up a frictionless plane that is inclined at 30 with the horizontal. Find the
acceleration of the crate.
5.27. An 800-kg car is towed up an 8 hill by a rope attached to a truck. The tension in the rope is 2000 N, and there is no
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frictional resistance to the car’s motion. How much time is needed to tow the car for 50 m starting from rest?

Answers to Multiple-Choice Questions

5.1. (d) 5.9. (b)

5.2. (a) 5.10. (b)

5.3. (b) 5.11. (a)
5.4. (c) 5.12. (c)

5.5. (c) 5.13. (c)

5.6. (d) 5.14. (d)
5.7. (c) 5.15. (d)

5.8. (d)

Answers to Supplementary Problems

5.1. The action and reaction forces always act on different bodies.

5.2. (a) The reaction force the ground exerts on its feet. (b) The force the horse exerts on it.

5.3. Yes, by an applied downward force in addition to the downward force of gravity.
5.4. (a) g (b) 0

5.5. (a) The tensions are the same.
(b) The front coupling is under the greater tension because it has a larger mass behind it to accelerate.