Chapter 7: Q.22CQ (page 259)
Do you do work on the outside world when you rub your hands together to warm them? What is the efficiency of this activity?
A very small amount of work is done in the outside world and the efficiency of this process the very high.
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What is conservative force?
(a) Use of hydrogen fusion to supply energy is a dream that may be realized in the next century. Fusion would be a relatively clean and almost limitless supply of energy, as can be seen from Table 7.1. To illustrate this, calculate how many years the present energy needs of the world could be supplied by one millionth of the oceans’ hydrogen fusion energy.
(b) How does this time compare with historically significant events, such as the duration of stable economic systems?
A 105-kg basketball player crouches down 0.400 m while waiting to jump. After exerting a force on the floor through this 0.400 m, his feet leave the floor and his center of gravity rises 0.950 m above its normal standing erect position.
(a) Using energy considerations, calculate his velocity when he leaves the floor.
(b) What average force did he exert on the floor? (Do not neglect the force to support his weight as well as that to accelerate him.)
(c) What was his power output during the acceleration phase?
Using energy considerations and assuming negligible air resistance, show that a rock thrown from a bridge 20.0 m above water with an initial speed of 15.0 m/s strikes the water with a speed of 24.8 m/s independent of the direction thrown.
The force exerted by a diving board is conservative, provided the internal friction is negligible. Assuming friction is negligible, describe changes in the potential energy of a diving board as a swimmer dives from it, starting just before the swimmer steps on the board until just after his feet leave it.
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