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Chapter 7: Q9 CQ (page 258)
What is conservative force?
The force for which the work done does not depend on the path taken is known as conservative force.
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A hydroelectric power facility (see Figure 7.38) converts the gravitational potential energy of water behind a dam to electric energy.
(a) What is the gravitational potential energy relative to the generators of a lake of volume , given that the lake has an average height of 40.0 m above the generators?
(b) Compare this with the energy stored in a 9-megaton fusion bomb.
Body fat is metabolized, supplying 9.30 kcal/g, when dietary intake is less than needed to fuel metabolism. The manufacturers of an exercise bicycle claim that you can lose 0.500 kg of fat per day by vigorously exercising for 2.00 h per day on their machine.
(a) How many kcal are supplied by the metabolization of 0.500 kg of fat?
(b) Calculate the kcal/ min that you would have to utilize to metabolize fat at the rate of 0.500 kg in 2.00 h.
(c) What is unreasonable about the results?
(d) Which premise is unreasonable, or which premises are inconsistent?
Discuss the relative effectiveness of dieting and exercise in losing weight, noting that most athletic activities consume food energy at a rate of 400 to 500 W, while a single cup of yogurt can contain 1360 kJ (325 kcal). Specifically, is it likely that exercise alone will be sufficient to lose weight? You may wish to consider that regular exercise may increase the metabolic rate, whereas protracted dieting may reduce it.
(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?
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