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Chapter 7: Q.26CQ (page 225)
If the efficiency of a coal-fired electrical generating plant is 35%, then what do we mean when we say that energy is a conserved quantity?
According to the law of conservation of energy, energy is a conserved quantity.
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Using values from Table 7.1, how many DNA molecules could be broken by the energy carried by a single electron in the beam of an old-fashioned TV tube? (These electrons were not dangerous in themselves, but they did create dangerous x rays. Later model tube TVs had shielding that absorbed x rays before they escaped and exposed viewers.)
(a) How much gravitational potential energy (relative to the ground on which it is built) is stored in the Great Pyramid of Cheops, given that its mass is aboutand its center of mass is 36.5m above the surrounding ground?
(b) How does this energy compare with the daily food intake of a person?
A large household air conditioner may consume 15.0 kW of power. What is the cost of operating this air conditioner 3.00 h per day for 30.0 d if the cost of electricity is $0.110 per kW⋅h?
A 100-g toy car is propelled by a compressed spring that starts it moving. The car follows the curved track in Figure 7.39. Show that the final speed of the toy car is 0.687 m/s if its initial speed is 2.00 m/s and it coasts up the frictionless slope, gaining 0.180 m in altitude.
Shoveling snow can be extremely taxing because the arms have such a low efficiency in this activity. Suppose a person shoveling a footpath metabolizes food at the rate of\(800{\rm{ W}}\).
(a) What is her useful power output?
(b) How long will it take her to lift\(3000{\rm{ kg}}\)of snow\(1.20{\rm{ m}}\)? (This could be the amount of heavy snow on\(20{\rm{ m}}\)of footpath.)
(c) How much waste heat transfer in kilojoules will she generate in the process?
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