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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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Using data from Table 7.5, calculate the daily energy needs of a person who sleeps for 7.00 h, walks for 2.00 h, attends classes for 4.00 h, cycles for 2.00 h, sits relaxed for 3.00 h, and studies for 6.00 h. (Studying consumes energy at the same rate as sitting in class.)
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?
a) Calculate the work done on a 1500-kg elevator car by its cable to lift it 40.0 m at constant speed, assuming friction averages 100 N. b) What is the work done on the lift by the gravitational force in this process? c) What is the total work done on the lift?
(a) Calculate the power per square meter reaching Earth’s upper atmosphere from the Sun. (Take the power output of the Sun to be\(4.00 \times {10^{26}}{\rm{ W}}\).)
(b) Part of this is absorbed and reflected by the atmosphere, so that a maximum of\(1.30{\rm{ kW}}/{{\rm{m}}^2}\)reaches Earth’s surface. Calculate the area in\({\rm{k}}{{\rm{m}}^2}\)of solar energy collectors needed to replace an electric power plant that generates\(750{\rm{ MW}}\)if the collectors convert an average of\(2.00\% \)of the maximum power into electricity. (This small conversion efficiency is due to the devices themselves, and the fact that the sun is directly overhead only briefly.) With the same assumptions, what area would be needed to meet the United States’ energy needs\(\left( {1.05 \times {{10}^{20}}{\rm{ J}}} \right)\)? Australia’s energy needs\(\left( {5.4 \times {{10}^{18}}{\rm{ J}}} \right)\)? China’s energy needs\(\left( {6.3 \times {{10}^{19}}{\rm{ J}}} \right)\)? (These energy consumption values are from 2006.)
Question: (a) Calculate the energy in kJ used by a 55.0-kg woman who does 50 deep knee bends in which her center of mass is lowered and raised 0.400 m. (She does work in both directions.) You may assume her efficiency is 20%.
(b) What is the average power consumption rate in watts if she does this in 3.00 min?
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