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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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Energy that is not utilized for work or heat transfer is converted to the chemical energy of body fat containing about \(39{\rm{ kJ}}/{\rm{g}}\). How many grams of fat will you gain if you eat \(10,000{\rm{ kJ}}\) (about \(2500{\rm{ kcal}}\)) one day and do nothing but sit relaxed for \(16.0{\rm{ hr}}\) and sleep for the other \(8.00{\rm{ h}}\)? Use data from Table 7.5 for the energy consumption rates of these activities.
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?
Boxing gloves are padded to lessen the force of a blow.
(a) Calculate the force exerted by a boxing glove on an opponent鈥檚 face, if the glove and face compress 7.50 cm during a blow in which the 7.00-kg arm and glove are brought to rest from an initial speed of 10.0 m/s.
(b) Calculate the force exerted by an identical blow in the gory old days when no gloves were used and the knuckles and face would compress only 2.00 cm.
(c) Discuss the magnitude of the force with glove on. Does it seem high enough to cause damage even though it is lower than the force with no glove?
(a) How long would it take a \(1.50 \times {10^5} - {\rm{kg}}\) airplane with engines that produce \(100{\rm{ MW}}\) of power to reach a speed of \(250{\rm{ m}}/{\rm{s}}\) and an altitude of \(12.0{\rm{ km}}\) if air resistance were negligible?
(b) If it actually takes\(900{\rm{ s}}\), what is the power?
(c) Given this power, what is the average force of air resistance if the airplane takes \(1200{\rm{ s}}\)? (Hint: You must find the distance the plane travels in \(1200{\rm{ s}}\)assuming constant acceleration.)
(a) How high a hill can a car coast up (engine disengaged) if work done by friction is negligible and its initial speed is 110 km/h?
(b) If, in actuality, a 750-kg car with an initial speed of 110 km/h is observed to coast up a hill to a height 22.0 m above its starting point, how much thermal energy was generated by friction?
(c) What is the average force of friction if the hill has a slope 2.5掳 above the horizontal?
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