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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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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.
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?
Question: Mountain climbers carry bottled oxygen when at very high altitudes.
(a) Assuming that a mountain climber uses oxygen at twice the rate for climbing 116 stairs per minute (because of low air temperature and winds), calculate how many liters of oxygen a climber would need for 10.0 h of climbing. (These are liters at sea level.) Note that only 40% of the inhaled oxygen is utilized; the rest is exhaled.
(b) How much useful work does the climber do if he and his equipment have a mass of 90.0 kg and he gains 1000 m of altitude?
(c) What is his efficiency for the 10.0-h climb?
Very large forces are produced in joints when a person jumps from some height to the ground.
(a) Calculate the magnitude of the force produced if an 80.0-kg person jumps from a 0.600鈥搈-high ledge and lands stiffly, compressing joint material 1.50 cm as a result. (Be certain to include the weight of the person.)
(b) In practice the knees bend almost involuntarily to help extend the distance over which you stop. Calculate the magnitude of the force produced if the stopping distance is 0.300 m.
(c) Compare both forces with the weight of the person.
(a) What is the available energy content, in joules, of a battery that operates a 2.00-W electric clock for 18 months?
(b) How long can a battery that can supply 8.00脳104 J run a pocket calculator that consumes energy at the rate of 1.00脳10鈭�3 W?
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