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Chapter 7: Q15 CQ (page 259)
Do devices with efficiencies of less than one violate the law of conservation of energy? Explain
No, devices with an efficiency of less than one do not violate the law of energy conservation.
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A shopper pushes a grocery cart 20.0 m at constant speed on level ground, against a 35.0 N frictional force. He pushes in a direction 25.0潞 below the horizontal.
(a) What is the work done on the cart by friction?
(b) What is the work done on the cart by the gravitational force?
(c) What is the work done on the cart by the shopper?
(d) Find the force the shopper exerts, using energy considerations.
(e) What is the total work done on the cart?
(a) How long can you rapidly climb stairs\(\left( {116/{\rm{min}}} \right)\)on the\(93.0{\rm{ kcal}}\)of energy in a\(10.0 - {\rm{g}}\)pat of butter?
(b) How many flights is this if each flight has\(16\)stairs?
In Example 7.7, we calculated the final speed of a roller coaster that descended \(20{\rm{ m}}\) in height and had an initial speed of \(5{\rm{ m}}/{\rm{s}}\) downhill. Suppose the roller coaster had had an initial speed of \(5{\rm{ m}}/{\rm{s}}\) uphill instead, and it coasted uphill, stopped, and then rolled back down to a final point 20m below the start. We would find in that case that it had the same final speed. Explain in terms of conservation of energy.
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) 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?
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