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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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The swimmer shown in Figure 7.44 exerts an average horizontal backward force of 80.0 N with his arm during each 1.80 m long stroke.
(a) What is his work output in each stroke?
(b) Calculate the power output of his arms if he does 120 strokes per minute.
Figure 7.44
How much work does a supermarket checkout attendant do on a can of soup he pushes \(0.600{\rm{ m}}\) horizontally with a force of \(5.00{\rm{ N}}\)? Express your answer in joules and kilocalories.
(a) How long will it take an 850-kg car with a useful power output of 40.0 hp (1 hp = 746 W) to reach a speed of 15.0 m/s, neglecting friction?
(b) How long will this acceleration take if the car also climbs a 3.00-m high hill in the process?
Describe the energy transfers and transformations for a javelin, starting from the point at which an athlete picks up the javelin and ending when the javelin is stuck into the ground after being thrown.
Explain why it is easier to climb a mountain on a zigzag path rather than one straight up the side. Is your increase in gravitational potential energy the same in both cases? Is your energy consumption the same in both?
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