47. The total work is the sum of the work done by gravity on the elevator, the work done by gravity on the counterweight, and the work done by the motor on the system: WT= We+ Wc+ Ws.Since the elevator moves at constant velocity, its kinetic energy does not change and according to the work-kinetic energy theorem the total work done is zero. This means We+Wc+ Ws= 0. The elevator moves upward through 54 m, so the work done by gravity on it is 25(1200 kg)(9.80 m/s )(54 m)6.35 10 J.eeWmgd=−
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