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A van of mass 1500 kg moving at 54 km/h is brought to rest in 10 s. If all the lost kinetic energy becomes heat in the brakes, find the average rate of thermal energy production (in cal/s).

1. 4000 cal/s
2. 3000 cal/s
3. 4200 cal/s
4. 3200 cal/s

Correct answer: 4000 cal/s

Solution

KE = (1/2)(1500)(15²) = 168750 J; rate = 16875 W = 16875/4.2 ~ 4017 cal/s, about 4000 cal/s.

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