A 4.0 kg brick is dropped from a height of 2.0 m into a river that is 1.0 m deep. If 80% of the gravitational potential energy ends up as thermal energy, find this thermal energy in calories.

21 cal
21.5 cal
23 cal
24.5 cal

Correct answer: 23 cal

Solution

Over a total drop of 3.0 m, the potential energy is m g h about 117.6 J; 80% of it (about 94 J) becomes heat, which is about 23 cal.

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