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A block of ice of mass m falls into a lake. On impact, a mass m/5 of the ice melts. Both the ice and lake are at 0 degree C. If L is the latent heat of fusion, find the minimum height from which the ice fell.

1. L/(5g)
2. 5L/g
3. g L/(5m)
4. m L/(5g)

Correct answer: L/(5g)

Solution

Equating the lost potential energy to the heat needed to melt m/5 of ice gives h = L/(5g).

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