Q: Hailstones fall from a certain height. If they melt completely on reaching the ground, find the height from which they fall. (g = 10 ms-2, L = 80 calorie/g and J = 4.2 J/calorie.)

Sol: On reaching the ground, a hailstone of mass M losses potential energy which is converted into heat energy required to melt it.

In SI, potential energy lost = heat energy required for melting the hailstone

$\large M g h = M L $

$\large h = \frac{L}{g} $

$\large h = \frac{80 \times 4.2 \times 1000}{10} $

= 33.6 × 1000 m =33.6km.