Question

A stone of mass 5 kg falls from the top of a cliff having height 50 m and buries 1 m in sand find average resistive force offered by sand and time takes to penetrate.

Answer 1

M = 5 kg
h = 50 m

velocity = v just before hitting the ground
v² = u² + 2 g h = 0 + 2 * 9.8 * 50 = 980

When the stone is penetrating the sand:
v² = u² + 2 a s
0 = 980 + 2 * a * 1
a = - 490 m/s²

Average resistance Force offered by sand = m a = - 5 * 490
=- 2, 450 N

The negative sign indicates the direction of the force is opposite;