Question

An elevator platform is going up at a speed 20 ms${}^{-1}$ and during its upward motion a small ball of 50 g mass falling in downward direction strikes the platform elastically at a speed 5 ms${}^{-1}$. Find the speed (in ms${}^{-1}$) with which the ball rebounds :

Answer 1

Given : Velocity of elevator $v_e=20$ $m{s}^{-1}$ (Upwards)

Velocity of ball before collision $u=5$ $m{s}^{-1}$ (Downwards)

Let the velocity of ball after the collision be $v$

Using Newton's Law of collision, $\frac{v-v_e}{-u-v_e}=-e$

$\frac{v-20}{-5-20}=-1$ $(\because e=1)$

$⟹v=45$ $m{s}^{-1}$