Question

A huge elevator platform is going up at a speed of $20\text{m/sec}$ and during its upward motion a small ball of mass $50\text{gm}$ falling in downward direction strikes the platform elastically at a speed $5\text{m/sec}$. Find the speed in m/sec with which the ball rebounds.

Answer 1

The situation is analysed in figure. We can consider mass of platform to be very large compared to that of ball, so we have

$\overrightarrow{v_1}=\overrightarrow{u_1}$ and $\overrightarrow{v_2}=2\overrightarrow{u_1}-u_2$
$v_2=45\text{m/s}$

Why this question? Tip: When a small body collides with a huge body, the change in velocity of the huge body can be neglected.