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.

The situation is analysed in the figure. We can consider mass of platform to be very large compared to that of ball, so we haveVelocity of the ball relative to the platform before collision =(20+5) m/s = 25 m/sSince the collision is elastic, velocity of the ball relative to the platform after the collision = 25 m/sTherefore , velocity of the ball relative to the ground after the collision = (20+25 ) m/s = 45 m/s (upward)