A marble ball is rolled on a smooth floor of a room to hit a wall. If the time taken by the ball in returning to the point of projection is twice the time taken in reaching the wall, the coefficient of restitution between the ball and the wall, is

ANS:B - 0.50

For a perfectly elastic body the coefficient of restitution is 1. If the marble is perfectly elastic the time taken a return to the point of projection is the same as the time taken to reach the wall. Here they give twice the time taken to reach the wall. So 1/2 = 0.5.