Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Conservation of the Energy and Momentum


To demonstrate conservation of energy and linear momentum.


two balls: one light (ping pong) and other heavy (rubber).


Two balls are dropped from a certain height (usually shoulder height) and bounces back to the shorter height (generally above the waist). The lighter ball is placed directly on top of the heavier ball and both are dropped simultaneously. The heavier ball rebounds to much smaller height (usually below the waist) but surprisingly, the lighter ball bounces back much higher than the point of release, sometime hitting the ceiling.



When a ball of mass \(m\) is placed at a height \(h\), it has gravitational potential energy \(mgh\). When the ball collides with the floor, some of its energy is lost to the floor and some is lost due to slight deformity of the ball during collision.

When lighter ball is placed on top of the heavier ball and both are dropped, the results are much different. The heavier ball loses the same amount of energy during the collision, but upon rebounding, the heavier ball now collide with the lighter ball. The lighter ball now has its original energy plus energy transferred by the heavier ball. All of this energy will send the lighter ball to a greater height.