Question

Suppose a ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. Thereafter, the ball begins to fall downward and attains the speed v again before striking the ground. It implies that the magnitude of the initial and final momentums of the ball are the same. Yet, it is not an example of conservation of momentum. Explain why?

Answer 1

The Law of conservation of momentum is applicable to isolated systems, i.e., systems in which no external force is applied. In the mentioned case, change in velocity is taking place due to the gravitational pull of the earth only, thus, it is not an example of conservation of momentum.