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 initial and final momentum of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?
The law of conservation of momentum is applicable to isolated system. An isolated system is a system that is free from the influence of a net external force. In the given example, the change in velocity is due to the gravitational force of earth. This is an external force. Therefore, it is not an example of conservation of momentum.