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 beigns to fall downward and attains the speed `v` again before striking the ground. It implies that the magnitude of initial and final momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?

Explanation

Law of conservation of linear momentum applies only to isolated systems where there is no external force. In this case, change in velocity of ball (upwards and downwards) is due to attractional pull of earth. That is why this motion of ball is not an example of conservation of momentum.