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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 momentums of the ball are same. Yet, it is not an example of conservation of momentum. Explain why?
 
Law of conservation of momentum is applicable to , i.e., systems in which  is applied.
 
In this case, change in velocity is taking place due to gravitational pull of earth only, thus, it is  of conservation of momentum.