Question

suppose apple ball of mass m is thrown vertically upward with an initial speed v, its speed decreases continuously till it becomes zero. There, 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 balls are same. Yet, it is not an example of conservations of momentum,. Explain why ?'

Answer 1

According to law of conservation of momentum: When no external forces acts on a system of several interacting particles, the total linear momentum of system is conserved. The total momentum is the vector sum of the all the linear momenta of all the particles present in the system.
Now according to your question
When the ball thrown it get the velocity v then the momentum will be mv which is different at different instant of time.And at last body came to rest therefore it's momentum become zero.If we sum all the momentum we found it to be zero but according to law of conservation of momentum final momentum must be equal to initial momentum so that total momentum should be equal.It is not the matter that it gain the same speed before coming to rest . Therefore your example is not the example of linear momentum.While coming down it gain it's velocity due to gravity which may be maximum then velocity through which you have thrown.
Hope it will help you.
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