Question

suppose A ball of mass M is thrown vertically upward with an initial speed V its speed decreases continuously till it becomes zero there after the Ball begins to fall down word 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 get it is not an example of conservation of momentum explain why

Answer 1

The conservation of linear momentum states that the momentum is conserved if no external forces acts on the system. The ball is under the influence of an external force called gravity. Hence the momentum is not conserved.

If the upward direction is taken as positive, initial momentum of the ball is

$MV$ and the final momentum is $-MV$. Though the magnitude is same, the direction is different. Hence we cannot apply conservation of momentum here.

Answer 2

Given data states that the ball of mass m is thrown upward with an initial speed V and after following the maximum height comes down and strikes with the same velocity which implied the momentum to be same for both instances.

As the initial momentum here is MV and final momentum is – MV due to the fact that the gravity acts against one side of motion which is an external force, do not makes the given condition to be an example of conservation of the momentum as the conservation of linear momentum states momentum to be conserved when no external force applied.