Suppose a ball of mass $m$ is thrown vertically upward with an initial speed $v$. Its speed decreases continuously until 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 the initial and final momentum of the ball are the same. Yet,it is not an example of the conservation of momentum. Explain why?

(N/A) The Law of Conservation of Momentum is applicable only to an isolated system where no external force acts on the system.
In this case,the ball is not an isolated system because it is constantly acted upon by the external gravitational force of the Earth.
This external force causes a change in the velocity and,consequently,the momentum of the ball.
Therefore,since an external force is present,the momentum of the ball alone is not conserved.