Work Step by Step
Momentum is conserved when no external forces act on a system. If the ball alone is our system, then momentum is not conserved when it rises and slows down. We must consider the whole Earth and the thrown ball as our system. Then, the change of momentum of the ball (downward impulse) is canceled by an equal and opposite change of momentum of the Earth (an upward impulse that results from the ball pulling up on the Earth). The system momentum is conserved.