Question
Consider the situation of the previous problem. Take "the table plus the ball" as the system. Friction between the table and the ball is then an internal force. As the ball slows down, the momentum of the system decreases. Which external force is responsible for this change in the momentum?

Answer

In this case as friction is internal force, The force applied by table offered to the ball is reason of decrease in momentum of the ball. (As Newton's third law as ball moves forward the table exerts some force on the ball that is in opposite direction)

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