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 momentum?
There is friction between the ground and the legs of the table, and this force prevents the table from sliding in the opposite direction of the motion of the ball. Due to this friction, the kinetic energy of the ball is converted into heat and so the ball slows down and eventually stops. So, friction between the ground and the table acts as an external force to change the momentum.