The gravitational force of the earth on the ball causes the ball to accelerate downwards and gain momentum. Now, Newton's 3rd law states that for every action there is an equal and opposite reaction. So there is also an equally strong force on the earth from the ball in the opposite direction. (We just don't appreciate that there is a movement of the earth caused by the gravitational pull of the ball on the earth because it is so massive in comparison to the ball, the acceleration of the earth towards the ball is tiny.)