Newton's first law of motion is often stated as. An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an external force. Now for example two people are playing a game of catch. The ball moves in the same direction and then falls down. so this external force must be gravity. But is friction between the air and the ball also involved?. Is friction also the external force? So in such a case two external forces are involved. One gravity and the other friction?