A force is a push or a pull on an object. Forces have a strength and a direction, and all the forces on an object add up to a net force. When the forces balance, the net force is zero and the motion does not change: an object at rest stays at rest, and a moving object keeps its speed and direction. This is Newton's first law. When the forces are unbalanced, the motion changes in speed or direction. Force arrows show the strength and direction of each push or pull.