As shown in the above figure, the horizontal force F is the friction force acting on the ball due to contact with the surface, during impact. This forceF is the cause of the velocity and spin (rotation) reversal. This friction force is generated by the gripping action of the ball with the surface. The direction of this friction force is opposite the direction of slip velocity between ball and surface, during impact. Slip velocity is the relative horizontal speed between the ball’s point of impact and the surface it is impacting.