Abstract: With the instance of lion running ball, the relationship among internal force, external force and motion is studied using the Newtonian laws. The friction which obstacles the relative motion between ball and land is the necessary condition for the motion of runner-ball system, and is equal to the ratio of system momentum in horizontal direction. However, the internal force between runner and ball in vertical direction is the real dynamic to drive the ball rotating and moving forward.

Key words: Newtonian laws, lion running ball, internal and external force, friction