Galileo's thought experiment considered rolling balls on inclined planes in the absence of friction or other resistant forces. The speed acquired by a body moving down a plane from a height was sufficient to enable it to reach the same height when climbing up another plane at a different inclination . As the angle decreases, the body should travel a greater distance. Galileo proposed that the body could travel indefinitely far as , contrary to the Aristotelian notion of the natural tendency of an object to remain at rest unless acted upon by an external force. Therefore, Galileo can be credited with introducing the concept of inertia, later exploited by Newton.