Galileo's Law of Inertia

A body moving on a horizontal surface will keep on moving with constant speed as long as there are no forces acting on the body.

Galileo's view is opposite to Aristotle's. Galileo says a force is needed to stop a motion while Aristotle says a force is needed to maintain it.

Note that if the earth were a hard smooth sphere without friction and if there were no air resistance, a hard smooth ball would keep rolling on it forever with constant speed. Galileo considered this motion to be force free. He did not yet have the notion of gravitational force that was introduced later by Newton. Thus, for Galileo, inertial motion was circular motion around a spherical earth. Newton's concept of inertial motion was different. See Page 3.