Galileo's thought experiment
Teaching Guidance for 14-16
Galileo thought that a ball, rolling or sliding down a hill without friction, would run up to the same height on an opposite hill.
Suppose that the opposite hill was horizontal. Would the ball's motion continue forever along the tangent, or forever parallel to the Earth's surface?
Galileo's conclusion from this thought experiment was that no force is needed to keep an object moving with constant velocity.
Newton took this as his first law of motion.