The development of the theory of impetus by medieval physicists provided Galileo, in the late 1500 A. D. with a starting point for his own speculations. The theory of impetus was set forth by the Parisian philosopher Jean Buridan. It was based on observations about the motion of objects that had been made by the Greek writer John Philoponus in the sixth century A.D. John's intellectual sense was on the right track, but he never fully developed the reason why it should be as he thought. Aristotle's assertion that it was the air that provided the motive force responsible for the violent motion of a body disturbed Philoponus. He reasoned if that were true agitating the air behind a stone should move that object, which of course it does not. Looking for an alternative explanation, he concluded that there must be a motive force that resides within the stone itself. When a force was imparted to a stone by another object, the stone absorbed the force, moving a little faster, and simply continues on until air resistance or a collision with another body stopped it. However, he gave no reason why that should be the case, it seamed to him an elementary fact. Buridan gave this hypothetical force of Philoponus the name "impetus" which depends upon both the speed and the quantity of the mass in a body. He reasoned that even after the body received this impetus force the body would maintain that absorbed motion long after it lost contact with the mover. In fact, the body being moved, under ideal conditions, would continue to move in a straight line, at uniform speed forever, without interference toward infinity. This idea is recognized as Newton's first law of mechanics. But neither Buridan nor Newton gave a reason how or why the impetus force should exist in the first place. Buridan also used his theory of impetus to explain the behavior of falling bodies. Unlike many of Galileo's contemporaries, Buridan claimed that rocks accelerate as they fall, but like Aristotle, he wrongly assumed that a falling object acquired a velocity proportional to its weight. He supposed that the weight added a quantity of impetus to the falling rock, which increased its velocity, as it continued to fall. The object consequently moved faster and faster as it descended, until at last it struck the ground. However, it is an experimental fact, all objects fall at the same rate of speed, on earth it is 32 ft. per second^2, regardless of their weight.

However, this same rock away from the earth, ponderable mass, does not immediately fall or move in any one direction; it simply floats around in space. Nevertheless, it still exhibits that motive force function imparted to it when it was attached to the earth, which operates within the rock. We call it kinetic energy, a motional force the rock received while attached to the rotating earth and now retains in space. The rock also obtained more motional force from the Space-Shuttle rockets that put the ship into orbit around the earth. The Space-Shuttle astronauts demonstrated this fact on one of their missions. However, the rock simply floats around inside the Orbiter, as though it is unaffected by the earth's gravitational field. Nevertheless, it is effected by the gravitational field of the earth since it and the Shuttle are being pushed toward the earth causing the Shuttle and all its contents to fall in an orbit around the earth.