Galileo’s Acceleration Experiment

Michael Fowler, UVa Physics Department

Summarizing Aristotle’s View

Aristotle held that there are two kinds of motion for inanimate matter, natural and unnatural. Unnatural (or “violent”) motion is when something is being pushed, and in this case the speed of motion is proportional to the force of the push. (This was probably deduced from watching oxcarts and boats.) Natural motion is when something is seeking its natural place in the universe, such as a stone falling, or fire rising. (We are only talking here about substances composed of earth, water, air and fire, the “natural circular motion” of the planets, composed of aither, is considered separately).

For the natural motion of heavy objects falling to earth, Aristotle asserted that the speed of fall was proportional to the weight, and inversely proportional to the density of the medium the body was falling through. He did also mention that there was some acceleration, as the body approached more closely its own element, its weight increased and it speeded up. However, these remarks in Aristotle are very brief and vague, and certainly not quantitative.

Actually, these views of Aristotle did not go unchallenged even in ancient Athens. Thirty years or so after Aristotle’s death, Strato pointed out that a stone dropped from a greater height had a greater impact on the ground, suggesting that the stone picked up more speed as it fell from the greater height.