Aristotle was the first to formulate a unified system to explain physical phenomena. His thinking dominated philosophy and science for an enormously long time, almost 1900 years. His ideas about physics remained unchallenged until the Middle Ages, when such scholars as Jean Buridan, William of Ockham (a.k.a. Occam), Nicolas Oresme, and Nicholas of Cusa started questioning and refining his assumptions.
Aristotle distinguished between “natural” and “unnatural” motion. Bodies moving naturally would move either up or down. Horizontal motion, as in a flying projectile, Aristotle termed “unnatural.” Natural motion of a body was a consequence of the amount of the prime substances –– earth, water, air, and fire –– present in that body. Each of these elements had a natural place to which it would move if unimpeded. A stone falls because it contains earth; steam rises because it contains fire. The natural motion of celestial objects he viewed as different from motion on Earth; it was circular motion, with no beginning or end.
Horizontal motion resulted from a pull or push acting on the object. To explain the motion of a projectile, which apparently is not pushed after release, Aristotle assumed that the air flowing around it and filling up the empty space created behind it would push the projectile continuously. Aristotle also noticed that bodies in a denser medium, such as water, fall at a slower speed than those in a less dense medium, such as air. From this he concluded that an object would fall at infinite speed in a vacuum; since he believed that infinite speed was not possible, an absolute vacuum could not exist.
In 1586 Simon Stevin showed that the speed of falling bodies is not proportional to their weight, as Aristotle had assumed, but that all bodies fall at an equal rate. Galileo did similar experiments a few years later, often timing balls rolling down on inclined planes to slow down their “falling motion.”
Galileo deduced the following laws from his experiments: 1) Any body moving on a horizontal plane will continue at the same speed unless a force opposes it. 2) In a vacuum, all bodies fall at the same rate, no matter what their weight or constitution. 3) A body falling freely or rolling down an inclined plane undergoes uniform acceleration.
Newton generalized Galileo’s ideas, showing that Aristotle’s “unnatural motion” is in matter of fact the norm, and rising and falling bodies act thus because of forces. Interestingly, since Aristotle’s ideas about motion correspond to what we observe in daily life, they remain as underlying assumptions for people unfamiliar with Newton’s laws. Since we do not observe motion in a vacuum or without friction, objects on Earth continue to appear to obey Aristotle’s laws of motion.