Part I.   Early Concepts of Gravity
B.   Aristotle’s Theory of Free-Fall

During the fourth century B.C., Aristotle (384 – 322 B.C.) reasoned that heavier objects fall toward the Earth faster than lighter ones. He conjectured that: “The downward movement of a mass of gold or lead, or of any other body endowed with weight, is quicker in proportion it its size.” [1]

In effect, Aristotle asserted and postulated that the magnitude of acceleration of a body’s fall toward the Earth is determined by, and is exactly proportional to, its mass or weight.[2] Thus, according to Aristotle, a 100 kg cannonball should fall 100 times faster toward Earth than a 1 kg cannonball. Aristotle did not reach this conclusion by experiments and observations, but rather only by reason and logic.[3] Nevertheless, his conjecture seemed quite logical and it was generally accepted for almost two millennia.[4]

There was isolated dissent to Aristotle’s postulate during this period. For example, Roman philosopher Titus Lucretius Cerus (96 – 55 B.C.) conjectured in his work, On the Nature of Things, that: “[T]hrough undisturbed vacuum all bodies must travel at equal speed though impelled by unequal weights.”[5]

In his 517 A.D. work, Physica, John Philoponus of Alexandria wrote:

“If one lets fall simultaneously from the same height two bodies differing greatly in weight, one will find that the ratio of their times of motion does not correspond to the ratio of their weights, but that the difference in time is a very small one.”[6]

Very importantly, notice that Philoponus compared the weight or mass of each falling body to the mass of the other falling body, rather than comparing the ratio of the weight or mass of each falling body to the enormous mass of the Earth. Down throughout history, everyone else, including Galileo, Newton, and Einstein, has also made this same critical mistake.

In 1554 A.D., Italian scientist Giovanni Benedetti demonstrated by experiment that bodies of different weight apparently free-fall with the same speed.[7]