By 1589, Galileo (now an instructor at the University of Pisa) had deduced that if light and heavy stones are suspended from ropes of equal length and they appear to take an equal time to fall back and forth in an arc, then they should also take an equal time to fall straight down if they were dropped simultaneously from the same height. However, this deduction was contrary to Aristotle’s conjecture that heavy objects fall proportionally faster than lighter ones. So it was generally not believed.
To prove his point, Galileo is said to have assembled numerous students and scholars from the University of Pisa in the courtyard below the famous Leaning Tower. He chose two cannonballs, one small and one large, and climbed the 294 steps to the top of the Tower. Then, at the same instant, he dropped both unequal cannonballs over the edge. They fell straight down and, much to the amazement of his assembled colleagues, they appeared to strike the Earth at the same time (Figure 3A).
Ever since his Leaning Tower experiment, Galileo has generally been credited with the empirical discovery that “[T]he velocity of free-fall does not depend upon the mass of the falling body” even though others before him had conjectured a similar result. Galileo later repeated his Leaning Tower experiment in slow motion by “diluting gravity;” that is, by rolling balls of different weight down an inclined plane in apparently equal times (Figure 3B).
In 1638, Galileo wrote in his great book entitled, Dialogues Concerning Two New Sciences:
“Aristotle says that an iron ball of one hundred pounds falling from a height of one hundred cubits reaches the ground before a one-pound ball has fallen a single cubit. I say that they arrive at the same time.”
“Having observed this I came to the conclusion that in a medium totally devoid of resistance all bodies would fall with the same speed.”
Thus, according to Galileo, gravitational acceleration is an absolute concept: two objects of unequal mass, such as an apple and a cannonball, both appear to free-fall at absolutely the same rate toward the Earth.