Imagine going on a “free fall” ride with Galileo at an amusement park. Despite the fact that Galileo packed a few extra pounds (he enjoyed Italian food and wine), you and he would fall at the same rate. Galileo, of course, would not be surprised by this result. Very early in his career, he knew that bodies of the same material fall together, regardless of differences in weight.
Galileo made enormous contributions to understanding the nature of free fall, but the above generalization is not one of them. Several of Galileo’s predecessors (including Giambattista Benedetti, Giuseppe Moletti, and Simon Stevin) had already understood this point and published it. From high places, they dropped bodies of the same dense material but different size and weight, and saw them hit the ground at the same time. They found this to be a convincing demonstration.
Of course, people also knew that a cannonball falls faster than a straw hat. One must appreciate the effects of air resistance in order to reach Galileo’s broad principle that all bodies accelerate at the same rate in free fall (“free” means “in the absence of friction”). I emphasized this point in my book, but I did so without citing the specific observations that contributed to Galileo’s appreciation of friction (for example, the experiments in which he dropped bodies through fluids). Strangely, this has led some of my critics to charge me with “historical inaccuracy.” In fact, it is simply one example of how I condensed and essentialized the material. I wanted to keep the emphasis on what is revolutionary about Galileo’s physics—and that is his brilliant quantitative experiments that led to mathematical laws.
My presentation of Galileo’s investigation of free fall is consistent with that of Stillman Drake, who gives little emphasis to the experiments on bodies falling through liquids. I suspect that Drake made this choice for the same reason I did.
I think it is amazing and sad that you have to write this. One does not even have to know what it means to essentialize to see that this criticism is unjustified. You never said or implied that Galileo never made these experiments or that they were irrelevant. What you said was that without an understanding of friction, people would more likely draw the wrong conclusion when they see a cannonball fall faster than hay. In fact, you explicitly said Galileo, more than others, worked to figure out the role of friction and how to minimize it, which was necessary to reach his laws of motion. It is only by quoting you out of context and/or not paying attention to what you say, that one can fail to understand this point.
I am very much enjoying these substantive discussions, and I hope they continue! By the way, I was thrilled to see a lunar astronaut in the documentary *For All Mankind* drop a light and a heavy object on the moon, in honor of Galileo.