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surprising things I found in article on "Laws of Motion"

James McWilliams is trying hard--too hard-- to convince the reader that Aquinas believed in momentum/inertia before Galileo ever came up with it.

I ain't takin' his word for it, but I will look up the passages that he cites.

They include the Commentary on the Physics VIII, lec. 22, where St. Thomas says that "the last quantity of energy is in the stone itself and is spent on the resistance of the object struck.  Here we have our whole doctrine of inertia."

Huh?

He also points out (on p. 13) that Aristotle was aware that more effort is needed to get an immobile thing moving than to keep an already-moving thing at the same velocity.  "In fact, that this phenomena was discussed  appears from the Mechanica (intended to complete the Physics), where we read: 'why is it that a body which is already in motion is easier to move than one which is at rest?'" (see ch. 32 858a3).

Fr. McWilliams also accuses Galileo of misrepresenting Aristotle's statement about the velocity at which things fall.  According to McWilliams, the velocity is slower in denser media than in less dense. (see Physics IV, 8).

Finally, page 16 offers some apparent evidence that St. Thomas Aquinas thought something would keep on moving in the same way forever if no resistance slowed it down:  In Phys. IV, lec. 11 (toward the end).  Says Aquinas:  "IF the motion be in a vacuum, one cannot assign any reason why the body in motion should stop anywhere. . . . Therefore, [in a vacuum] either every body is at rest and nothing in motion, or if anything be in motion, it must continue in motion forever..."

Comments

Anonymous said…
Wow!

I agree this is a pretty extraordinary claim, but the Aquinas quotation is really, really awesome.
8:35 PM
Leo White said…
Book IV, lecture 11 (par. 526) does seem to talk about momentum/inertia as being the case in a void. But it also argues that there is no void. So his reasoning about a void is like Galileo's reasoning about a fricitionless ramp.
11:01 AM
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