In Thursday's class we did an activity on Aristotle's physics. My goal in this activity is for them to become familiar with Aristotle's ideas about matter and motion, and also to see how reasonable those ideas are if you are describing things qualitatively rather than quantitatively. I want them to understand this because Aristotle's physics and cosmology were the main reason people initially rejected the Copernican theory. I want my students to see that this rejection was not unreasonable, given what was known at the time. In fact, it may have been the most rational thing to do at the time. Hindsight, as they say, is 20/20.
I have read bits and pieces of Aristotle's Physics and On the Heavens, as well as small parts of his Meteorologica. I am very much not an expert on Aristotle. But what I have read seems very qualitative. I know that later commentators tried to interpret Aristotle's words in a more quantitative way, but from what I have read it is not clear if Aristotle was really making quantitative statements. This is particularly true when you consider that our "modern" mathematical definitions for quantities like velocity or acceleration simply did not exist at that time.
So Aristotle seems to be saying that objects fall with a constant (instantaneous) speed. But he almost certainly didn't have a good mathematical definition for instantaneous speed. So you can read some of his statements as just saying that when you drop a certain object from a certain height, it always takes the same amount of time to fall to the ground, and therefore its average speed during its fall is always the same. But I perhaps I just haven't read enough of Aristotle, or read him closely enough, to see that he is actually saying more than this.
Likewise with the idea that heavier bodies fall faster. Later "Aristotelians" read Aristotle as saying that the speed of a body's fall is proportional to the body's weight. But it is not clear to me that he was stating such a definite quantitative relationship. And there is no doubt that, in general, heavier objects fall faster than lighter ones. I have my students drop a metal ball and wad of paper towel. The two objects are about the same size, but the metal ball is heavier. When you drop them from the same height, the metal balls hits first. The greatest challenge in doing this activity is getting the students to look at what really happens. They have all heard in school that objects fall at the same rate, so they want to claim that the metal ball and the paper wad hit at the same time. But they don't. I usually have to make them repeat the experiment and watch closely to see what really happens.
Of course, this experiment completely refutes the more quantitative notion that the speed is proportional to weight. The metal ball weighs several times what the paper wad weighs, yet it hits the ground only a fraction of a second sooner. But qualitatively, it is a fact that the metal balls hits before the paper wad.
Lest anyone worry that I'm going to convince my students that Aristotle's physics is "correct", let me reassure you that we will get to Newtonian physics by the end of the course and show that Newton does a better job of explaining things in a quantitative way. Even at the qualitative level we discuss some problems with Aristotle's ideas. He does pretty clearly indicate that no object can move without a force causing it to move (excepting natural motions like falling, which he treats differently). The classic case against this is the flight of an arrow. The bowstring pushes the arrow initially, but once the arrow leaves the bow the bowstring can't continue pushing it. So what keeps the arrow moving?
Well, it turns out that Aristotle's theory of motion implies that no vacuum can exist in Nature. Because without the resistance of some medium, an object could move with infinite speed. Aristotle found this notion ridiculous (and modern physicists would agree), so he claimed that there could be no vacuum. Well, when the arrow moves from one place to another it no longer occupies the space it used to be in. To avoid a vacuum, air (or whatever the medium is) must rush in to fill that vacated space. Aristotle felt that the air rushing in might push on the back of the arrow and keep it moving along. Not a very satisfying answer, perhaps, but it does illustrate the coherence of Aristotle's ideas. He is able to fix an apparent flaw in his theory of motion (the moving arrow) by appealing to a conclusion (Nature abhors a vacuum) that derives from that very same theory of motion.
Aristotle's cosmology has a similar coherence. His notion of the five elements (four terrestrial, one celestial) ties in very closely with his ideas about the spatial structure of the universe. And both tie in to his ideas about natural motions. With a handful of ideas he built up a pretty comprehensive picture of the world. These ideas were each subject to criticisms, but the problem was that if you tossed out one of the ideas the whole thing would start to fall apart. People don't want to throw the baby out with the bathwater, and because Aristotle's ideas were so reasonable it took a long time before people started to realize that there was no baby in this bathwater. Serious criticism of Aristotle, and new proposals about motion, began to surface during the Middle Ages and by the 14th century there was a robust new theory of motion (the impetus theory). But it was not until Galileo that someone came along with was willing to not only discard Aristotle's theory of motion (and replace it with a better, more quantitative theory) but also Aristotle's cosmology.
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