Wednesday, July 25, 2007

Aristotle, Gravity and Wile E. Coyote

Wile E. Coyote’s greatest enemy is gravity. He just can’t beat it. But somehow he does manage to resist it for longer than most of us. We all know the scenes where he inadvertently runs of a cliff edge but keeps going until he stops. Only then does he fall. (Many people mistakenly believe that he falls when he realises he is only supported by thin air, but this just shows the danger of equating correlation with cause. He realises he is standing on thin air when he comes to a halt, but he actually falls because he is stationary.)

There is a crazy logic to this. As a very small kid, I remember it seemed plausible that this would happen if you did run off a cliff. Luckily I never tried it. Even though I realise today that this isn’t true, the humour of the situation depends on the fact that it might have been. We can relate to a world where gravity doesn’t take hold until your previous motion has stopped.

Chuck Jones, director of the Roadrunner cartoons, probably never read Aristotle’s Physica but he did manage to model Aristotelian dynamics nevertheless. The Physica was the main source of natural science in Christian Europe's universities during the Middle Ages until the seventeenth century. According to Aristotle, there are two kinds of motion – forced and natural. Natural motion means falling under gravity. Forced motion is anything else, for instance when you throw a ball. Now Aristotle believed that the two kinds of motion could not exist in the same object at the same time. A ball cannot move under the influence of gravity and the motion you impart by throwing it simultaneously. Thus, according to Aristotle, when you throw a ball, it travels in the direction you propelled it, gradually slowing down due to air resistance. At some point the air resistance means that it will stop. Then, gravity will take hold and the ball will drop straight down to the ground. So, only when forward momentum is used up does gravity come into effect. This is exactly what happens to the unfortunate Wile E. Coyote.

This suggests to me that Aristotle’s idea isn’t as daft as we historians often assume. Firstly, it is actually true of a ball thrown straight up into the air. The ball really does stop, stationary for an instant at the apex of its trajectory, before gravity drags it back down to earth. Secondly, Aristotle idea does have a twisted logic that a small child watching a cartoon clearly comprehends. We should be slow to mock it.

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