The Demon Theory of Friction


by Eric Rogers

How do you know that it is friction that brings a rolling ball to a stop and not demons? Suppose you answer this, while a neighbor, Faustus, argues for demons. The discussion might run thus:
You     I don't believe in demons.
Faustus I do.
You     Anyway, I don't see how demons can make frction.
Faustus They just stand in front of things and push to stop them from moving.
You     I can't see any demons even on the roughest table.
Faustus They are too small, also transparent.
You     But there is more friction on rough surfaces.
Faustus  More demons.
You     Oil helps.
Faustus Oil drowns demons.
You     If I polish the table, there is less friction and the ball rolls further.
Faustus You are wiping the demons off; there are fewer to push.
You     A heavier ball experiences more friction.
Faustus More demons push it; and it crushes their bones more.
You     If I put a rough brick on the table I can push against friction with more and more force, up to a limit, and the block stays still, with friction just balancing my push.
Faustus Of course, the demons push just hard enough to stop you moving the brick; but there is a limit to their strength beyond which they collapse.
You     But when I push hard enough and get the brick moving there is friction that drags the brick as it moves along.
Faustus Yes, once they have collapsed the demons are crushed by the brick. It is their crackling bones that oppose the sliding.
You     I can not feel them.
Faustus Rub your finger along the table.
You     Friction follows definite laws. For example, experiment shows that a brick sliding along a table is dragged by friction with a force independent of velocity.
Faustus Of course, the same number of demons to crush however fast you run over them.
You     If I slide a brick anong a table again and again, the friction is the same each time. Demons would be crushed on the first trip.
Faustus Yes, but they multiply incredibly fast.
You     There are other laws of friction: for example, the drag is proportional to the pressure holding the surfaces together.
Faustus The demons live in the pores of the surface: more pressure makes more of them rush out and be crushed. Demons act in just the right way to push and drag with the forces you find in your experiments.

By this time Faustus' game is clear. Whatever properties you ascribe to friction he will claim, in some form, for demons. At first his demons appear arbitrary and unreliable; but when you produce regular laws of friction he produces a regular sociology of demons. At that point there is a deadlock, with demons and friction serving as alternative names for sets of properties - and each debater is back to his first remark.

You realize that friction has only served you as a name: it has established no link with other properties of matter...  And now we can state the full case against demons: they are arbitrary, unreasonable, multitudinous, and over-dressed. We need a special demon with peculiar behaviour to explain each natural event in turn: therefore we need many kinds and vast numbers of them. And we have to clothe them with  special behaviours to fit all the facts.

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