Eric Rogers Gets the Last Word: The Demon Theory of Friction

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 neighbour, 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 friction.

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.

238FAUSTUS

Yes, once they have collapsed the demons are crushed by the brick. It is their crackling bones that oppose the sliding.

YOU

I cannot feel them.

FAUSTUS

Rub your finger along the table.

YOU

Friction follows definite laws. For example, experiment shows that a brick sliding along the table is dragged by friction with a force independent of velocity.

FAUSTUS

Of course, same number of demons to crush, however fast you run over them.

YOU

If I slide a brick along the table again and again, the friction is the same each time. Demons would be crushed in 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 to push and be crushed. Demons act in just the right way to push and drag with the forces you find in your experiments.