ITo those considering why a block sliding on a table top comes to a
stop, I commend Eric Roger's "Demon Theory of Friction"
The Demon Theory of Friction
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,
Fauystus, 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.
Faustua 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.
Abridged from: Physics for the Inquiring mind By Eric Rogers pp343-345.