Re: work done by friction

[John Denker <jsd@MONMOUTH.COM>]

At 01:57 PM 10/25/99 +1000, Brian McInnes wrote:
> Ah, but in the model I use (as does Arons and as does Knight)
> frictional forces do not do work.  They are localized and do not move
> with the body and so the line integral of the frictional force is
> zero. The frictional forces are the means whereby kinetic energy is
> reduced and thermal energy increased.

Uhhhh, that seems overstated.

1) The no-work claim is only valid for a particular subtype of frictional
forces.

More generally, friction between A and B does work on A if B is moving.
For example, there have been millions of joules of work done by the
friction between the clutch face and pressure plate in my car.

2) The claim that friction produces only thermal energy is also not true in
general.  Consider the rolling friction between tires and the road.  As the
vehicle decelerates, this friction allows the vehicle's kinetic energy to
be converted to something else.  That "something" is typically heat in the
brakes, but more generally there could be a regenerative braking system
that transfers the energy to a flywheel or battery.

============

This is not just nit-picking word games.  There's some real physics here.

In particular, Mr. Joule himself realized that there are cases where adding
a little friction of the right type makes the system *less* dissipative
than otherwise.  Joule-Thompson expansion is less dissipative than a sudden
expansion.

______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com