Re: MOMENT OF INERTIA

[John Mallinckrodt <ajmallinckro@CSUPOMONA.EDU>]

On Sat, 19 Dec 1998, Jerome Epstein wrote:

> I am utterly unable to see how friction can do positive work on a body.
> Friction (kinetic friction) always acts opposite to velocity. Thus
> slowing it down. I missed the early part of this thread, but there has
> to be some confusion here.

As mentioned in an earlier post of mine which Epstein may have missed,
whether positive, negative, or zero work is done by any specific force in
any specific situation depends on the work definition being used and the
reference frame in which the calculation is performed.

Using the most commonly held notion of work and the most obvious reference
frame, I think most of us would agree that static friction does positive
work on--and thereby "increases the speed of"--a box resting on the flat
bed of an accelerating truck.

Less obviously perhaps, even kinetic friction might appear to do positive
work on a car with locked brakes when calculated by observers in another
car which was initially running side-by-side at the same velocity.

John
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