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Re: positive and negative work



On Sun, 11 Nov 2001 09:45:30 -0500 Gene Mosca wrote:
The car exerts a force on the road surface, which is
stationary, and the tire is rolling so the frictional force is static, not
kinetic. It seem to me that the car is not doing any work on the road
surface.

Right.

Is there anything else the car is doing work on?

Yes.

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

Tangentially-related point: In general one should be very careful when talking about
"work" in real-world situations, for two reasons:

1) Force is defined as F dot ds, which is only meaningful if the
object is pointlike -- either a point particle or something with
no internal degrees of freedom. For complex objects, in order
to make progress you need to decompose it into pointlike elements
and compute the work on each element separately.

2) Work is not a conserved quantity. You cannot in general speak
of the work transferred from A to B. You cannot assume that
positive work on A is associated with negative work on B, even
if they are the only participants.

Energy is conserved. Work has dimensions of energy, but that
doesn't mean that work is conserved. See
http://www.monmouth.com/~jsd/physics/thermo-laws.htm#sec-eschew-w+q