Re: positive and negative work

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

Gene Mosca  asked:

> ...  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.  Is there anything else the
> car is doing work on?

I usually say "work done by a force". Suppose you are on
a boat pushing against the bottom with a long stick. The boat
is moving. The force with which the stick acts on the ground
does very little work because the mass of the earth is so large.
But the reaction to this force (earth acting on the boat through
the stick and your body) does work, for example 1000 J per
push. Each force (action and reaction) does positive work
but only one work is numerically significant.

To illustrate negative work consider yourself on a deck
while the boat is moving toward you. With the same stick
you are pushing against the advancing boat. The work done
on the boat is negative at first. For a small boat the work
may first be negative and then positive. Suppose the net
work is zero (you stopped pushing at the right time). The
momentum of the boat also changed the sign but the final
speed is not as large as the initial speed.

Please do not ask me to redefine work as W' (using the First
Law formula: dU=W'+Q). Would W' calculated from this
formula, if calculation were possible in this example, yield the
same numerical answer as for W? Would it also be zero?

The wheels of a car are also "force transmitters." You are
correct, very little work is done to push our planet, most work
is done to push the car. Using its wheels the car pushes itself
forward or backward with respect to stationary ground.
Ludwik Kowalski