Re: positive and negative work

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

"Waggoner, Bill" wrote:

> I guess you are also going to say that the floor wouldn't crush
> an egg that was dropped from a height, as it does no work?

"John S. Denker" responded:

> What law of physics equates "participates in crushing" with
> "doing work". Is that the first law?  The second law?  The
> third law?  It's not any law I've ever heard of. Momentum is
> not the same as energy.  The floor contributes quite to the
> momentum budget without contributing any F dot ds.

If you prefer not to use the "impulse-momentum" approach,
Bill, then you can say this: The egg accelerates toward M with
g and M accelerates toward the egg with a=g*m/M. The contact
interaction develops. If the egg was practically rigid then it
would penetrate into the floor and kinetic energy would be
turned into heat. But the egg is not rigid and internal forces
develop. They crush it. To elaborate you could say that in the
frame of the CM the force with which your floor acts on the
egg, during the process, does positive work on the egg.

The same is true for a person jumping from a chair. The
floor does positive work on that person in the CM frame.
At the same time the leg muscles do positive work on the
floor (F*ds*cosA is positive when directions of F and ds
are identical).

What is applicable to jumping should also be applicable
to seating. So my answer to the original question is:
positive work is done by the "force=agent (muscle)."
Note that in the energy sequence outlined in:

>  http://alpha.montclair.edu/~kowalskiL/energy.html

work is not redefined when the First Law is introduced.
The ambiguities about work, mentioned by JohnM, do
not occur when objects are assumed to be rigid but they
must be addressed in more advanced courses. In a first
physics course we do not analyze what happens in a body
of a skier on ice, or what happens inside an egg. A skier
is modeled as a rigid object (a particle approximation is
actually sufficient in many problems).
Ludwik Kowalski