Re: positive and negative work

["John S. Denker" <jsd@MONMOUTH.COM>]

On Fri, 09 Nov 2001 15:18:55 -0600, "Waggoner, Bill" wrote:

> Assuming no change in KE of the system, I would tell a student the
> normal force of the floor did negative work on the system.

That's highly unconventional and a disservice to the student, to say the
least.
 -- In physics, "work" usually means F dot ds.  If you are using the word in
some other way, if you want to communicate with physicists you will have to
(re)define your terms very very explicitly and carefully.
 -- One normally assumes that the analysis is being conducted in the lab
frame, unless otherwise stated.  If you are using a coordinate system where
the ds of the floor is nonzero, if you want to communicate with physics you
will have to explain your unconventional choices in much more detail.

> Does this necessarily mean the reaction forces of the feet pushing on the
> floor did postive or negative work on the floor. That depends on how you
> model that surface, and the initial conditions. Is the floor a "spring" as
> well?

The springiness of the floor is utterly negligible.  Remember the problem is
to analyze the energetics of sitting.

> Another fun variation would be what happens if the person just squats as
> if getting ready to jump. Would we model the "system" as being spring
> like?

No.  And even if we did, it would be irrelevant to the original question.

Then on Fri, 09 Nov 2001 15:59:36 -0600:

> 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?
Huh?

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.