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Re: positive and negative work



Excuse me for posting how I would have addressed this conceptual
question with a student taking introductory physics, for expressing my
views about the worthiness of this conceptual question. and for not
addressing it using a context more suitable to John and other physicist
with similar views.... it wont happen again.......

-----Original Message-----
From: John S. Denker
Sent: Fri 11/9/2001 8:45 PM
To: PHYS-L@lists.nau.edu
Cc:
Subject: Re: positive and negative work



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.