Re: positive and negative work

["Waggoner, Bill" <BWaggoner@METROPO.MCCNEB.EDU>]

From an energy point of view the energy of the "system" is decreasing.
By system I mean the entire person.

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 is if
we don't view the "system" as a spring that's being compressed.)  

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?

This is an excellent discussion question. I personally like to use
energy bar charts as part of a multiple representation problem solving
scheme I teach the students.

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? 

-----Original Message-----
From: Gene Mosca [mailto:mosca@USNA.EDU]
Sent: Friday, November 09, 2001 1:31 PM
To: PHYS-L@lists.nau.edu
Subject: Re: positive and negative work


I agree with John Denker that the leg muscles do negative work.  If a
muscle
contracts under tension it does positive work, if it lengthens it does
negative work.  The muscles in the front of the thigh are under tension
and
lengthen as you sit.


On 11/8/01 8:42 AM, "Justin Parke" <FIZIX29@AOL.COM> wrote:

> A conceptual question from the fifth edition of Serway and Faughn
College
> physics  (ch. 5 number 2):
>
> "Discuss whether any work is being done by each of the following
agents and,
> if so, whether the work is positive or negative:
>
> d)  the leg muscles of a person in the act of sitting down."
>
> I say the muscles are doing negative work since they exert an upward
force (to
> prevent the person from simply falling into the chair) while the
motion of the
> person is down.  The answer in the book is positive, with no
explanation
> given.
>
> Could someone explain this for me?
>
> Justin Parke

--
Eugene P. Mosca
Physics Department
MI350-B
United States Naval Academy
(410) 293-6668