Re: positive and negative work

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

But gravity doesn't do the positive work until the center of mass has been
moved--by the leg bending--behind the point of support.  It seems that the
sitting  process is more complex than the authors considered.  I would say
that there is some positive work in the process of moving the COM backwards,
and then possibly some negative work as described by Justin and Robert.  The
latter depends on the height of the seat and just how one lowers oneself
into the seat.  At about this time of year, after a full day of classes, it
is often just 'collapsing' onto the couch for me--legs doing almost nothing!
;-)

Rick

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Richard W. Tarara
Professor of Physics
Saint Mary's College
Notre Dame, IN 46556
rtarara@saintmarys.edu

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----- Original Message -----
From: "Robert Cohen" <Robert.Cohen@PO-BOX.ESU.EDU>


> On the way down, gravity is doing positive work.  If there is no change in
> kinetic energy, something else must be doing negative work.  If there is a
> change in kinetic energy, we can compare the change with what the change
> would be with gravity acting alone.  If the change in kinetic energy is
less
> than what it would be with gravity acting alone, something else must be
> doing negative work.
>
> I'm with Justin - I would think the muscles must be doing negative work.
>
> --------------------------------------------
> Robert Cohen           rcohen@po-box.esu.edu
> 570-422-3428         http://www.esu.edu/~bbq
> Department of Physics
> East Stroudsburg University
> East Stroudsburg, PA  18301
> --------------------------------------------
>
> > -----Original Message-----
> > From: kowalskil [mailto:kowalskil@MAIL.MONTCLAIR.EDU]
> > Sent: Thursday, November 08, 2001 12:45 PM
> > To: PHYS-L@lists.nau.edu
> > Subject: Re: positive and negative work
> >
> >
> > Justin Parke asked:
> > > 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?
> >
> > Let me try. Model the situation with a vertical spring
> > supporting a mass m; the system is in equilibrium. You
> > are the "agent" inside the spring (muscle). You have
> > to pull the ends of the spring toward each other. By
> > compressing the spring your two forces do positive work.
> > Ludwik Kowalski