Re: positive and negative work

[Rick Tarara <rtarara@SAINTMARYS.EDU>]

I guess my point was that it all depends on the details.  One can go through
the 'sitting' motion while suspended upside down from the feet.  There would
be an upwards (and backwards) displacement of the COM all due to the leg
muscles (positive work).  In normal sitting, there is a combination of leg
muscles moving the COM backwards (and perhaps downwards--albeit helped by
gravity) and also a resistance to the downward acceleration--unless one just
falls into the chair (where the chair must do negative work to stop your
fall).  Whether or not the horizontal motion results in a _net_ work of zero
again depends on the details of the motion and whether or not the chair
stops any of the motion.  I tend to agree that the problem as stated seems
to have it backwards--for the reasons given, but (as is the wont of this
group) we CAN make the whole thing extremely complicated!  <g>

Rick

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


> > -----Original Message-----
> > From: Rick Tarara [mailto:rtarara@SAINTMARYS.EDU]
> > Sent: Thursday, November 08, 2001 10:08 AM
> >
> > 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.
>
> OK - during this "horizontal acceleration" period the leg muscles do
> positive work.  However, the legs must also stop this horizontal motion at
> some point, no?  Wouldn't the net work be zero if we only considered the
> horizontal motion?
>
> >  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.
>
> Possibly?
>
> >  The
> > latter depends on the height of the seat and just how one
> > lowers oneself
> > into the seat.
>
> Explain?
>
> > 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!
>
> Translation: No work by legs ---> large change in kinetic energy.
>
> --------------------------------------------
> 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
> --------------------------------------------