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Re: positive and negative work



-----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
--------------------------------------------