Re: positive and negative work

["RAUBER, JOEL" <JOEL_RAUBER@SDSTATE.EDU>]

Here's my quick take on the student's question:

> > A student asked to day "If I don't do any work holding a
> heavy object, why
> > do I get tired after a while?"  Is John's answer something
> I should pass
> > on?

An individual muscle fiber as it twitches is doing work on itself.

Sub-divide the fiber into elements and consider that each element has forces
acting on it and the point of application of those forces is "shifting
position" giving non-zero work.  The energy for doing this work comes at the
expense of chemical potential energy when you do those ATP -> ADP
conversions.

Note: the fiber is not doing work on the block you are holding as that last
"element" that is applying a force to the block does not have its point of
application displaced.

Joel R