Re: positive and negative work

["Carl E. Mungan" <mungan@USNA.EDU>]

> "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."

Answer the following questions and I hope things will be clearer:

1. What is the agent doing the work - a part of the muscle, the whole
set of muscles, or the entire leg?

2. What is the recipient of the work - another part of the muscle,
the leg excluding muscles (ie. the bone and flesh), or the rest of
the person (ie. the upper torso)? Note that if you mean the upper
torso, there is more than just the leg muscle at play - there are
also reaction forces at the joints which keep your torso from
dropping off the top of your leg!

3. Do you wish to calculate work in a mechanical work-energy context
(ie. probably the pseudowork) or in a thermodynamics context (ie. the
proper thermodynamic work)? That is, are you interested in internal
energy conversions to/from chemical, or solely in mechanical energy
and net forces?

4. Do you wish to assume the person sits down quasistatically, ie.
slow enough that there are no sudden jerks, gains of KE, etc?

I think the different answers we have heard on this list reflect
different answers to these questions.
--
Carl E. Mungan, Asst. Prof. of Physics  410-293-6680 (O) -3729 (F)
      U.S. Naval Academy, Stop 9C, Annapolis, MD 21402-5026
mungan@usna.edu    http://physics.usna.edu/physics/faculty/mungan/