Re: positive and negative work

[Brian Whatcott <inet@INTELLISYS.NET>]

At 08:42 AM 11/8/01 -0500, you wrote:
> 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?
>
> Justin Parke


I'll take a shot at it.
Heterodox view coming up.
 Avoid the idea of muscles as springs, like the plague.
You slither into ideas of negative work, before you can say 'Jack Robinson'.
If you must, think of muscles as screw jacks driven by a heat engine.
   It takes fuel to extend the screw, and it takes fuel to retract the screw.

You can demonstrate this experimentally. with a flow meter for the breath,
and a blood analyzer looking for  blood glucose.

These screw jacks can exert a force. The force may reduce gravitational
acceleration - which is the physics conception of negative work,
or the force may increase the acceleration provided by gravity - which is the
physics conception of  positive work.
The physiological mechanism - the heat engine driven screw jack doing work
- works the same either way.
Even when the muscular force makes no displacement (physically,
work = zero) it is like an car with an auto transmission holding on the
throttle,
stationary on an incline: the engine is turning against the resistance of
the fluid flywheel - and that is work.

But this is a heterodox view. Did I mention that?








Brian Whatcott
  Altus OK                      Eureka!