Re: positive and negative work

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 05:38 PM 9/26/2003 -0600, Larry, you wrote (I think>):
> The work done _by_ the muscles is negative.  (The work
> >> done _on_ the muscles would be positive,
> >> but that's not the question that was asked.)
> ///
> 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?  What's the chemistry of getting tired?
>
> Thanks,
> Larry

I missed the beginning of the thread - perhaps while blocked with worms -
still, I like to use the mechanistic metaphor:
what would happen with a freely hinged arm, for which a torque motor
can provide torque at the elbow?
If a weight is hooked onto the hand end, the torque motor could hold the
elbow angle sensibly constant at the cost of electrical energy dissipated as
 heat. There is a little more dissipation as the arm lifts the weight and so
does work on it, a little less as the arm allows the weight to sink and the
weight does work on the arm.

After a while, as likely as not, the motor becomes warm from the I squared R
and the motor winding resistance rises: so if the drive voltage were
constant -
the arm would droop; goniometric feedback allows the angle to be restrained
 from drooping at increasing electrical cost. You might say the motor is
tiring
or that's it is exceeding its duty cycle - and it would like to sleep a while.
So that's the kind of "chemistry" to which Larry refers here, I suggest....

Brian Whatcott    Altus OK    Eureka!