Re: positive and negative work

[Brian Whatcott <inet@INTELLISYS.NET>]

Hmmm... here Richard Tarara is here the voice of reasoned moderation,
and below,  Michael Edmiston is evidently speaking ex cathedra.

Let me explore the topic a little further, in order to provide some shading
in the
black and white sketches so far provided.

We use Professor Leigh's favorite model: the car rolling on an incline.
Let us suppose that Michael is sitting in this car, which has started to roll
backwards, downhill. He wants to proceed slowly astern, so he applies a little
throttle while he is in a forward gear. This prevents the car from
careering away.

 The work done by the car, we say, is negative.   There is no discussion,
no debate.
This is the convention.

Now we look inside the car, make it less concrete a model, more abstrract,
you might say.

We see two shafts: one connected to the engine, and one connected to the
rear wheels.
They meet at the torque converter - the fluid flywheel - the slipping
'clutch' if you like.
We see that the engine is rotating, and rotating against a viscous resistance.
We say it is doing [positive] work.

But wait! We see the rear wheels are driving the rear shaft into the torque
converter -
in the other direction.
But this shaft is rotating, and meeting resistance too. So *it* is doing
[positive] work.

Meanwhile, the fluid in the torque converter is getting hotter, and hotter.

How can it be that the car is doing negative work, but the car's engine,
and the transmission from the rear wheels are both also doing positive work?

Oh yes - negative work is just an accounting convention we are dealing with
 - we must apply it by rote - and if we expect it to have a real relation to
concrete objects  - like muscles or fluid flywheels, we find
the accounting convenience is a fiction.

Oh dear!


Brian W   :-)

At 01:57 PM 11/9/01 -0500, Richard Tarara wrote:
> If we look at the HS level (pretty much the same as our 'liberal arts'
> level), I'm not sure that I see that keeping with a simple (but standard)
> sign convention complicates things or is counter-productive.  We've just
> finished this unit.  The formulation was easy:
>
> For NET-Work ON an object---positive if the KE increased, negative if it
> decreased.
>
> For WORK done BY an agent--positive if force and displacement are in the
> same direction, negative if opposite directions.
>
> That's it!  Keep the examples simple--as with most of the semester, a ball
> thrown straight up and later caught at the original height provides ample
> physics with which to work--and this should be unambiguous.
>
> Rick
>
> **********************************************
> Richard W. Tarara



> > on 11/9/01 9:39 AM, Michael Edmiston at edmiston@BLUFFTON.EDU wrote:
> >
> > snip
> > > I would answer this by saying it's not important, don't bother to teach
> > > students any arbitrary conventions.  The only reason we would need to
> > > memorize arbitrary conventions would be if we ever intended to
> communicate
> > > clearly with other people.

Brian Whatcott
  Altus OK                      Eureka!