Re: positive and negative work

["John S. Denker" <jsd@MONMOUTH.COM>]

John Mallinckrodt wrote:

> As I understand it, you want to promote a single definition of
> work.

Yes, that would be an improvement, other things being equal.

> For that definition to be useful, we need to know what to do
> wih it after we calculate it.

I'm not assuming that work is a useful concept.  Sometimes it is useful, but
sometimes it isn't.  There is no 11th commandment that requires it to be useful.

> That would seem to me to require
> knowing how it is related to the change in some independently
> calculable energy of the system.

I agree with the sentiment, but I wouldn't have stated it as a "requirement".  I
would have said if (!) we can relate work to energy, then work is useful.  Energy is
primary and fundamental.

> To determine that we need to
> know the answers to my two questions.

I don't think so, especially in cases where work is not useful.

> The answer to this first question will, for instance, determine
> whether or not work is related to changes in the bulk
> translational kinetic energy of an object.  Lots of people think
> it is,

Well, lots of people make mistakes.  I have posted numerous counterexamples to this
notion, notably carts with flywheels on board.

> Consider for instance the result of pushing gently on an otherwise
> freely floating balloon and doing a *thermodynamic* analysis of
> its contents.  In a case like this, we generally say that no work
> has been done on the gas even though its bulk translational
> kinetic energy changes.

What do you mean "we", Kemosabe?

 -- If the push is truly gentle, it accelerates the balloon without disturbing its
internal degrees of freedom, so we can treat the balloon as a large but essentially
simple object, and apply the notion of "work" to its overall motion.
 -- If that's not true, then I don't even know what F dot ds means for such a
complex system, and therefore I don't know how to apply the notion of "work" to the
overall system.  I would not expect my colleagues to know what "work" means in such
a context, and I certainly wouldn't expect students to learn it.