Re: Why work before energy in texts

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

From: crawford j maccallum <mccallum@UNM.EDU>
> >
> > Work is easily defined.  Energy is not .. it's just something that we can
> > always find some way of accounting that makes it conserved.

At 11:24 PM 10/15/01 -0400, Joe Heafner wrote:
> That's it! There's nothing more to it! Read Feynman's story of Dennis the
> Menace.

No, that's not it.

I've read Feynman's story and I come to the opposite conclusion.  The point
of Feynman's story is to develop an understanding of energy in particular
and conservation in general, _without_ basing it on any prior notion of
work.  His point is _not_ that conservation is hard to define;  his point
is that even a child can understand it, abstract though it may be.

If work is so easily defined, and so readily usable to explain other bits
of physics, then why have I just read about the distinctions between
 -- work
 -- real work
 -- pseudowork
 -- system specific external work
 -- frame specific external work
 -- et cetera.

How many more epicycles do we need to add to this pre-Copernican view of
work before we decide that there's something fundamentally broken?


At 11:41 AM 10/15/01 -0700, kowalskil wrote:
> "Simple concepts first" is a good rule.

That's not a bad rule, as far as it goes, but I prefer the version that
says "Learning proceeds from the known to the unknown."  This focuses
attention on some additional points:
 -- The "first" ideas need to be not only simple, but known.
 -- There must be a path whereby learning can "proceed".  A hodgepodge of
simple concepts doesn't necessarily lead anywhere, if the concepts can't be
connected.

So, I'm happy with the premise that learning proceeds from the known to the
unknown.  Let that be the major premise.  What can we deduce from that?

There seems to be an implicit minor premise that "work is a simple
concept".  I don't buy that.  Similarly I don't buy the minor premise that
work is a well-known concept to the students.  Most of all I don't buy the
minor premise that there is a path whereby work leads to an understanding
of the general concept of energy -- certainly not in the real world, where
non-conservative force-fields are everywhere.

So I definitely cannot deduce that work should come first.

As possibly-constructive alternatives, let me point out:
 -- Students generally have a reasonable understanding of money as a
conserved quantity (in ordinary day-to-day transactions), and this can
serve as a very simple introduction to the notion of energy as a conserved
quantity.
 -- If you prefer, you could use Dennis's blocks as an ultra-simple
starting point.

==================

It has been known for 175 years that you can't define energy as "the
ability to do work" -- not at any nonzero temperature.  And it's been known
for almost 100 years that you can't even do it at T=0, as Chuck Britton
pointed out at 09:19 PM 10/15/01 -0400:
> Isn't Quantum Mechanics another good reason to put a stop to this
> 'ability to do work' chestnut?
> What conceivable WORK can the zero-point energy of an oscillator do?

Indeed!

> I prefer to think of Work as being a PROCESS of changing energy from
> one form into another.

Right, but let's be clear that it is not just any process, but a very
particular process, namely F dot dx.