Re: ENERGY BEFORE Q

[Ludwik Kowalski <kowalskiL@MAIL.MONTCLAIR.EDU>]

1) Those who followed this thread (about a week ago) may
recall the plan for approaching the energy topics gradually:

Model 1 --> idealized textbook world without friction.
Model 2 --> a little more realistic world with dissipative forces.
Model 3 --> a more realistic world with engines, etc.

All this was to be limited to what is reasonable in the first
physics course. I hoped we would quickly agree on concepts
and vocabulary of Model 1 and proceed to Model 2. Then we
would again agree on the vocabulary and go to Model 3. In
my opinion this is a reasonable way of revisiting unresolved
issues connected with the first law. Is this worth doing or not?

2) Side issues developed (in parallel threads). One of them
has to do with a possibility of introducing various forms of
energy without leaning on the concept of work and force.
I suggest that we leave these options aside; they are certainly
worth discussing. But for the time being I suggest we use
a traditional sequence: force--> work --> forms of energy,
as in common textbooks. Is this acceptable?

3) The following way of defining potential energies (as
introduced in Model 1) was presented:

PEgrv==work done against weight (or -work by weight).
PEspr==work done against a spring (or -work by spring)
PEelctr==(later) is work against the electric field ( or ...).

This is not different from what one can find in most texts.
Unexpectedly one highly knowledgeable phys_L-er objected.
He wrote:

> I stand by my assertion:  Defining work in terms of
> potential energy [this was NOT my suggestion] would
> be a blunder. Defining potential energy [as above] in
> terms of work would be an even bigger blunder.

Repeated calls for a clarification were not answered. Why?
I am puzzled. I do not want to go from Model 1 to Model 2
without resolving the above issue. The three lines above
represent the very essence of Model 1.  Why were they
labeled as a "big blunder." Please help to resolve this dilemma.

4) It would certainly be wrong to define a "frictional potential"
as work done again friction (or air resistance, etc.) because
that work depends on the length of the path and not only on
locations if the initial and final points. But why should potentials
associated with conservative forces not be defined in terms of
these forces? How should the above three potentials introduced
in the first physics course?

5) We want to discuss the law of energy and the law of entropy.
These laws were discussed many times and we had some
disagreements. Some of them had to do with vocabulary. That
is why we should be very careful about how concepts are named
as we proceed "from what is known to what is not known." If
it was up to me the material up to Model 1 would be:

1) Traditional calorimentry
2) Kinematics
3) Work done by a force, W. It is defined as a product of two
    vectors. Work is positive when cos(TET)>0 and negative
    when cos(TET)<0. We should not redefine work in Model 2
    and Model 3. We may need to use new names for new concepts,
    as needed. Calling six different things work, for example, can
    lead to unnecessary confusion.
4) Kinetic energy, KE; its definition is to be introduced via
    kinematics supplemented by F=m*a. KE is never negative.
5) PEgrv defined as work done against a weight (m*g)
6) PEspr defined as work done against a spring (k*x).

The topics we are addressing are not new and we should not
feel bad if what we are trying to establish has already been
established. Rediscovering old good approaches may be
useful. Please do not spoil the game by trying to show how
much you know. Keep in mind that we are discussing the
very first physics course here.
Ludwik Kowalski