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Re: Energy



We use many terms in physics that the "public" uses in a way that might be
inappropriate for the physics (e.g., velocity, mass, acceleration, force,
wave, etc.). We all know how this impedes student learning and we strive to
make the distinction clear rather than just avoiding the use of such terms.
Is the word "flow" any different?

I have learned that I shouldn't assume the students are using *any* of the
terms the way I intend them to be used. With "flow", as with any term, I
wouldn't assume the students are using it correctly but, at the same time, I
wouldn't avoid using it if I felt I could make the distinction clear enough.
That is why self-evaluation is important. If the students have been trained
to identify situations where they don't understand - I can then probe to
find if the cause is a difference in definitions (which many times is the
case).

----------------------------------------------------------
| Robert Cohen Department of Physics |
| East Stroudsburg University |
| rcohen@po-box.esu.edu East Stroudsburg, PA 18301 |
| http://www.esu.edu/~bbq/ (570) 422-3428 |
----------------------------------------------------------

-----Original Message-----
From: Jim Green [mailto:JMGreen@SISNA.COM]
Sent: Sunday, September 23, 2001 3:31 AM
To: PHYS-L@lists.nau.edu
Subject: Re: Energy


Happily this thread is nearly dead. Perhaps I can offer a
benediction of sorts:

Some of the same people who asked me to offer my opinion
which began this
thread have asked why I have not participated in the
discussion of the past
week. A couple of reasons:

I am just too old and senile to enjoy disputes and feuds and also the
moderator/owner of this list publicly chastised me for
fomenting discord
with the original post so I am a little gun shy -- but that
may just have
been because he is quite opposed to my views and wants me to
be silent.

Yet, I face the same difficulty Thomas Moore faced; Does
silence argue for
acquiescence? And I am surely not acquiescent!

My observations -- for what they are worth:

There has been the thought that exact physics is not all that
necessary for
most intro classes -- eg "energy flowing" is good enough. What a
frightening thought! It is true that our students have been
prepared by
cartoons, Bill Nye, and under-prepared teachers. However, it is not
totally the teachers fault; they learned physics from like
teachers; and
they from like teachers, and they...

Now good instructors will step in and try to stem this
pitiful momentum

There has been the thought that the students are just not
mentally prepared
for a legitimate physics class -- that the students can only
understand
faulty concepts like "energy flow". Well then, why not cave in
substantially -- why not teach eg a projectile path like the
one depicted
for Wiley Coyote as he runs off a cliff along with the concepts of
impetus. This would certainly be in keeping with the student's
preparation. No, it is our task to mold their minds as best we can --
albeit with only occasional success. Good teachers will
strive to do this.

Folks, it is just not difficult in the least to present the
work/energy
theorem (You have to anyway) and expand on this. Energy is a
mathematical
invention to describe an abstract property of a system and can only be
changed by doing work on that system. There is nothing there
that intro HS
students can't understand -- even in my local HS.

A comment or two about the rationale of college instructors:

The overriding theme of this list is the teaching of physics
to a spectrum
of students. And thus, effective communication is paramount.

The impression I have experienced this last week is that most
participants
feel that "energy flow" is what they learned many years ago,
that they are
comfortable with the concept, that it is easy to teach, and
that they just
don't want to change. But some have seen a different
approach. As a result
there has been on the part of some a general effort to
contort the common
language of the students into a special convoluted language which is
logical and in one's mind justifies the older pictures.

Now finally a few comments re John's post of this morning
which left me
yelling at my monitor:

The "flow" definition I gave on this list is perfectly
consistent with a
commonsense notion of how matter flows. It is also perfectly
consistent
with a correct notion of how energy flows.

A rare intro student would agree that the proffered definition is
commonsense. And the notion of how energy "flows" is pedagogically
abysmal. Energy does not flow or move in the manner proclaimed.

Larry Smith asked three times how any correct idea of "energy
flow" might
be given in a class. The question was never answered.

I simply asked if you tell your students what "flow" means
when you use the
term vis a vis energy.

I never knew I had to tell them what flow means.

Now the nub of the dispute:

I consider energy to be a particular type of incorporeal
stuff. I consider
flow to be more-or-less a particular type of moving,
although "moving" has
slightly different connotations than "flowing".

This and the following would certainly appear to intro students as
incomprehensible hand waving.

I have repeatedly formulated the local conservation law as
change(stuff inside boundary) = -flow(stuff outward
across boundary)
in which the RHS quite explicitly refers to stuff flowing. I still
consider this formulation to be correct and clear.

No. I was wrong. The following is the nub:

My goal is to write so that people who want to understand will
understand. If there are others who want to misunderstand,
I'm sure they
can find a way to misunderstand; that's not my problem.

I can not believe that a conscientious teacher would say this about
students or, for that matter, anyone else!

Amen


Jim Green
mailto:JMGreen@sisna.com
http://users.sisna.com/jmgreen