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Re: [Phys-l] internal/external conservative/nonconservative forces!?!?





John Clement <clement@hal-pc.org> wrote:


But one can do experiments to find out where the energy is located. For
example you can prove there is no energy in gasoline by putting into a
closed container. There is no way to get thermal energy from it then. So
the energy is in the system of gasoline and Oxygen. Similarly gravitational
energy only exists when you have both the Earth and a ball. If either of
these is absent, poof no gravitational energy. But this experiment only
reveals that the energy can not be located in one of the two objects, so one
appeals to the idea of a system.

As to whether energy is "something" is certainly debatable. But we act as
if it is something by saying it is conserved. As to conservative and
non-conservative forces, that is a fairly destructive nomenclature. Of
course non-conservative forces conserve energy by putting it into a
different place. So friction puts it into the individual molecules as
molecular kinetic energy. The idea that energy is merely moved to a
different location has been found to help students visualize conservation.
Energy transformation on the other hand makes them think that energy is a
different thing in each case. Of course this gives rise to all kinds of
misconceptions, and students will often allow energy to disappear in
problems. But if it must be put into a different location, they tend to
understand and use conservation. Perhaps we should change the
conservative/non-conservative labels. It really is talking about mechanical
energy conservation, but it leads students to think that energy disappears.

I would think that to a physicist energy would be much more than just an
abstract quantity. It is one of the most important ideas, and its
conservation is considered to be inviolable. OK, you can say it is just an
abstract idea, but making it more real by having it reside in a location
seems to help students deal with it much better.

The distinctions between contact and non contact forces are very valuable
when teaching. Without the idea of contact students make up mythical forces
in physical systems. But once they have the rule that you have to touch
something, they can find the analysis much easier. So far the research
shows that this idea is extremely important to student acquisition of
Newtonian concepts. Similarly the concept of interactions is also an
important concept which helps students learn NTN3. This goes way back to
Robert Karplus who might be considered the founder of PER and the author of
the "Lerning Cycle" in science.

The force vs acceleration psychology is important when teaching students.
Your concepts do not come whole cloth from new ideas. Rather they are
constructed from existing concepts. This happens in math as well as in
physics. So using the idea that force causes acceleration can be helpful to
students. So yes, in a mathematical sense force does not cause
acceleration, but it still can be useful as a teaching device. Many
teachers seem to hold the big misconception that you can communicate
abstract ideas to students directly. This is not true. They have to
construct the ideas by putting together or learning to apply existing
paradigms. So I think the force causes acceleration formulation is a useful
bridge. I will admit that I don't know of any experiments that show this.

Look at the anchor and bridging analogies. They start with something the
student believes and moves them over to more abstract concepts by a series
of steps. They are extremely successful in teaching students NTN concepts.
If you don't know about them go back to JRST and read the research by John
J. Clement (no relation to me) at U.Mass Amherst. This was published as I
recall in the 80s and 90s, so why isn't it used routinely by teachers?????

My suggestion which seems to be ignored by many is to read the research on
the topic of teaching physics. A string theorist would read the research in
his field, but why do teachers suppose they can teach physics without
reading the PER research. Teaching can be a science if you treat it as a
science and get into the research. This sort of thing also goes on in the
cognitive science field where reputable researchers fly off and try to
refute specific teaching methods when it is obvious they have not read the
relevant research.

John M. Clement
Houston, TX


I don't see why this is an issue. "Energy" is an abstract quantity, not a
thing-in-the-world. You can say it "resides" wherever you like. Can you
design an experiment that favors one view over another? Or a problem that
can be solved if you hold one of those beliefs but not the other? This
seems to me to be philosophy or psychology, not physics. [Not to bring
this issue up again, but it also seems like philosophy/psychology when we
debate whether the force causes the acceleration of the acceleration
causes the force.]

As for the earlier question: I think I know why it is important to
distinguish between conservative and non-conservative forces, and think I
can design an experiment to see if a force is conservative. But I have no
understanding at all of the terms "internal" and "external". I'd never
seen them before this thread started and I can't imagine teaching them. I
don't even make a big deal about "contact" vs. "non-contact" forces -- in
the end, aren't all forces "at a distance"?

-----Original Message-----
From: phys-l-bounces@carnot.physics.buffalo.edu [mailto:phys-l-
bounces@carnot.physics.buffalo.edu] On Behalf Of William Robertson
Sent: Wednesday, December 15, 2010 1:14 AM
To: Forum for Physics Educators
Subject: Re: [Phys-l] internal/external conservative/nonconservative
forces!?!?

Stating that the energy properly ascribed to a system resides instead
in one object of the system is not an approximation. Rather, it is a
small lie. Small lies are fine as long as we explain to students what
the lie is, and why it's okay to proceed with the small lie. And here
I am not talking about using mgh rather than the universal law of
gravitation.


On Dec 14, 2010, at 5:51 PM, John Denker wrote:

On 12/14/2010 05:19 PM, William Robertson wrote:
I believe that the concept of system is given short shrift in
too many physics or other science texts. There exists research
showing
that understanding or not understanding a choice of system can
dramatically affect one's problem solving ability in physics.

Well, that cuts both ways.

Yes, physics is about principles. But physics is also about
applications
and approximations.

In the physics course, I want students to learn the principles. But
just as importantly, I want them to learn how to make well-controlled
approximations.

If we are talking about the earth/moon system, the gravitational
energy
is clearly in the system, not "in" the moon. The principles of the
thing
are clear, and the same principles apply -- in principle -- to every
other
gravitating system.

On the other hand, in a very wide range of practical applications,
including soccer balls, planes, trains, and automobiles, we find "m
g h"
is an exceedingly good approximation, and is significantly simpler
than
"G M m / r". Treating the earth (and the earth's gravitational field)
as imperturbable is an approximation. Like all approximations,
sometimes
it is appropriate and sometimes it is not.

Deciding what approximation to use in this-or-that situation requires
judgment and skill.

It is ultra-super-important that students understand we are not peeved
about the approximation but rather about certain _inappropriate uses_
of the approximation.
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l

_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l
_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l


_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l