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

["John Clement" <clement@hal-pc.org>]

We can argue all day about where the energy does or does not reside.  But
the Modeling program finds that using the idea of energy as merely being
transferred from one place to another helps students understand conservation
of energy.  When you use the word potential, the student thinks that is
somehow different from other types of energy.  But when it is labeled
according to where it resides, they get a more unified view.  Students tend
to treat mgh as a magic incantation.  But if they understand it is
expressing how much energy is available in the Earth-object system then they
will not make as many mistakes.

As to the energy being partially in you, that is true when you push it.  The
push transfers a small amount of horizontal kinetic energy, but it does not
contribute any vertical kinetic energy.  If the student recognizes the
system and visualizes the idea that there is a connection between the Earth
and object they will gain a more coherent energy paradigm.

My example of bonds is very important to understanding chemistry.  Because
students will tell you there is energy in bonds and when you break them you
get energy.  So if you dissociate gasoline the student will tell you that
releases energy.  The student will tell you the same thing about the Krebs
cycle.  This misconception blocks understanding of chemical and physical
reactions.

Of course it is possible by heating gasoline to a sufficiently high
temperature to dissociate it, but that is not an experiment we would do in
class.  The student who says that Oxygen is needed to dissociate the
gasoline is of course missing the point of having a spark, which starts the
process by dissociating a small sample.  Then when that small sample
combines with the Oxygen you have a chain reaction.  So one has to ask why
you need a spark or another flame to start the reaction.

Incidentally Feynman is not a good guide to teaching practices, because he
admits his lectures were a failure.

My point is that the concept of energy as residing somewhere and merely
being transferred by interactions, helps the students understand energy, and
improves their problem solving.  Whether you put the energy in the
Earth-object system, or in the gravitational field connection between them
are both fairly good ideas.  But when you put it in the object, this is
ignoring the interaction.  All energy transfers are because of interactions,
so when one cart hits another the interaction transfers energy.  So students
can visualize that an elastic interaction just transfers energy from one
cart to another, but an inelastic also transfers some energy to molecular
motion.  I really don't care about where energy "actually" resides, but I do
care about making the concepts workable for students.

You don't have to believe what I just said, but if you look at the PER
research you will find some very good justification for what I have said.
The U.Mass Amherst group UMPERG and the Modeling group are all very strong
on the idea of systems.  Modeling in particular uses the idea of energy
transfer, and it does get good results.  Again the idea is that you are
taking a pre-existing concept of energy as "something" and making this work
for the students.  Remember that concepts do not appear of a vacuum, but are
constructed from students from pre-existing concepts.  Students are not
"tabula rasa".  And the concept needs to come before the equation, otherwise
they become equation hunters rather than problem solvers.  For goodness
sakes read Arons book on  "Teaching Introductory Physics".

John M. Clement
Houston, TX

> If I lift a block from the floor and place it on the desk, does it have
> potential energy?  I sure teach that it does.  But I can't extract the
> potential energy without pushing the block off the desk.  So does that
> mean the energy "resides" partially in me?