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Re: [Phys-l] teaching energy

I've been skimming through the posts and I think there are three points
of view:

1. Energy is associated with fields.
2. Energy is associated with objects.
3. Energy is associated with systems of objects.

I believe Joel R is asking why #3 is "wrong". I have the same question.
It seems to me that #1 and #3 are equivalent. Further, it seems to me
that #3 is easier for high school students to understand, although I
have no hard evidence of that.

After wading through the emails, it seems that the Modeling paper is
objecting to #2, not #3, in comparison to #1. I think John Denker made
this point, if I followed his arguments correctly.

Taking liberty with snipping lots of his post, John Denker wrote:

Viable choices could have been the PE of an object on
a high shelf, the KE of a moving object, the electrostatic
energy of a capacitor, ... almost any form of energy.

It seems that JD is using language #2 here.

Obviously you can raise the energy of the earth/moon
gravitational system by raising the moon in the earth's
gravitational potential, i.e. by doing work on it by pulling
it against the gravitational force.

It seems that JD is using language #3 here.

It is traditional and convenient from a /laboratory/ point of
view to speak of the gravitational energy "of" an object when
it is subject to an applied gravitational field ... as if the
energy "belonged" to the object. For example, we speak of
the GPE "of" a book on a high shelf. However, from the point
of view of /universal/ gravitation, this is highly
problematic. For starters, equation [1] is symmetric w.r.t
the roles of (m) and (M), and attributing the energy to just
one of the objects would break this symmetry.

Note that JD is commenting on the weaknesses of using language #2.

JD goes on, though, to comment on local conservation, which raises some
other questions in my mind.

Local conservation demands that any decrease in energy in any
region must correspond with a simultaneous increase in energy
in some _adjacent_ region(s).

Does that mean that energy must be assigned to a location (i.e., that #3
is not viable)?

So, unless you want to overthrow the most fundamental
principles of physics, we have to assume that there is energy
flowing in the field in Region 2. (The energy must be in the
_field_, because there isn't anything else in Region 2.)

As opposed to energy be in the "system" of the two objects? Does this
mean that language #3 is also weak?

----------------------------------------------------------
Robert A. Cohen, Department of Physics, East Stroudsburg University
570.422.3428 rcohen@po-box.esu.edu http://www.esu.edu/~bbq