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Re: [Phys-L] defining energy



This is a simple idea. If you have a ball in space without any
gravitational attraction, you can not have gravitational potential energy.
But if you have a ball on Earth you can define the gravitational potential
energy. But if you have no ball, again this is not possible. So you must
have both the Earth and the ball to know the gravitational potential energy
of this system. If you wish to then think of it being in a place, it is
neither in the ball nor the Earth. You can think of it as being in the
system. For students you can have them picture the ball and the Earth as
being attached by "something" that might be called the gravitational field.
It is like a rubber band which does not increase in strength when stretched.
In which case the energy is in the field.

This of course is a first model for gravitational energy. By getting
students to visualize where the energy is "located", it can be helpful in
establishing conservation. If the students are all at the "theoretical
level" with a good grasp of math, this type of thing may not be necessary.
But since the majority of students who enter an intro physics class are
below the formal operational level, they need ways of visualizing where the
energy is located. This is also true in intro calculus based classes except
possibly at some top schools like MIT, RPI... But even at those schools
only a very few are at the theoretical level as defined by Lawson.

Similarly gasoline does not "contain" energy because if you seal it in an
air free container, you can not get the energy from it. You need oxygen,
but the Oxygen can not contain energy either. It is in the system of
gasoline/Oxygen. This type of picture is important for students to begin to
comprehend.

The idea of a system is strongly hit in the Minds-on-Physics books, but is
not really stressed in most other books.

John M. Clement
Houston, TX




On 2013, Oct 29, , at 22:05, "John Clement"
<clement@hal-pc.org> wrote:

In other words one has to have
both the Earth and the object to have energy and neither alone has
gravitational energy.


I'm confused: The g field in energyless?

bc thinks he's going to learn something.
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