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Re: [Phys-l] teaching energy




----- Original Message ----- From: "Dan MacIsaac" <danmacisaac@mac.com>
To: "Forum for Physics Educators" <phys-l@carnot.physics.buffalo.edu>
Sent: Tuesday, October 03, 2006 1:21 PM
Subject: Re: [Phys-l] teaching energy


On Oct 3, 2006, at 12:21 PM, John Clement wrote:

No, viewpoint number 2 is not valid. If gravitational potential
energy can not be in the ball because the presence of the
Earth is necessary for there to be GPE. Now it is certainly
associated with objects (note the plural),
but then you have #3.

SNIP
I agree with eliminating #2, even for kinetic energy (below).

Now with kinetic energy, the energy is in the object. With thermal
energy, the energy is in various physical places such as the two
objects that are rubbing against each other...

SNIP
I disagree with your first sentence. KE is not in the object but
also depends on other objects; velocity is measured relative to a
system. A thrown baseball on a moving train has a different KE in
the frame of the train than it does according to an external
stationary observer. Even KE requires a system.

Hi Dan, nice to see you participating ;-)

Wouldn't it be more accurate to say that quantifying the energy contained in the object is subject to your choice of frame? Choosing a frame doesn't require a system of objects necessarily. In the case you cite above, for example, it seems to me that the train itself is irrelevant, only the frame chosen is a determining factor. If that IS the case, then there is still nothing wrong with asserting that WHATEVER the quantity of kinetic energy calculated may be, it is STILL "contained" in the object. Yes? No?