Re: Energy, etc

[Robert A Cohen <bbq@ESU.EDU>]

On Tue, 3 Aug 1999, brian whatcott wrote:

> Permit me to offer a straw man to this debate - in a form often
> mentioned here - called kinetic energy.
>  This is indeed a prickly topic for some physicists.
>    The formulaic 1/2 m.v^2 is the nub of it.

   I'm hesistant about entering this debate but I've always been
uncomfortable with my understanding of the term "energy".  Personally, I
am fond of Newton's 2nd law, so I tend to prefer W=delta KE, than delta
(PE + KE) = 0.  In other words, I prefer "a force (field) has a potential
to do work" rather than "potential energy associated with a force
(field)."
   As such, I prefer to say that an object's kinetic energy increases
because some force does work on the object rather than say that the
kinetic energy increases because the energy "flows"  from some potential
energy supply. For example, for a pulse of light, I prefer to say that the
field associated with the pulse has the potential to do work.  As the
pulse (a wave in the electromagnetic field?) hits the wall, the field does
work on the wall molecules, increasing their kinetic energy and causing
the wall to warm up.

>        Folks remind themselves that v is usually relative (?),
> so that kinetic energy is relative, and by changing their frame,
>  they shift energy between the moving article to the scenery which
>  was thought to be immobile. This means they believe they know how to
> 'relocalize' it!

   As another example, if one's frame of reference is not inertial, then
an object's kinetic energy will change.  I wouldn't say the energy flowed
into or out of the object, but rather that the apparent forces (due to the
non-inertial frame) appeared to do work on the object.

   Are my interpretations incorrect or improper?

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| Robert Cohen               Department of Physics       |
|                            East Stroudsburg University |
| bbq@esu.edu                East Stroudsburg, PA  18301 |
| http://www.esu.edu/~bbq/   (570) 422-3428              |
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