Re: Why work before energy in texts

[Joe Heafner <heafnerj@VNET.NET>]

> From: kowalskil <kowalskil@MAIL.MONTCLAIR.EDU>
> "John S. Denker" wrote:
> > I stand by my assertion: Defining work in terms of potential
> > energy would be a blunder.  Defining potential energy in
> > terms of work would be an even bigger blunder.
>
> YES, work is defined as a dot product. But:
I would argue that work is *not* defined as a dot product, but rather *calculated* as a dot product. Work is but one of several *processes* by which energy is moved from one place to another. Other processes are heat and ratiation.

> PEspr==work done against a spring (or -work by spring)
> PEelctr==(later) is work against the electric field.
> PEelctr==(later) is work against the electric field.
>
> In general work done against a conservative force
> BECOMES POTENTIAL ENERGY ASSOCIATED WITH THAT FORCE
> (WITH RESPECT TO A CHOSEN REFERENCE LEVEL).
The concept of *potential energy* is only valid within the context of two or more particles undergoing a mutual interaction. It is not correct to speak of *the potential energy of a ball*. Rather one should say *the gravitational potential energy of the ball-Earth system*. Similarly for springs and so forth.

Regarding the chosen reference level, relativistically all potential energy must go to zero when the interacting particles are infinitely far apart. This interesting realization is but one of many wonderful and rich outcomes of Chabay and Sherwood's superb text entitled "Matter & Interactions I" (Wiley, 2002). I've used this text for three year now (two preliminary editions and now the first edition). Chapters 4-6 in Volume I give the most correct and consistent (with respect to correct terminology) treatment of energy methods in print to date.

I'm *VERY* surprised at all the interesting (?) debate over these issues I've seen here. Is it any wonder our students end up confused over this stuff when most practicing physicists and instructors don't even get it?!? The traditional textbooks don't get it right, so why should we expect our students to? Read Arons for many unique tidbits. Then check Chabay and Sherwood to see how this topic should be treated. Many professional colleagues have learned a thing or two from this truly innovative text!


Cheers,
Joe

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