Re: Energy as ability to do work

[Ludwik Kowalski <KowalskiL@MAIL.MONTCLAIR.EDU>]

Your block (see below) did gain an "ability" (Serway calls it "capacity")
equal to 10 joules of work. In practice you can not get 10 J of work out
of the block (thinking in terms of a Carnot engine) unless the c*m of
the block is approaching zero, or unless a zero K sink is available.

But conversions of heat into work (sorry for using forbidden words) is
not the issue. Mechanical energy is introduced when we are dealing with
bullets, springs, pulleys and satellites. At this level a traditional definition

of energy as "an ability to do work" can be as useful as the concept of
frictionless motion of a hockey puck. I am in favor of continuation of
introducing work before energy. Poj thinks that energy can be introduced
without first defining work. I am waiting for his reply to my three
questions (asked yesterday). Perhaps I will be convinced of advantages
of the alternative approach.
Ludwik Kowalski

John Denker wrote:

> At 09:59 AM 10/26/99 -0500, Lemmerhirt, Fred wrote:
> > Lately there have been several emphatic rejections of the old idea of energy
> > as the "ability of a system to do work".  I'm wondering if those who oppose
> > this approach do so because:  1) they feel it is just plain "wrong";  2) it
> > is incomplete and lacks generality;  or  3) they just don't find it as
> > "intellectually satisfying" as a more abstract approach.  (or maybe for some
> > other reason)
>
> I vote for "just plain wrong".
>
> Consider a block of brass with some initial temperature.  Then I dump ten
> joules of heat into it.  The incremental ability of the system to do work
> with that energy is somewhat less, possibly very much less, than the
> incremental energy.
>
> ______________________________________________________________
> copyright (C) 1999 John S. Denker             jsd@monmouth.com