Re: PseudoWork ????

[John Denker <jsd@MONMOUTH.COM>]

At 09:51 AM 10/29/99 -0700, Jim Green wrote:
> > ... when you push off from a stationary
> > wall, the wall does work, even though the resulting energy of bulk motion
> > came only from internal sources, not from the wall!

This is a highly nonstandard conception of "the wall doing work".

> I
> would add the it is also true that there is no such thing as a stationary
> wall.  If the pusher moves, so does the wall.  Now given the mass of the
> wall plus the Earth to which it is attached, the wall does not move very
> much, but it does move.  The wall does indeed do work on the pusher.

Yes, the earth+wall moves.  This is important for conservation of momentum.
 But as you point out, it doesn't move very much.  Consequently, its
contribution to the energy balance is utterly negligible.  The work, under
any reasonable definition of work, is equal to P squared over 2 M, where M
is the mass of the earth.  This amount of work is very nearly zero on any
reasonable scale.  Saying the wall does no work is an exceedingly
reasonable approximation.

> The list has previously gone over this ad nauseum.  And we would agree I
think.

Evidently not everyone agrees.  I don't buy it one bit.  And I assure you
there are vast numbers of professional physics researchers who would be
apalled by many of the things that pass for "consensus" on this list.

> It is also true that energy does not move!

Whaaat?  This is exceedingly nonstandard.

> I would like to say that in no case is there an increase of mechanical
> energy (I really would like to say energy of any kind but I have not
> thought that through) unless work is done on the system -- but surely
> someone will give a counter example -- I look forward to it. (:-)

Your wish is granted:

1)  Regarding an increase in mechanical energy:  Nuclear energy is,
according to standard definitions, not mechanical energy.  An object
containing a nuclear reactor can convert that to mechanical energy without
any significant work being done from outside.  If you want this mechanical
energy to be in the form of kinetic energy, momentum balance will require
pushing against some heavy object (and if you want to get really fussy the
net work per unit momentum can be made strictly zero by playing off a
combination of light and heavy objects).

2) Regarding an increase in total energy:  A heat transfer, according to
standard definitions, is not work.  Shining a heat lamp on an object
increases its total energy without an input of work.



______________________________________________________________
copyright (C) 1999 John S. Denker             jsd@monmouth.com