Re: [Phys-l] momentum dissipation?

[John Denker <jsd@av8n.com>]

On 04/25/2010 03:20 PM, Bob Sciamanda wrote:
> Consider the case of a motorboat accelerating by exchanging momentum with 
> the fluid water.  It would seem that in such cases it is legitimate to speak 
> of the dissipation of  momentum by the water.  Similar cases occur with 
> aircraft/atmosphere interactions.

Well, you could say that, but I wouldn't recommend it.

First of all, this thread started with a statement about
momentum not being dissipated "as heat".  I hope nobody
is suggesting that motorboats or aircraft dissipate
momentum "as heat".

Secondly, to my ears, dissipation refers to a rather
specific set of phenomena, all of which involve the
production of entropy, as in the celebrated experiments
on the mechanical equivalent of heat:  Rumford, Joule,
et cetera.  So the word dissipation by itself, to my
ears, implies of energy "as heat" anyway.

This is an important conceptual point, because when
energy is dissipated (as heat!) you can't get all of
that energy back, because you are constrained by the
second law of thermodynamics.

No such constraint applies to momentum!  Absolutely
100% of the momentum that flows from the airplane
into the airmass flows from the airmass into the
ground.  The momentum spreads out a little bit, and
some of it is stored in the air for a short time,
but not for long.  This does not even remotely 
resemble what I call dissipation, i.e. dissipation
of energy.

Here's the picture:
  http://www.efluids.com/efluids/gallery/gallery_images/citation_2a.jpg