In article ,
brian whatcott wrote:

Alan Baker wrote:
In article ,
Jim Logajan wrote:

Any change in pressure is *by definition* a change in the number of
particles in the fluid that are impacting the surface.

That assertion is incorrect. You are no dummy so I'm sure you'll correct
it
when you realize the errors.

Sorry, but it's not. Pressure is created by particle collisions.

Hmmm...looks like Jim expected too much from you: the kinetic theory of
gases has it that pressure may be computed from the temperature AND the
density of gases... that is to say, by retaining the SAME molar quantity
of gas, and raising its temperature (which translates to a higher
velocity), the pressure is increased P.V = R.t and all that....

Put it another way: each "hotter" molecule reverses direction at a
surface with greater force.

Brian W

I'm perfectly aware of that, but that hardly matters for the scope of
our discussion of the effect of pressure on a wing. The point I'm making
is that all else being equal, more collisions means higher pressure and
fewer means lower pressure, but that pressure is therefore always a
positive value that acts toward the surface to which it is applied.

It is *never* acting away from that surface; i.e. "pulling".

That is the only reason I mentioned a vacuum, because it is a situation
in which there is *by definition* zero absolute pressure.

--
Alan Baker
Vancouver, British Columbia
http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg