In article ,
Jim Logajan wrote:

Alan Baker wrote:
Because the push is caused by the impact of countless air molecules
with the surface of wing. If those collisions fall to zero (i.e. in a
perfect vacuum) then there is zero push.

But there is no set of circumstances that can make the number of
collisions be negative.

Pedantically speaking, outgassing would occur for a while that would create
a force on your wing surface when it is exposed to a vacuum. Pedantically
speaking, I don't see why those couldn't be called negative collisions.

The force they'd create would be in the same direction as the force of
regular collisions: toward the surface. If the outgassing molecules have
momentum away from the surface then the surface must experience a change
in momentum in the opposite direction.

--
Alan Baker
Vancouver, British Columbia
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