Mike, I believe you described the ping pong ball accurately but came to the
wrong conclusion. If you give the ball a top spin with the paddle so that
someone from the left side of the table would see a ball rotating counter-
clockwise the ball will have a downward force due to a difference in
pressure. But to explain this effect you cannot view the ball in motion
since that is not a steady flow. You must look at the inverse flow
where the ball is not in motion and the air is flowing past it, then
the rotation of the ball, through friction, will decrease the speed of
the air over the top but increase it over the bottom, again as seen
from the left. The air over the top will therefore exert a greater
pressure than the air over the bottom and there will be a net force
downward besides that of gravity. This force will be the same in the
system in which you watch the ball in motion and therefore, with the
lightness of the ball, a definite curving downwards of the ball. It
has been claimed by some that Bernoulli's equation only holds for
incompressible fluids, but that is not quite right. It holds for any
fluid as long as the fluid is not compressed in the process. In this
case the compression of the air is negligible and the Bernoulli
effect will take place. All this breaks down at higher speeds when
the air does get compressed such in supersonic flight. So as long
as the air speed is not too great the compression of the air is
negligible and Bernoulli's equation can be used.