Aim the hair dryer straight up and turn it on. Gently
place the ping pong ball in the airstream and let it go, you
should be able to keep the ping pong ball floating in the airstream
above the hairdryer. With a little practice, you should be able
to keep the ball in the airstream even if you hold the hairdryer
at an angle!
The jet pushes the ball up and gravitational force pulls it down - so that the ball ends up hovering in a
space where the flow is passing at the ball terminal velocity - the point where the drag of the ball is exactly
the same as the weight of the ball. The more subtle point to note, though, is what happens as the ball moves
to the edge of the flow (when the hair dryer is angled or when the ball is pushed). At the edge of the flow,
one side of the ball will see roughly no flow (atmospheric pressure) whereas the other side will see the high
speed jet flow. In fact, this flow will curve around the ball and therefore the ball will see a sub-atmospheric
pressure on this side (Euler's n-equation). The net pressure difference will push the ball back toward the
center of the flow. This curving flow will also start to rotate the ball - so as the hair dryer is angled,
the ball will start to rotate but will be held in the jet until the gravitational force becomes greater than
this force due to curvature and then the ball will fall out of the stream.