This activity is based on Bernoulli’s Principle, named after the Swiss mathematician and scientist Daniel Bernoulli (1700-1782) more than 250 years ago. It states that as a fluid’s velocity increases, the pressure exerted by that fluid decreases.
In this activity, the force of the "wind" from the hair dryer holds the ping pong ball up - but why doesn't the ball fly off to the side whenever the "wind" isn't directly centered on the ball?
If the ball is pushed to one side, the air flowing near the ball must take a longer path to flow all the way around the ball - compared to the straight path of air that is not near the ball. To "catch up" with the air that travels the shorter, straight route, the air flowing around the ball must move faster. Faster moving air has lower pressure, so the side of the ball towards the middle of the air flow has lower pressure on it. This causes the ball to be pushed back into the middle of the air flow whenever it strays towards the edge.
When the tube is placed on top of the ball, the air flowing around the ball and into the tube must follow a longer path, so it must flow faster. The air curving around the top of the ball and into the tube must flow fastest, so that is where the pressure is lowest. The low pressure above the ball causes the ball to be pushed up into the tube. Once the ball is in the tube, air coming in behind cannot flow around the ball, so the flowing air pushes the ball along the tube like a shell being fired from a cannon. The ball zooms up the tube and out the top!
A good rhyme when it comes to air: High pressure to low, high pressure to low, that is where the air tries to go!