The Magic Ping-Pong Ball
"I'll need an ordinary blow dryer," Skinny says.
You produce your HairGun Pro 3500 hair dryer, and he plugs it into the wall outlet and turns it on full power. Pointing it straight up and holding the ping-pong ball in the air stream, he asks "What will happen when I let go of the ping-pong ball?"
"It will fly up and out of the air stream, then fall. And in under thirty seconds," you reply.
He releases the ping-pong ball, and contrary to intuition, it doesn't fly out of the air stream. It bounces around a bit, but remains fixed in the path of the air flow, over a foot away from blow dryer's nozzle.
"Impressive, but you promised that only air would be between the ping pong and the ground. There's also that hair dryer."
"Correct, but watch."
Skinny slowly tilts the hair dryer to one side, so that instead of pointing straight up, it is at about twenty degrees away from vertical. The ping-pong ball moves as well, as if invisibly tethered to the nozzel. Now nothing but air is between it and the floor.
"Cool," you observe as Skinny pockets your twenty.
OK out there in web land, if you've never seen this experiment performed, you may want to take a break and try it for yourself.
The science behind the magic is known as Bernoulli's principle. Simply put, Bernoulli's principle states that the pressure of a fluid (like water or air) on a surface decreases as its velocity along the surface increases.
The stream of air coming out of the blow dryer is moving quickly relative to the air in the room. Toward the center of the flow, where air is moving faster, the pressure is lowest. The ping-pong ball is pushed by the higher pressure slow moving air into the low pressure fast moving air right in the center.
Gravity keeps the ball from launching up and out of the stream, and the force of the air stream keeps the ball from falling into the blow dryer.
Bernoulli's principle has a number of practical applications in everyday life. Probably the most important is keeping airplanes from crashing to the Earth. If you've ever looked at an airplane wing, you may have noticed that bottom is flat, but the top is sloped. This gives the top a larger surface area. As a plane moves forward, air passes above and below the wing. The air directly over the wing is forced to move faster because of the larger surface area. This means that the air pressure below the wing is higher than the air pressure above it- and this is what keeps the plane up.