The Bernoulli Effect

• A hair dryer
• Some light balls; ping pong balls or very light foam balls. They need to be smaller than the nozzle of your hair dryer.

1. Turn the hair dryer on to a cool setting and point it upwards.
2. Take your light ball and place it in the stream of moving air. Watch as your ball apparently levitates over the stream of air, and does not get blown off…
3. If you want to see your ball spinning in the air you might want to draw on it.
4. Now tilt your hair dryer at an angle and see how the ball still appears to defy gravity.

The hair dryer produces a fairly uniform stream of air in a column. While the air in the stream is moving reasonably quickly, the air around it is pretty much stationary.

At first glance you might think that the air would simply blow the ball up and out of the air flow (and in fact this does happen occasionally). Or, if the ball is perfectly balanced in the air flow, you might expect the ball to fall if the hair dryer is tilted to one side. Instead, the ball bounces around in the air flow as if it were contained in an invisible cylinder.

The reason the ball levitates is due to the force exerted up on the ball by the moving air, balancing the weight of the ball. This is why you need a lighter ball if you are using a small hair dryer, or if your ball is too light it will get blown off. This balanced force, however, does not explain the invisible cylinder which seems to keep the ball in place as the hair dryer is tilted.

Bernoulli's principle states that faster moving fluids are at a lower pressure than slower moving fluids. So the air flowing out of the hair dryer is at a lower pressure than the still air around it. The higher air pressure around the cylinder of moving air pushes on the ball and keeps it in place if the ball tries to escape. It is relatively stable even when you add another ball.

Be careful not to burn yourself on the hot setting of the hair dryer, and always be careful when using equipment plugged into mains electricity.