Things you need:
Set the hair dryer on the cool and high setting. Turn it on and point it upward. Carefully put the ping-pong ball in the stream of air. That step may take a little practice, but don't give up! Hold the hair dryer steady. What happens to the ping pong ball? Now try turning the hairdryer from left to right. Where does the ping pong ball go? How far left or right can you turn the hair dryer before you lose the ball?
How does it work?
You've just seen an example of what's called Bernoulli's principle. Bernoulli, a Swiss scientist who wanted to find out how these things work, discovered this effect more than 250 years ago. He found that the faster air slips past the surface of something, the less the air pushes on that surface (and so the lower its pressure). So, when you place the ball in the stream of air created by the hair dryer, you force the air to flow around the ball and create an area of lower pressure. The still air surrounding the air stream has more pressure and pushes the ball to keep it snuggled in the stream. Gravity pulls the ball downwards while the pressure below the ball from the moving air forces it upwards. This means that all the forces acting on the ball are balanced and the ball hovers in mid-air.
You can make the ball follow the stream of air as you move the hair dryer because, if the ball tries to leave the stream of air, the still, higher pressure air will push it back in - so the ball will float in the flow no matter how you move.
Airplanes can fly because of this principle. Air rushing over the tops of airplane wings exerts less pressure than air from under the wings. So the relatively greater air pressure beneath the wings supplies the upward force, or lift, that enables airplanes to fly.
Things to try:
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