### Naked Scientists - Kitchen Science Dr Hugh Hunt

26 Nov 2006 - Transcript of the live radio show

"the aerodynamics of a ping pong ball"

KITCHEN SCIENCE - The Aerodynamics of a Ping Pong Ball - With Derek Thorne, Hugh Hunt and Nick and Christian from Norwich School.

If you've ever wanted to make a ping pong ball levitate in mid air - this kitchen science is for you. This week Derek and Hugh Hunt are with Nick and Christian from Norwich School trying to investigate how we can make something levitate without using dodgy magic tricks!

To do this experiment, you will need:

A ping pong ball
A hairdryer
This experiment works best if the nozzle on your hairdryer is about the same size as a ping pong ball. If it's not try making a funnel with a drinks bottle. Another thing you could use is a vacuum cleaner with a "blow" setting. Make sure it's on the "cool" setting!

How to do the experiment:

1 - Switch your hairdryer on and point the air stream vertically upwards.
2 - Try to balance the ping pong ball into the air stream.

So what's going on?

The ball levitates in the air stream, a few inches above the hairdryer!

Why?

This is an example of what is called the Coanda effect. Air, when if flows near a curved surface, tends to follow that surface and stick to it. For example, if you blow over the top of a horizontal tube you find that the air from your breath is deflected downwards as it follows the curve of the tube.

In our experiment, air is flowing upwards out of the hair dryer and around the ping pong ball equally in all directions. If the ping pong ball fell out of the flow and moved to the left, then the air around the right of the ball will want to follow the ball and so the air will also tend to move to the left.
There's a very important law called Newton's 3rd law of motion, which says that every action has an equal and opposite reaction. So if air moves around the ball to the left, the ball must be being pushed to the right. So the coanda effect holds the ball in place by pulling it left if it moves right and right if it moves left. That's why the ball moved backwards and forwards a bit but managed to stay levitated in the centre!

In fact you can even tilt the hairdryer and the ball will stay levitated in the angled air stream. What a great magic trick to show your friends!