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# Bernoulli Balls

A simple hair-dryer becomes a magical levitation device through an understanding of the principles of fluid flow.

## Ingredients:

• hair dryer (make sure you have an appropriate power supply available!)
• small light balls (such as polystyrene balls available at most craft shops, or ping pong balls)

## Instructions

1. Orient the hair dryer so that the outlet is pointing directly upwards. Turn it on.
2. Place a ball carefully in the flow from the hairdryer. It will balance in the air, appearing to levitate!
3. Gently move the hairdryer from side to side – the ball will stay in the air stream, i.e. will also move back and forth. Repeat this process moving the hairdryer up and down.
4. Carefully tilt the hairdryer – the ball will still stay in the airstream, hanging in mid-air with nothing directly underneath it.
5. Try using balls of differing sizes, and challenge your audience to see how many they can place in the airstream at once.

## How Does it Work?

The upward pressure from the hairdryer balances the downward force of gravity, keeping the ball 'levitating'. The more impressive part of this trick is being able to move the ball along with the hairdryer and angle it. The stream of air sticks to the surface of the ball. This is a demonstration of the Coanda Effect, which emerges when fluids like air or water pass over curved surfaces. When the ball approaches the edge of the stream, air is curved round the ball and directed out of the stream. This has the effect of pushing the ball back into the stream. This is the process that enables the ball to balance inside the airstream and stay in the airstream as the hairdryer is moved around.

## Tips for Success

Try to find a hairdryer with a 'cool' setting – it will last longer and allow you to perform the trick for much longer in one sitting, without the hairdryer overheating. Make sure that the balls aren't larger than the output of the hairdryer or it won't work. Tilting the hairdryer to too great an angle will cause the ball to fall out of the airstream – although most audiences enjoy seeing the effect!

## Serving Suggestions

This is a good eye-catching demonstration that can keep audiences amused for a significant period of time – everyone wants to have a go, especially when challenged to make the largest number of balls stay within the airstream. However, it does require a power supply so is generally limited to indoor venues.

## Did You Know?

The Bernoulli Effect underlies the principle of the aerofoil. By encouraging air to flow more quickly over the top surface of a wing an upward pressure is produced by the slower moving air beneath. This phenomenon can also be demonstrated by holding up two sheets of paper and blowing between them. Instead of moving apart, they are drawn together. If you thought anyone could have worked this out, remember that Daniel Bernoulli was awarded his masters degree at the ripe old age of 16.