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Levitating card

By David Shaw, 5 September 2019

To hold a card up in this activity, you blow down on it. Seems strange, huh?

Normally, when you blow on a piece of card, it flies away. But in this activity, you can blow to pull the card towards you with the science of air pressure!

Safety: In this activity, you are placing your mouth on a spool of thread. Make sure it is clean before each use.

You will need

• Wide spool of thread, ours was 2.5 cm across
• Thin card
• Scissors
• Drawing pin
• Pencil (optional)

What to do

1. Look at your spool. There should be a small hole running through the middle of it, about 5 mm wide. If there’s a sticker over the hole at either end, carefully poke through it with a pencil.
2. Hold the spool up to your mouth and blow through the hole. You should be able to feel the air coming out the other end in a thin stream.
3. Cut a square of card, about 7 cm wide.
4. Carefully poke the drawing pin through the middle of the card.
5. Hold one hand out with your palm up. With the pointy end of the pin facing upwards, put the card on the palm of your hand.
6. Put the spool on the card, so the pin goes through the centre hole of the spool.
7. Secure the spool by holding the sides with your other hand as well. Keeping the palm of your first hand flat, bring the items to your mouth so you can blow through the spool’s hole.
8. As you blow through the hole, slowly lower your first hand’s palm. With practice, you’ll be able to do it without the card falling!
9. If you want to give someone else a go, remember to clean the spool so you don’t spread germs.
10. When you’re finished, remember to put the pin away somewhere safe, so nobody pricks themselves!

What’s happening?

Air pressure and flow

To hold a card up in this activity, you blow down on it. Seems strange, huh? Gravity is already pulling on the card, and your breath will push down as well. So what’s pushing the card back up and stopping it from falling?

The answer is air pressure. The air all around us is pushing against every surface it touches, and that force isn’t light. It’s about one kilogram of force on each square centimetre of surface area, pushing down on floors, up on ceilings, and it squeezes a sphere or ball in every direction.

In this activity, air pressure is pushing up on the bottom of the card. It’s also pushing down on the top of the card. Usually, that balances exactly. But your breath travels through the spool of thread, out the bottom, and then moves towards the edges of the card. All the air on top of the card is moving, and that movement makes the difference.

Bernoulli to the rescue

Way back in 1738, a scientist named Daniel Bernoulli realised that when a fluid (such as air) was moving, its pressure decreased. This effect is now known as Bernoulli’s principle, and it explains how the card in this activity can stay up.

Since the air on top of the card is moving, the pressure there is lower. That means there is lower air pressure on top of the card than usual. Meanwhile, air under the card is not moving much, so the air pressure pushing up is normal. The difference in these two forces is enough to keep the card up.

You might think this is just a fun toy, but similar devices are used in high tech factories. Known as a Bernoulli grip, these devices pick up flat objects without touching them. That makes them useful in factories making computer chips or solar panels.

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