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Air cannon

By David Shaw, 22 August 2019

Can you hit the target with your air cannon?

Want to reach out and touch something with a puff of air? This tiny air cannon shoots vortexes of air that can be felt from metres away!

Safety: This activity uses a sharp scalpel or craft knife. You will need an adult’s help.

You will need

• Paper cup (if re-using a cup, wash and dry it first)
• Pen
• Scalpel or craft knife
• Thin plastic wrap (we reused packaging)
• Rubber band
• Scissors
• Scrap paper, tinsel, or feathers
• Adult to help

What to do

1. Turn the cup upside down and draw a circle on the bottom, about one centimetre from the rim.
2. Get an adult to carefully cut out the circle with a scalpel or craft knife.
3. Turn the cup the right way up and cover the top with plastic wrap.
4. Hold the plastic wrap on the cup with a rubber band.
5. Carefully cut off the excess plastic with scissors so it doesn’t get in the way.
6. Get some scraps of paper and fold them in half. Then stand them around to use as targets. You could also use feathers or old tinsel.
7. To fire your air cannon, point the hole at a target and tap the plastic wrap. Start about 50 centimetres away and gradually move further back.
8. For strong pulses, tighten the plastic before each tap. With practice, we could use our air cannon to hit a target from two metres away!

What’s happening?

It’s surprisingly difficult to make a gust of air. Without an air cannon, flicking your finger will only push air a centimetre or two. Blowing as hard as we could, we couldn’t move a feather two metres away with just our lungs. So why does this air cannon work so well?

The first step is amplification. Your fingertip is small and doesn’t push much air. When you tap the plastic, you’re suddenly pushing a much larger area, and that means you’re pushing much more air.

A pulse of air escapes the cup, all through the hole. Since the hole is smaller than the plastic, the air speeds up to get out – so you get a pulse of air that’s bigger and faster!

But there’s one more trick to this device. As the short, sharp pulse of air leaves the cup, it sets up a donut-shaped air current called a vortex ring. This air shape is very stable.

Larger examples of vortex rings can travel many metres before they dissipate. Exactly how they work is very complicated, and not fully understood.

Some species of dolphin blow bubble vortex rings and play with them. Even some volcanoes have been spotted blowing rings filled with smoke or steam!

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