Balloon Jets

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You need...

  • A very long piece of thin string
  • A straw
  • Some sticky tape
  • A balloon
Tie one end of the string to the highest point you can
find - maybe a tree or a door frame...
Thread a straw onto the other end.
Stick 2 pieces of sticky tape to the middle of the straw.
Blow up your balloon and hold the end tightly.
Stick the balloon to the straw.
..... 5 - 4 - 3 - 2 - 1 - LET GO!
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How it works...

When you let go of the balloon it flies up the string.

You may think this is because the escaping air is pushing against the atmosphere, a bit like a row boat is pushed

forward by the oars pushing back against the water - but you would be WRONG!

It's the air trapped inside the balloon that makes it fly, not the air escaping out of the balloon.


This is what happens...

The pressure of the air in the blown-up balloon (with your fingers holding it tightly closed) is the same in all directions.

But when you let go, there is nothing for the compressed air inside the balloon to push against at the neck of the balloon.

So the air escapes. But the pressure on the fat end of the balloon is still there.

Since there is no counter pressure to balance it, the pressure at the fat end can move the balloon.

So the balloon flies across the room in the opposite direction from the escaping air.


Muddled? This picture may help you...

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Strange as it may seem, this is how all jet engines and rockets work!

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Want to find out more?

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Meet Sir Isaac Newton and make a Balloon Racer