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The balloon rocket

Rockets and landing · level 1, lesson 2 of 6

A question to start

Blow up a balloon and let go of it without tying the end. It zooms round the room. Nobody pushed it. What made it go?

The idea

Make a balloon rocket. You need a long piece of string, a drinking straw, sticky tape and a balloon.

  1. Thread the string through the straw and tie the string across the room, tight, between two chairs.
  2. Blow up the balloon and hold its end shut with your fingers. Do not tie it.
  3. Tape the balloon to the straw, with the open end pointing along the string.
  4. Let go!
air rushes outballoon goes

The balloon shoots along the string. The stretched rubber squeezes the air inside, and the air rushes out of the open end backwards. As the air is pushed out backwards, it pushes the balloon forwards. The faster the air rushes out, the harder the push.

That is exactly how a real rocket works. A rocket does not push on the ground or on the air. It throws stuff out of its back, very fast, and that pushes the rocket the other way. A real rocket burns fuel to make hot gas, and the hot gas rushes out of the engine.

Worked examples

1. A bigger balloon. Blow the balloon up more and it goes farther along the string: more air to push out, for longer.

2. Letting go with a pinch. If you let the air out slowly by pinching the end, the balloon moves slowly. A big open end lets the air out fast, and the balloon zooms.

Try it

  1. Which way does the air go when you let go of the balloon? Which way does the balloon go?
  2. What happens if you blow the balloon up less?
  3. How is a real rocket like the balloon?
  4. Point the balloon's open end the other way. Which way does it go now?
Answers
  1. The air goes backwards; the balloon goes forwards, the opposite way.
  2. It does not go as far.
  3. Both push stuff out of the back to move forwards: air for the balloon, hot gas for the rocket.
  4. The other way: always opposite to the air.

Common mistake

"The air pushes on the room behind it." The balloon works because it pushes its own air out. It would work in a room with no air at all. That is why rockets work in space.

Where it leads