What it shows
Air rushing out of the balloon’s neck pushes the balloon the other way along the string. The bigger the balloon, the further it travels. This is Newton’s third law, the same rule that lifts a real rocket.
The science
When the balloon is blown up, the stretched rubber squeezes the air inside. Let the neck go and the rubber pushes the air out backwards; by Newton’s third law the air pushes the balloon forwards with an equal force. The string and straw keep it travelling in a straight line instead of looping around the room. A bigger balloon holds more air and pushes it out for longer, so it travels further. A rocket works the same way, pushing burning gas out of its tail so fast that it is pushed up into space, even where there is no air.
Materials
Cost: under Rs 50. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- Balloons, long ones work best
- About 5 metres of smooth string or fishing line
- A drinking straw
- Tape
- A measuring tape
- Two chairs to tie the string between
Safety
For this project: Balloon pieces are a choking hazard for very young children; keep them away from the face when the balloon is released.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Thread the string through the straw and tie the string tightly between two chairs, pulled taut and level.
Blow up a balloon, hold its neck pinched shut and tape it to the straw with the neck pointing towards one chair.
Slide the balloon to the end nearest its neck, let go and watch it race along the string.
Measure how far it went before stopping and write it down.
Repeat with the balloon blown up half as much and measure again.
Try a round balloon and a long balloon with the same number of breaths and compare the distances.
Tilt the string so the balloon has to climb, and see how much shorter its run is.
Record all your runs in a table and draw a chart of distance against balloon size.
What you should see
The balloon zooms along the string every time, and fuller balloons travel further, often the whole 5 metres. Climbing a slope shortens the run.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1What pushes the balloon forwards?
The air rushing out backwards pushes the balloon forwards with an equal and opposite force.
2Which of Newton’s laws is this?
The third law: every action has an equal and opposite reaction.
3Why does a fuller balloon travel further?
It has more air to push out, so the push lasts longer.
4How can a real rocket work in space with no air?
It pushes its own burning gas out backwards; it does not need air to push against.
Take it further
Fix two balloons to the same straw, one behind the other, and release the second only when the first is nearly empty to make a two-stage rocket.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
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