The science idea
Blow up a balloon, let go of its neck and it darts around the room. The balloon is not pushing against the floor or the wall; it is pushing against the air that escapes from it. In this project you put the balloon on wheels and point its jet backwards through a straw, so the car travels in a straight line and you can measure what it does.
The science is Newton’s third law of motion: whenever one object pushes on another, the second pushes back on the first with a force of the same size in the opposite direction. The balloon (together with the car) pushes the air backwards, and the air pushes the car forwards. The two forces are equal in size, but they act on two different things, one on the air and one on the car, so they never cancel each other out. The car accelerates because the forward force acts on the car, and the air accelerates because the backward force acts on the air.
Why does the balloon push the air out at all? The stretched rubber squeezes the air inside it, so the pressure inside is a little higher than the pressure outside. Everywhere there is a wall, the air pushes on the wall and the wall pushes back, and these pushes balance. At the nozzle there is no wall: the air is free to go, and it leaves. The wall on the opposite side of the balloon still gets pushed, and nothing balances that push, so the whole balloon is pushed forwards. This is exactly how a rocket works, and rockets work in empty space, where there is no air to push against, because the thrust comes from throwing mass out of the back, not from pushing on the surroundings.
You can also see it as conservation of momentum. Before you release the nozzle, the car and the air are both at rest and the total momentum is zero. Afterwards the air has backward momentum (its mass times its speed), so the car must gain the same amount of momentum forwards to keep the total at zero. A lighter car gains more speed for the same amount of air, which is why every gram you save helps.
The car does not go on for ever because the push only lasts while air is leaving. After that, friction in the axles, rolling resistance of the wheels and air drag slow it down until it stops. Newton’s second law, force equals mass times acceleration, tells you what to improve: a bigger forward force, a smaller mass, or less friction will each make the car go farther. That makes this a good project for fair-test experiments, where you change one thing at a time and measure the result.
Materials and tools
Cost: Free
Everything is waste or household material. If you have no balloon, one costs about Rs 2 to Rs 5.
| What you need | How many | Free substitute or where to find it |
|---|---|---|
| Corrugated cardboard (stiff) for the chassis | 1 piece, 20 cm × 10 cm | Cut from any carton or shoe box. Choose a thick, stiff piece that does not bend. |
| Plastic bottle caps (all the same size) for wheels | 4 | Collect from water, cold-drink or milk bottles. Four caps from the same type of bottle. |
| Wooden skewers or thin stiff sticks for axles | 2 skewers, cut to 14 cm each | Thin, round, straight, stiff sticks, such as the sticks of an old coconut-leaf broom. Flat ice-cream sticks do not work as axles. |
| Drinking straws for the axle sleeves (they must be wider than the skewer) | 2 pieces, each 10 cm long | Straws from juice cartons or cold drinks. |
| Balloon (a round one, normal size) | 1, plus 1 spare | Reuse a balloon from a birthday party. It must still stretch well. |
| Straw for the nozzle (a different, narrower straw if you have one) | 1, about 12 cm long | Any straw. A bendy straw works once the bend is cut off. |
| Cellophane tape or masking tape | 1 roll | Parcel tape saved from a delivery box. |
| Rubber band (to seal the balloon on the nozzle) | 1 small | A thin strip of cycle tube, or a few turns of tape. |
Tools
- Scissors (an adult cuts the skewers and pierces the caps)
- A sharp nail or an awl and a small piece of scrap wood (adult help)
- Ruler and pencil
- Tape measure or a 1 metre ruler
- A watch or phone stopwatch
Safety first
Read this before you start
- An adult must cut the skewers and make the holes in the bottle caps. Skewer tips are sharp: snip or file them blunt before you start.
- Blow up the balloon gently through the clean nozzle straw, and stop if you feel dizzy. Do not hold a balloon in your teeth.
- Keep the balloon and balloon pieces away from small children: a burst balloon can be swallowed and block the breathing pipe.
- Test the car on the floor, never on a table edge, a staircase or a road.
- People with a latex allergy should use a non-latex balloon or ask an adult to inflate it.
Parts and layout
These drawings show the parts and how they fit together. Keep them beside you while you build.
Scroll sideways to see the whole diagram.
Scroll sideways to see the whole diagram.
Step-by-step build
12 steps. Do them in order, and test each part before you go on to the next.
Mark and cut the chassis
Draw a rectangle 20 cm long and 10 cm wide on the corrugated cardboard and cut it out. Cut it carefully so the long edges are straight and the corners are square.
Tip: Press on the cardboard: if it bends easily, glue two layers together so the chassis does not flex when the balloon pushes.
Mark where the axles will go
On the underside, draw two lines across the width, each 3 cm in from the front and the rear edge. Check that both lines are exactly 14 cm apart, measured from the front line to the rear line (20 cm minus 3 cm minus 3 cm).
Common mistake: Measuring the two lines from only one edge. Measure from both ends so the axles are parallel to each other and to the short edges.
Tape the axle sleeves in place
Cut two straws to 10 cm, the same as the chassis width. Lay each straw on a pencil line and tape it down along its whole length so it cannot twist.
Tip: Run the tape on both sides of each straw. A sleeve that wobbles makes the wheels wobble.
Cut and check the axles
An adult cuts two skewers to 14 cm. Slide one through each straw sleeve: it should turn freely, and about 2 cm should stick out at each end.
Common mistake: Using a skewer that is too thick. If it sticks, pick a thinner stick or a wider straw. The axle must spin with almost no effort.
Make the wheels
An adult pushes a nail through the exact centre of the flat top of each bottle cap, on scrap wood. Twist the nail to make the hole just large enough for the skewer to be pushed through firmly. Make four wheels.
Tip: Wind one turn of tape around the skewer end before pushing it into the cap. The wheel then grips the axle, so the axle and wheels turn together.
Fit the wheels
Push one cap onto each end of each skewer, flat side outwards. Leave a gap of about 2 to 3 mm between the cap and the chassis so the wheel does not rub.
Common mistake: Pushing the wheels right up against the cardboard. The wheel rubs, slows the car and may stop it.
Spin test
Turn the car upside down and spin a wheel with your finger: it should run for several seconds. Now push the car along the floor: it should roll straight for at least a metre with no balloon.
Tip: If it veers to one side, check that both straws are parallel and that the wheels are the same size. Fix this now; it is easier before the balloon goes on.
Make the nozzle
Cut a straw about 12 cm long. Push about 2 cm of it into the neck of the balloon. Wrap the neck tightly onto the straw with a rubber band or several turns of tape so that no air escapes round the sides.
Common mistake: Leaving a leak at the neck. If you can feel or hear air, rewrap it until the only way out is through the straw.
Mount the balloon and nozzle
Lay the balloon along the middle of the chassis with the nozzle pointing backwards, and tape the straw to the chassis so that about 3 cm sticks out past the rear edge. Tape the straw in two places so it points straight back along the centre line.
Tip: Keep the nozzle straight and level with the ground. A nozzle pointing up or sideways pushes the car up or round in a circle.
Inflate and launch
Blow into the end of the straw to inflate the balloon (about 3 to 5 gentle breaths). Pinch the end of the straw shut, put the car on the floor, and let go.
Tip: Do not over-inflate. A balloon that is only moderately full works and does not burst. Use the same number of breaths for every run so the test is fair.
Run three trials and measure
Mark a start line with tape. Release the car, and measure how far it travels from the line to where it stops, and the time from release until it stops. Do three runs and write them in the table. Speed = distance divided by time.
Tip: Use the same floor each time. A tile floor and a carpet give very different results, and that is an experiment in itself.
Improve and test one change
Change only one thing: a longer or shorter straw, a lighter chassis, a slightly bigger balloon, or a dab of cooking oil in each sleeve. Run three more trials and compare the averages.
Common mistake: Changing two things at once. If you do, you cannot tell which one made the difference.
How it should look when it is finished
A flat cardboard car about the size of a school geometry box lid, running on four equal bottle-cap wheels. A balloon lies along its middle with a straw pointing out of the back. When you release it the balloon shrinks while the car rolls forward in an almost straight line for several metres, with a thin hiss of air at the back.
This is a description, not a photograph. Compare your own model with it and with the figures above.
How it works
The balloon is a small pressure vessel. The stretched rubber squeezes the air, so the pressure inside is higher than the pressure of the air around the car. When the pinch on the straw is released, this extra pressure drives the air out of the only opening, the nozzle, at high speed in a backward direction.
To throw the air backwards, the car must push it backwards, and by Newton’s third law the air pushes the car forwards with an equal force, called thrust. The thrust acts on the balloon and through it on the chassis, so the whole car accelerates. The thrust is largest at the start and drops as the balloon empties and its pressure falls.
The wheels turn freely on the axles inside their sleeves, so only a small friction force acts against the motion. When the balloon is empty, there is no thrust left and friction and drag slow the car until it stops.
Scroll sideways to see the whole diagram.
Troubleshooting
| What you see | Likely cause | What to do |
|---|---|---|
| The car does not move at all | The wheels rub on the chassis, the axles are stiff in the sleeves, or the balloon leaks at the neck. | Do the spin test again. Widen the gap between wheel and chassis, use a wider sleeve or thinner axle, and re-seal the neck. |
| The car veers to one side | Axles are not parallel, wheels are different sizes, or the nozzle is not on the centre line. | Re-measure the sleeve positions from both edges, use four identical caps and tape the straw along the middle. |
| The car jumps up or spins round | The nozzle points upwards or sideways, or the straw is loose. | Tape the straw flat along the centre line, level with the chassis. |
| The balloon empties but the car only rolls a short way | Too much friction or too heavy a car. | Trim the chassis, remove extra tape, and place a drop of oil or a smear of soap in each sleeve. |
| The wheels slip on the axle | The hole in the cap is too big. | Wrap a turn of tape around the skewer end or fill the hole with a little glue. |
| The balloon bursts or will not inflate | It is old and weak, or blown too big. | Use a fresh balloon and stop at a moderate size. |
Test and record
Do three trials for each version of the car. Use the same number of breaths each time and the same floor. Speed = distance ÷ time.
| Trial | Change made (for example: none, longer straw) | Distance (cm) | Time (s) | Speed (cm/s) |
|---|---|---|---|---|
| 1 | ||||
| 2 | ||||
| 3 | ||||
| Average |
Copy the table into your notebook and write each reading as you take it, in pencil, with the date. Repeat every test at least three times and use the average. Then draw a graph or chart of your results.
Viva questions and answers
A teacher or a judge will ask you how your project works. Practise these eight questions and answer them in your own words.
- Which law of physics explains how the balloon car moves?
Newton’s third law of motion: for every action there is an equal and opposite reaction. The car pushes the air backwards and the air pushes the car forwards.
- The action and reaction forces are equal. Why do they not cancel out?
They act on different objects. One acts on the air and the other acts on the car. Forces only cancel if they act on the same object.
- Would the car work in space, where there is no air?
Yes. The car (or rocket) pushes the escaping gas backwards, and the gas pushes it forwards. It does not need outside air to push against.
- Why does the car stop after the balloon is empty?
The thrust has stopped, and friction in the axles and wheels and air drag still act. The net force is backwards, so the car slows down.
- What happens if you make the car heavier?
For the same thrust the acceleration is smaller (a = F / m), so it speeds up less and usually goes a shorter distance.
- Why must the axles be parallel?
If they are not, the wheels point in slightly different directions and the car is pushed to one side, so it veers off its straight line.
- What did you change in your experiment, and what did you keep the same?
Give your own answer. A fair test changes only one thing (for example, the straw length) and keeps the balloon size, floor and car mass the same.
- Where is the same idea used in real life?
Rockets and jet engines throw gas backwards. A garden hose jerks backwards when the water comes out fast, and a squid or jellyfish moves by squirting water.
Written report outline
Use these headings in your project file, and write each part in your own words. Add a title page, your diagrams, the precautions you took and the books or sites you used.
- Aim
- To make a car driven by escaping air from a balloon and to show Newton’s third law, by measuring the distance and speed of the car.
- Apparatus
- Cardboard, four bottle caps, two skewers, two straws, a balloon, a nozzle straw, tape, ruler or tape measure, stopwatch.
- Procedure
- Describe how you built the chassis, axles and wheels, fixed the balloon on the nozzle, and then ran the car on the same floor three times, writing the distance and time each time. Add the labelled parts diagram.
- Observation
- The table of distance, time and speed for each trial, and for each change you tested.
- Result
- The car moved forwards when air escaped backwards from the balloon. Give the average distance and speed for each version of the car.
- Conclusion
- The force of the air pushing back on the car is equal and opposite to the force of the car on the air, so the car moves forwards (Newton’s third law). State which change improved the distance and why, using friction and mass in your explanation.
Take it further
- Fair test: measure how distance changes with the number of breaths (2, 3, 4, 5). Plot a graph with breaths on the x-axis and distance on the y-axis.
- Compare nozzle diameters: a thin straw, a wide straw, and no straw at all. Explain your results.
- Make the car lighter and heavier (add coins) and plot distance against mass.
- Build a two-balloon car, and test whether double the push means double the distance.
- Make a balloon-powered boat from a milk-carton and see how water resistance changes the result.
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