Balloon Rocket Experiment: Newton's Third Law on a String

Ask most people why a rocket moves and you get some version of “the exhaust pushes against the air.” It is wrong, and a balloon rocket experiment on a string is the cheapest way to start dismantling it. The balloon is not pushing on the room. It is throwing air backwards out of its neck, and the balloon goes the other way because something has to.

That distinction matters, because it is the reason rockets work in space, where there is nothing to push against at all.

Key Takeaways

• You will tape a balloon to a drinking straw threaded on a taut string and time it over a measured distance.
• The concept is Newton’s third law, and specifically that thrust comes from the momentum of what you throw away.
• Works from age 5 as a demonstration, from about 8 as a measurement with a stopwatch.
• The thing that goes wrong most: a slack string. A sagging line kills the run before it starts.
• Uninflated balloons and burst fragments are a serious choking hazard for toddlers. Count them in and count them out.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Long party balloons	6 to 10	Supermarket, party aisle	Round balloons work but fly less straight
Drinking straws	5	Kitchen drawer	A 5 cm length of a plastic pen barrel
Smooth string or fishing line	6 m	Hardware shop	Smooth nylon kite string. Avoid fuzzy wool, which drags
Masking tape	1 roll	Stationery drawer	Any tape that peels off a balloon without popping it
Tape measure	1	Toolbox	A metre stick used five times, marked with tape
Stopwatch	1	Any phone	Two kids counting “one-banana, two-banana” is less precise but works
Paper clips	10	Desk drawer	Small coins taped on, for the mass variation
Bulldog clip or clothes peg	1	Stationery drawer	Pinch the neck with your fingers, which is less consistent
Notebook	1	Anywhere	Required for the measurement version

Latex note: party balloons are usually natural rubber latex. If anyone in the house has a latex allergy, use foil balloons or skip this one; a reaction to latex is not something to improvise around.

Time: prep 10 min, experiment 30 min, cleanup 5 min Cost: a bag of balloons and a ball of string Mess level: low

Do the Experiment

Step 1 — Rig a taut line and measure it

Tie one end of the string to a door handle. Thread a 5 cm piece of straw onto it. Tie the other end to a chair back or another handle, pulling it genuinely tight, not just straight. Measure the distance between the two ends and write it down. Five metres is a good length.

Step 2 — Make the rocket

Inflate a balloon and pinch the neck closed, or clip it. Tape the balloon lengthwise to the straw with two short pieces of tape, one near the neck and one near the nose, with the neck pointing backwards along the line. Keep the tape off the neck itself so the opening cannot kink.

Step 3 — Standardise your inflation

Decide how you will measure “full.” Counting breaths works, and so does inflating until the balloon reaches a chalk mark on a wall. Write down the method. Without this, every later comparison is noise.

Step 4 — Release cleanly and time the run

Hold the balloon at one end of the line with the neck pinched. Count down, let go without pushing, and start the stopwatch at release. Stop it when the balloon stops moving forward. Record the time and how far along the line it got.

Step 5 — Three runs per condition

Repeat three times with the same inflation. Average the times, and note the spread between the fastest and slowest. That spread is your measurement error, and you need it before any comparison means anything.

Step 6 — Change exactly one thing

Run three more trials with a narrower nozzle: tape a thinner straw into the balloon neck so the air has to escape through a smaller hole. Then three with four paper clips taped to the straw. Then three with the line tilted uphill by about 20 cm over its length. One variable at a time, always three trials.

What you should see: a full long balloon on a taut 5 m line usually covers the whole distance in roughly one to two seconds, which works out to around 3 to 5 m/s. It accelerates hard at the start and visibly slows as it empties, because thrust drops as the pressure falls. The narrow nozzle gives a slower, longer push and often travels further. The added paper clips slow it noticeably. The uphill line is the one that most often fails to complete the run.

What’s Actually Going On

A balloon is a pressure vessel with a hole in it. The stretched rubber squeezes the air inside to a pressure above the room, and when the neck opens, that air accelerates out through the opening. Momentum is the thing to track. The air leaves carrying momentum backwards, and because the balloon and its air started with zero total momentum, the balloon has to carry the same amount forwards (The Physics Classroom on momentum conservation).

Said the other way, through forces: the balloon pushes the air backwards, so the air pushes the balloon forwards with an equal and opposite force. That is Newton’s third law, and NASA’s aeronautics guide makes the rocket case explicitly, defining thrust as the reaction to accelerating a working fluid out of the back (NASA Glenn on Newton’s third law).

Now the myth. “The exhaust pushes on the outside air” predicts that a rocket cannot work in vacuum, which is false and has been known to be false for over a century. Thrust depends on the mass of propellant you throw per second and how fast you throw it, which NASA’s thrust page states as the mass flow rate multiplied by the exhaust velocity, with a correction term for the pressure difference at the nozzle exit (NASA Glenn on thrust). Ambient air shows up only in that small correction term and in drag, which slows a rocket down. Outside air is a hindrance, not the source of the push.

That equation also explains your nozzle result. A wide opening lets a lot of air out per second, so the thrust is high, but the balloon empties in a fraction of a second. A narrow opening lowers the mass flow rate, so the thrust is smaller, but it lasts much longer. Total momentum delivered is roughly the same either way, since it is set by how much air you put in and how fast it comes out. What changes is whether you get it as a brief shove or a sustained push, and against friction and drag the sustained push usually wins more distance. That is a real engineering trade, and it is the same one that separates a solid rocket booster from an ion thruster.

Friction on the straw and aerodynamic drag on a fat, blunt balloon both eat into the result, which is why your balloon never reaches the speed a frictionless calculation would predict.

How to Explain It to Your Kid

Ages 5–8: Throw something backwards and you go forwards

Sit them on a swivel chair or a skateboard and have them throw a heavy cushion away from themselves. They will roll the other way. That is the entire idea, and they just did it with their body. The balloon throws air instead of a cushion, and air is light, so it has to throw a lot of it very fast.

Ages 9–12: Thrust is how much you throw and how fast

Introduce the two numbers: the amount of air leaving each second and the speed it leaves at. Then give them the nozzle puzzle before you run it. A small hole or a big hole: which goes further? Most kids pick the big hole. Let them be wrong, then let them explain the result themselves, because the explanation is where the learning is.

Ages 13+: Momentum conservation and the vacuum question

Do it properly. Total momentum of balloon plus air is conserved and starts at zero. Write thrust as mass flow rate times exhaust velocity. Then ask the vacuum question directly: if the exhaust needed air to push on, how did the Apollo service module manoeuvre? Work out why outside air appears only as a pressure correction and as drag. This is also the moment to point out that “equal and opposite forces cancel” is false here, because the two forces act on different objects.

The question to ask: “If we did this on the Moon, would the balloon go faster, slower, or the same?” (Faster. No air drag, and a slightly larger pressure difference at the nozzle. If they say it would not move at all, you have found the misconception and the whole experiment just earned its keep.)

Why Teaching This Early Pays Off

“Rockets push against the air” is one of those explanations that survives into adulthood because it sounds reasonable and nobody tests it. Replacing it early costs one afternoon. The bigger payoff is the habit underneath: the balloon forces you to ask what is being thrown and where its momentum went, which is the question that makes jet engines, recoil, octopus propulsion and reaction wheels on satellites all the same problem.

A word on how to run it, though. Handing a kid a bag of balloons and walking away will not do this. Kirschner, Sweller and Clark’s review of minimally guided instruction argues that discovery without explicit guidance tends to underperform direct instruction, particularly for learners who do not already have the relevant knowledge (Kirschner, Sweller & Clark, 2006). The prediction before the nozzle swap, and the conversation after it, is where this becomes teaching rather than entertainment.

Set expectations honestly. One balloon rocket does not build a physicist. It does mean that when a classroom hits Newton’s third law, your kid has a concrete memory to hang it on instead of a sentence to memorise. If they want the louder version, see our guides to model rocketry for families and to water rocket physics for ages 8 to 14.

Before You Start: What Parents Should Know

Best age	5 and up as a demonstration, 8 and up for the timed version. Below 5 the string rig is more hazard than lesson
Supervision	An adult inflates the balloons and rigs the line at adult chest height. Keep the line away from eye level for small children
Real hazards	Choking. Uninflated balloons and burst pieces are among the most dangerous common toys for children under three. Also latex allergy, and a taut string across a room at face height
Don’t	Don’t let a toddler anywhere near the balloons, inflated or not, and don’t tie the line across a doorway people will walk through

Count your balloons at the start and count the pieces at the end. A burst balloon fragment is thin, clingy and exactly the wrong shape for a small airway. Bin them immediately rather than leaving them on the floor.

When It Doesn’t Work

The balloon travels half a metre and stops. The string is slack. Taut means you can pluck it like a guitar string. Retie both ends to something solid, not to a chair that slides.

It judders and sticks. Wrong string. Wool and fuzzy twine grab the straw. Switch to fishing line or smooth nylon, and shorten the straw to about 5 cm so less of it can bind.

It spins on the line instead of flying straight. The neck is off-axis or the tape is pulling the balloon crooked. Re-tape so the neck points directly back along the string, and use two tape points rather than one.

Results scatter wildly between runs. Your inflation is not standardised and your release is pushing. Fix the inflation with a count or a wall mark, and release by opening your fingers rather than letting go with a flick.

The neck collapses and the air dribbles out. The nozzle has kinked, usually because tape was wrapped over it. Keep all tape at least a centimetre from the opening, and for the narrow-nozzle trial use a straw inserted into the neck rather than pinching it.

Take It Further With Your Balloon Rocket Experiment

Map the nozzle trade-off. Use three nozzle diameters: no straw at all, a normal straw, and a narrow coffee-stirrer straw taped into the neck. Three runs each. Record time, distance and whether it finished the line. Plot nozzle size against distance. You are measuring the trade between thrust and burn duration, which is the same choice real propulsion engineers make.

Add mass and find the limit. Tape paper clips to the straw, two at a time, three runs at each mass, until the balloon can no longer complete the line. Plot mass against distance travelled. The curve is not straight, and working out why is a good conversation.

Tilt the line. Raise one end by 10, 20 and 30 cm over five metres and record how far the balloon climbs. You are now measuring thrust against a known fraction of weight, which is the beginning of thrust-to-weight ratio, the number that decides whether a real rocket leaves the pad at all.

What to Watch For Over the Next 3 Months

• Week 4: Your kid explains a jet engine or a firehose kicking back using the words “throws” and “backwards” rather than “pushes on air.”
• Month 2: The vacuum question comes back. Often it arrives as “but how do they steer in space?” which is exactly the right follow-up and has a great answer involving small thrusters and reaction wheels.
• Month 3: They propose a variable you did not run. A second balloon in series, a different gas, a balloon on wheels. Say yes to whichever one is cheapest.

Frequently Asked Questions

Does a bigger balloon always go further?

Not reliably. A bigger balloon carries more air and more momentum, but it is also heavier and has more frontal area, so drag rises. On a short line the extra thrust usually wins. On a long one you often find a sweet spot, which is worth finding by measuring rather than guessing.

Why does my balloon slow down before the end of the line?

Thrust depends on the pressure inside, and that falls as the balloon empties. Once the push drops below friction and drag, the balloon decelerates. You are watching a burn profile, which is the same shape a real rocket motor produces.

Can we use a long thin balloon or does it have to be round?

Long balloons work much better. They are more streamlined and they resist rolling on the string, so your runs are more repeatable. Round balloons will fly but they wobble and your spread between trials gets large.

Does the balloon push against the air in the room?

No, and this is the point of the experiment. The balloon pushes on the air leaving its own neck. Room air only slows it down through drag. A rocket in vacuum works slightly better than the same rocket at sea level.

What age can do this alone?

Treat eight as the floor for unsupervised running, with an adult still doing the inflating. The real constraint is not skill, it is making sure every balloon and every fragment is accounted for when you finish, and that is an adult job.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. NASA Glenn Research Center. “Newton’s Third Law: Action and Reaction.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-third-law-action-reaction/
2. NASA Glenn Research Center. “Thrust.” Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/thrust-force/
3. The Physics Classroom. “Momentum Conservation Principle.” https://www.physicsclassroom.com/class/momentum/Lesson-2/Momentum-Conservation-Principle
4. The Physics Classroom. “Newton’s Third Law.” https://www.physicsclassroom.com/class/newtlaws/Lesson-4/Newton-s-Third-Law
5. Kirschner, P. A., Sweller, J., & Clark, R. E. (2006). “Why Minimal Guidance During Instruction Does Not Work: An Analysis of the Failure of Constructivist, Discovery, Problem-Based, Experiential, and Inquiry-Based Teaching.” Educational Psychologist, 41(2), 75–86. https://doi.org/10.1207/s15326985ep4102_1
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165