Experiment 6: Balloon Rocket — Newton's Third Law

A balloon rocket is Newton's third law you can hear: the balloon shoves air backwards, and the air shoves the balloon forwards. In this experiment you will fly balloon rockets along a string track, measure how air amount affects distance and speed, and learn why the rocket slows down before it stops instead of cruising to the end of the string.

The physics behind the practical

Newton's third law: when object A exerts a force on object B, B exerts an equal force back on A in the opposite direction. The pair here is balloon and air. Stretched rubber squeezes the air inside; released, the balloon pushes air out of the neck (action), and the escaping air pushes the balloon the other way (reaction). Real rockets work identically, throwing exhaust mass backwards to gain forward motion — no pushing against the ground or the air is involved.

More air means more mass to throw and a longer-lasting push, so fuller balloons go faster and farther — but not without limit: a bigger balloon also has more drag, and the string adds friction. The thrust also is not constant: as the balloon empties, the pressure falls, the jet weakens, and drag plus string friction eventually win, which is why the rocket decelerates into the last part of its run. Average speed from stopwatch data is simply v = distance ÷ time, and it hides this rise-and-fall — one reason scientists prefer speed measured over a short middle section.

Variables and controls

In the virtual lab

The sim's single control is Air in balloon, with a Release! button and readouts for Distance and Top speed. Run it at 60% first and watch the jet blast backwards as the balloon accelerates forwards — action and reaction drawn for you. Then run 20% and 100% and compare the three distances on the chart, where the orange dot marks your latest launch against the distance-vs-air curve. Notice two things the readouts let you check: top speed grows with air amount, and the balloon visibly decelerates as it empties, because the push fades while drag does not. Log each launch (air %, distance, top speed) in the Observation Log — three launches minimum, and you have the virtual version of your bench data.

Hands-on: the string track

Thread about 4 m of string through a straw and tie it taut between two chairs. Inflate a balloon, pinch the neck (do not tie it), tape the balloon to the straw with the neck pointing backwards, slide it to one end, and release. A partner times the flight while you mark where it stops; measure the distance. Three trials half-inflated, three fully inflated — inflating "to the same size" by eye is unreliable, so wrap a tape measure loosely around the widest point and inflate to a target circumference. Keep the flight path clear and never inflate a balloon near your face.

A worked investigation

One group's results, with speed computed per trial:

Step 1 — Mean speeds. Half: (2.00 + 2.12 + 2.06) ÷ 3 = 2.06 m/s. Full: (2.76 + 2.82 + 2.79) ÷ 3 = 2.79 m/s.

Step 2 — Mean distances. Half: (3.2 + 3.6 + 3.4) ÷ 3 = 3.4 m. Full: (5.8 + 6.2 + 6.0) ÷ 3 = 6.0 m — note the string is only ~4 m of free run between chairs in many setups, so this group used a longer hallway track; check your own track length before promising a distance.

Step 3 — Interpret. Doubling the air raised mean distance by 2.6 m and mean speed by about 35%. Ranges within conditions (0.4 m and 0.17 s worst case) are far smaller than the difference between conditions, so the effect is real for this setup.

Step 4 — Uncertainty. Stopwatch reaction error of ±0.2 s on a 1.65 s flight is ±12% — the dominant error, dwarfing the tape measure's ±1 cm. That is why the sim's top-speed readout and slow-motion phone video both exist: timing short, fast flights by hand is poor metrology, and honest evaluation says so.

Conclusion and evaluation

More air gave more distance and more speed, exactly as the third law predicts: more mass ejected, and ejected for longer. The rocket stopped because thrust faded as the balloon emptied while drag and string friction did not — when the backward forces won, it decelerated to rest. The weakest part of the method is judging "half" and "full" inflation; circumference targets fix most of it. A subtler flaw: the same balloon reused all session slowly stretches, so later "full" balloons actually hold more air — rotating two or three balloons, or resting them between rounds, controls that drift.

Safety

Never inflate balloons near your face or ears, stop if a balloon looks overstretched or damaged, beware latex allergies in the group, and keep the taut string at a height where nobody can walk into it.

Common mistakes

• Sagging string — a slack track adds friction and sideways swing; tie it drum-taut.
• Judging inflation by eye — "half" drifts between trials; measure circumference.
• Timing the whole flight by reflex — reaction error of ±0.2 s can exceed the difference you are hunting.
• Tape in different places or covering the neck, changing both mass balance and the jet direction.
• Blaming the third law when the balloon spirals — a twisted straw mount aims the thrust off-axis; that is engineering, not physics failing.

Exam Experience

Question 1 — analysis [4 marks]. A student flies a balloon rocket three times with the same inflation. The distances are 4.2 m, 4.6 m and 4.4 m, with times 1.9 s, 2.0 s and 2.0 s. Calculate the mean distance and the mean speed, and comment on the consistency of the trials.

Scoring guide:

• Mean distance = (4.2 + 4.6 + 4.4) ÷ 3 = 4.4 m [1]
• Speeds: 2.21, 2.30, 2.20 m/s; mean speed ≈ 2.24 m/s [1]
• Range of distances = 0.4 m, small compared with the mean [1]
• Trials are consistent (scatter under 10%), so the mean is a reliable summary [1]

Question 2 — evaluation and design [4 marks]. A group wants to test whether nozzle size (the balloon neck taped more open or more closed) changes the rocket's distance. Design the test, identifying the variables and one improvement to their timing method.

Scoring guide:

• Independent variable: nozzle opening (defined, e.g. neck taped to 1 cm vs 2 cm diameter); dependent: distance (and/or time) [1]
• Controls: same balloon model inflated to the same circumference, same string track, same tape positions [1]
• Three or more trials per condition with means and ranges compared [1]
• Timing improvement: slow-motion video timing, timing gates, or timing only a middle section — anything that cuts reaction-time error [1]

Multiple choice 1. The action–reaction pair in a balloon rocket is:

• A. The string pulls the straw; the straw pulls the string
• B. The balloon pushes air backwards; the air pushes the balloon forwards
• C. Gravity pulls the balloon down; the string holds it up
• D. The tape sticks to the straw; the straw sticks to the tape

Answer: B. Third-law pairs act on different objects in opposite directions: balloon on air, air on balloon.

Multiple choice 2. The rocket slows and stops before the string ends mainly because:

• A. Newton's third law stops working at low speed
• B. The balloon runs out of air so thrust fades, while drag and string friction keep opposing the motion
• C. Gravity increases along the string
• D. The air inside turns into a vacuum

Answer: B. Thrust depends on air still escaping; once it fades below the resisting forces, the net force is backwards and the rocket decelerates.

Going deeper: thrust is momentum per second

Strip the balloon rocket to bookkeeping: the balloon throws air backwards; every gram thrown carries momentum, and the balloon collects the equal opposite share. Put numbers on it: suppose the balloon ejects 2 g (0.002 kg) of air at about 10 m/s. The air's backward momentum is 0.002 × 10 = 0.020 kg·m/s, so the balloon-plus-straw (mass about 10 g) gains 0.020 ÷ 0.010 = 2.0 m/s of forward speed if it all happened at once. Real balloons eject over a second or so, and drag and string friction nibble the gain continuously — landing the observed top speeds (around 2.8 m/s for a full balloon) in exactly the right neighbourhood. The same ledger scales up without alteration: a chemical rocket throwing tonnes of exhaust at kilometres per second is this balloon with better plumbing.

That is also why the rocket's speed does not grow without limit even while air remains. Drag grows roughly with the square of speed: double the speed, quadruple the air resistance. Early in the flight, thrust dwarfs drag and the balloon accelerates; as speed climbs and the balloon empties (thrust falling), drag catches up, the net force crosses zero at the top speed, and then the resisting forces — the only ones left — bring it to rest. The sim's separate "top speed" readout exists precisely because the average hides this rise-and-fall shape; whenever a motion has a story, a single average is a summary, not an explanation.