Watch opposite directions
Air leaves backward as the balloon rocket gains forward speed.
Balloon Rocket
Release measured air puffs to connect backward exhaust with forward rocket motion.
Grades 3–5 · Physics
Your goal
Fill with 20 g of air and choose an exhaust speed of 10 m/s. Release all the air in measured puffs and compare the forward rocket speed with the backward exhaust.
Reduced motion is on. The controls still update the scene.
Air remaining is 40 g. Rocket speed is 0.00 m/s forward after 0.0 s of release. Release a puff to see the forward response.
An ideal variable mass rocket carries a 100 g empty balloon and carrier. Each puff expels 10 g in 0.1 s at the selected speed relative to the rocket. The model uses the ideal rocket equation. Real balloons have changing exhaust speed, drag and string friction. Arrows represent the last puff, including after the air is empty.
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Learn with Balloon Rocket
Balloon Rocket invites elementary students to explore thrust with measured releases of air. Choose the starting air supply and the speed of the backward exhaust, then release one puff at a time. The balloon shrinks as its forward speed increases, and the feedback reports remaining air, release time and the thrust of the last puff. Children can compare equal puffs at different exhaust speeds or different stages of the release. The activity connects opposite directions of motion with changing rocket mass using an explicitly ideal model of a balloon on a guide string.
What to look for
Compare exhaust speeds
With the same air mass, faster exhaust produces a larger speed gain.
Remember changing mass
As air leaves, the rocket becomes lighter, changing the effect of the next puff.
For teachers and curious learners
Explain how expelled air and exhaust speed produce rocket thrust and speed gain. Ask students to predict what will happen, change one setting, and explain what they observe. Learn gives a starting point. Practice and Challenge offer new goals with the same scene.
An ideal variable mass rocket has a fixed 100 g empty mass. Each action releases 10 g of air over 0.1 s at constant exhaust speed relative to the rocket. Drag and guide friction are ignored.