The big idea
Push air out the back, and the balloon goes the other way.
The stretchy balloon squeezes the air out. The air pushes back on the balloon, and that push sends it zooming!
Every push comes with an equal push back.
When the balloon pushes air out of its neck, the air pushes back on the balloon just as hard. That push, called thrust, points forward, away from the escaping air.
Your turn
Push your hands on the desk, and feel yourself lean back.
Push your hands on the desk, and feel yourself lean back. The desk pushes back on you just as hard. Pushes always come in pairs.
Why does a bigger breath go farther?
More air inside means more air to push out. The push lasts longer, so the balloon goes farther.
A fuller balloon is stretched tighter, so it squeezes harder. It also holds more air, so it pushes for longer. More push for longer means more speed and more distance.
Round or long?
A round balloon pushes hard, but not for long. A long, skinny balloon pushes softer, but for longer, and it slips through the air. Race them and see!
The round balloon squeezes harder, so it starts faster. The long one squeezes more gently, so its air lasts longer, and its narrow front has less air drag. Over a long string, the long one catches up.
From balloon to rocket
A rocket works just like your balloon. It pushes gas out the bottom really hard, so it goes up!
A rocket engine burns fuel to make hot gas and throws it out the nozzle very fast. The gas pushes back on the rocket. When that push is bigger than the rocket's weight, it lifts off.
It even works in space, because the rocket pushes on its own gas, not on the air.
Try this
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- ✓Make the balloon reach the flagPump it up big, then let go.You did it! A big breath gives a long push.
- ✓Win a race with the small breath (can you?)In Race, give one balloon a small breath and the other a big one.Nope, it can't! The small breath runs out of air first. More air, more push.
- ✓Race a round balloon against a long oneGive them the same breath, but different shapes. Who wins?The round one zooms off first, but the long one keeps pushing and wins.
- ✓Make the rocket flyPress Now a real rocket, then Launch.Liftoff! It pushes gas down, so it goes up.
Make one for real
- Thread a string through a drinking straw.
- Tie the string tight across the room, chair to chair.
- Blow up a balloon and pinch the end shut.
- Tape the balloon to the straw. Then let go!
Newton's third law
When one thing pushes on another, the second thing pushes back just as hard, in the opposite direction. The balloon squeezes the air backward, and the air pushes the balloon forward.
How the model works
The rubber squeezes the air inside to a pressure p a few thousand pascals above the room. Air leaves the neck (area A, about 5 mm² ×10) at speed u = √(2p/ρ), so:
As the air runs out, p and F drop. Air drag ½ρCdAv² and the straw rubbing on the string slow it down; the air still inside adds to the mass that has to speed up. A round balloon here squeezes up to 2.4 kPa; a long one stays near 1.1 kPa but holds more air and has a narrower front.