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What Makes a Balloon Car Zoom? Build Your Own Balloon Racer!

2026-10-30 · Watch on YouTube

Summary

A little car with no engine, no batteries and no fuel zooms across the floor. What is pushing it?

Transcript

0:00The Zoomy Mystery

0:00This little car has no engine, no batteries and no fuel. Not one single drop! Yet when you let go of its balloon, it zooms across the floor like it's late for lunch. So here is the mystery: what is pushing it? 0:13You can't see a hand. You can't see a motor. Something invisible is giving this car a big shove, and by the end of this video, you will know exactly what it is. 0:22But first, we need a clue from a very famous thinker who lived long, long ago.

0:28Newton's Secret Clue

0:28Over three hundred years ago, a scientist named Isaac Newton wrote down some rules about how things move. One of them is our secret clue: every push has an equal push back. Try it yourself. 0:40Press your hand flat against a wall and push. Can you feel the wall pressing back on your hand? It is pushing back just as hard as you are! Wow fact: Newton worked out his rules without electricity, without computers, 0:53and without even a light bulb to read by. Keep that rule in your pocket, because we are about to build a car that runs on it.

1:02Build Your Racer

1:02Here is what you need: an empty plastic bottle, four bottle caps, three straws, two wooden sticks, some tape and a balloon. Step one: tape two straws across the bottom of the bottle, one near the front and one near the back. 1:16Step two: a grown-up snips the pointy tips off the sticks and pokes a small hole in the middle of each cap. Step three: slide one stick through each straw and press a cap onto every end. Those are your wheels! 1:30Step four: tape the balloon tightly onto one end of the third straw, then tape that straw on top of the bottle so its open end points out the back. Wow fact: rolling on wheels is so much easier than sliding that the wheel is one of the most useful inventions people ever made. 1:49Now let's fill it up and see if it really goes!

1:52Ready, Set, Whoosh

1:52Time to fill the balloon. Have a grown-up nearby whenever you use balloons, and let them help fill it up through the straw. Then pinch the straw shut so no air sneaks out. 2:02Set the car down on a smooth floor. Ready? Three, two, one... let go! 2:07Whoosh! The balloon shrinks, air rushes out the back, and the car zooms forward. Wow fact: air might look like nothing at all, but it is real stuff, and it even has weight! So the air goes out the back, and the car goes forward. 2:20But why? Most people make the very same guess, and it is a sneaky one.

2:26Testing the Guess

2:26Here is the guess: the air shoots out, hits the floor or the wall behind the car, and bounces back to push it along, like blowing on a paper boat. It sounds smart, right? Lots of grown-ups think so too! 2:39But let's test it like real scientists. If the air needs a wall to push against, then the car should go slower in the middle of a big empty room, far away from any walls. So you try it right in the middle of the room... 2:52and it zooms just the same! Wow fact: scientists call this kind of test an experiment, and even a wrong guess is useful, because it tells you where not to look. So what is really pushing the car? 3:06Our clue from Newton is about to crack this case wide open.

3:10Pushes All Around

3:10Let's look for Newton's rule out in the real world. When a swimmer pushes off the side of a pool, the swimmer pushes on the wall, and the wall pushes the swimmer away. Zoom! 3:20When someone rows a boat, the oars push the water backward, and the water pushes the boat forward. Wow fact: every time you jump, you push the whole Earth down a tiny bit, and the Earth pushes you up. 3:31The Earth is so enormous that its move is far too small to ever notice! Swimmers, boats, jumpers. Every time, two things push on each other. So in our balloon car, what are the two things? 3:44Get ready, because here comes the big twist.

3:47The Big Twist

3:47Here it is: the two things are the balloon and the air inside it. The squeezy balloon pushes the air out backward, and the air pushes the balloon forward, just as hard. The balloon is taped to the car, so the whole car comes along for the ride! 4:02That means the car isn't being pushed by the floor or the wall at all. It is pushing itself, by shoving air out the back. Wow fact: this is exactly how rockets fly in space, where there is no wall to push on at all. 4:17Rockets shove gas out the back, and the gas pushes the rocket forward. Your bottle car is a tiny rocket on wheels! But can we prove that nothing is needed behind it? Watch this.

4:29Balloon Rocket Proof

4:29With a grown-up's help, stretch a long string between two chairs, and thread a straw onto it before you tie the second end. A grown-up fills a balloon and holds the neck closed while you tape the balloon under the straw. 4:42Now let go, and the balloon races along the string, all the way across the room, with nothing behind it but empty air! No wall, no floor, no hand. Just air going one way and the balloon going the other. 4:54Wow fact: squids swim this way too. They squirt water out backward, and the water pushes them forward, fast. Okay, rocket scientist, now let's make your car even zoomier. What do you think happens with more air?

5:10More Air Test

5:10Test number one: a little air versus a bit more air. Mark a start line on the floor with tape. Run one: a small balloon. Run two: a fuller balloon. 5:21Measure how far your car rolls each time. More air means the push lasts longer, so the car keeps speeding up and rolls farther! Wow fact: the push from a balloon is gentle, but it keeps going the whole time air is coming out, and all that pushing adds up. 5:37Remember to stop filling well before the balloon feels tight, and if a balloon ever pops, a grown-up throws away every piece right away. Next, let's change the car instead of the balloon.

5:50Lighter Car Test

5:50Test number two: the same balloon, but a lighter car. Try a smaller, lighter bottle, or peel off any extra tape and decorations. Heavy car versus light car, with the same push from the same balloon. Which one wins? 6:04The light car zooms faster! The same push moves a lighter thing more easily, just like it is easier to push an empty wagon than a wagon full of books. Wow fact: that is why people who design real race cars work hard to keep them light. 6:19Now you have all the pieces of the mystery. Let's put them together.

6:24Mystery Solved

6:24So here is what you discovered: every push has an equal push back. Your balloon pushes air backward, the air pushes your car forward, and more air or a lighter car makes it zoom even better. Now look at that little car again. 6:39No engine, no batteries, no fuel, and no invisible hand. The answer to our mystery was hiding in plain sight the whole time: the car is pushing itself. Every whoosh of air out the back is a shove forward, and that little 6:54bottle on wheels is really a rocket, small enough to park under your bed.

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Series: science (English)