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A balloon car made from cardboard, skewers and straws.
Balloon cars are fun and fast!

Turn hot air into motion! You could even make 2 or more of these cardboard cars and run a race.
Safety: sharp hazard iconBe careful with scissors and with the pointy end of skewers. Always ask an adult for assistance if you’re unsure.

You will need

  • Toilet paper roll (or paper towel roll cut in half)
  • Scissors
  • Corrugated cardboard
  • Masking tape
  • 2 straws (paper or plastic)
  • 1-2 skewers
  • Balloon
  • 1 rubber band

What to do

  1. Cut a straw in half (so each piece is about 8-10cm long).
  2. 2 short straws taped to either end of a toilet roll.Tape the straw pieces near each end of the toilet paper roll. Do your best to make them parallel with one another.
  3. Tracing around a lid to make circles shapes on cardboard.Cut out 4 cardboard wheels about 4-6cm in diameter. We recommend using something round to trace the circles, like a jar lid or a drinking glass.
  4. Cut a skewer in half (so each piece is about 12cm long). If your skewer is too hard to cut, use 2 skewers and snap them down to length.
  5. Pushing a skewer through the centre of a cardboard circle.Use the pointy end of the skewer to make a hole in the centre of each cardboard wheel. Be very careful not to poke your fingers – don’t place your fingers near the centre of the wheel where the skewer will come out. Ask an adult to help with this step.
  6. Skewers inserted through straws which are taped onto either end of a toilet roll.Slide one skewer piece into each of the plastic straws taped to the paper roll.
  7. Pushing cardboard circle onto skewer setup.Attach a wheel to each end of the skewers.
  8. Strip of card poking out of toilet roll car.Cut a long strip of cardboard, about 3 cm wide and 15 cm long. Flip the car so it stands on its wheels and tape the strip inside the paper roll so that about 10 cm sticks out of one end.
  9. Yellow balloon tied over the end of a straw.Put a new straw through the mouth of the balloon. Then put a rubber band around the neck several times to hold it tight.
  10. Balloon straw taped to top of car setup.Tape the balloon straw on the top of the car so the balloon sits on the cardboard strip.
  11. Balloon inflated ready to go.Blow into the straw to fill up the balloon and then block the end of the straw. Set the car down on the ground and, when you’re ready, let go and watch it go!

What’s happening?

Wherever there is motion, there is energy! When your car is moving, it’s packed with moving energy, which scientists call kinetic energy. But that energy had to come from somewhere, and originally it came from your breath!

When you blow up the balloon, you expand the elastic rubber, stretching it like a spring. If left to its own devices, the balloon will spring back to its deflated state and release the stored energy. In this car, the energy goes into pushing air out of the straw.

Did your balloon car drive off in the direction you expected? The last step in powering our car comes from Newton’s 3rd law of motion. This law can be written in many ways, but the most famous is, “for every action there is an equal and opposite reaction.”

The balloon pushing air out of the straw is an action. The equal and opposite reaction is the air pushing back on the balloon, in the opposite direction. The same law explains how rockets work. They shoot powerful jets of gas and flame out of the back of their engines to push them forward to space.

Re-engineering

Your balloon car isn’t very powerful, so it’s important not to waste any energy. The wheels and axles can have a lot of resistance if they don’t spin smoothly. What can you do to make them work better?

What purpose does the strip of cardboard (from step 8) serve? Can you redesign your car so that it’s not needed?

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