Balloon Racers

Challenge

How far can a balloon make your car roll?

Supplies

• • ~4"x5" Polystyrene tray^[1] or corrugated cardboard

  • 4 Kelvin wheels or similar

  • 2 Plastic drinking straws

  • 2 Bamboo kitchen skewers

  • Straw with bendy elbow

  • Balloon

  • Rubber band

  • Tape

  • Scissors

What to do

Did you try the Downhill Racers yet? They are a great experiment to try before making these. The basic racers are the same, but this one has a more advanced propulsion system.

• • Tape your straws (bearing) on the bottom of the polystyrene or cardboard chassis

    • • One towards the front

      • One towards the back

      • Make sure the straws are parallel

  • Remove the pointy ends of the skewers

  • Put a wheel on the end of a skewer

  • Run the skewer (axle) through a straw

  • Place a wheel on the other end of the skewer

  • Rubber band the balloon to the bendy end of the straw

  • Tape the bendy straw to the top of the racer with the long section pointed back

  • Inflate the balloon by blowing through the straw

  • Set the racer down and watch it drive

Inquiry questions

• • What happens if you put more air in the balloon?

  • Can you reduce the friction or rubbing between the wheels and the chassis?

  • Does adding balloons help?

  • Can you increase the pressure from the balloon?

What’s happening?

Like the Downhill Racers, the Balloon Racers start with potential energy and convert it to kinetic energy to make them go. This racer uses elastic potential energy rather than gravitational potential energy which means how stretchy the balloon is governs how far the racer can go. The other change is related to air resistance or drag. The larger you inflate the balloon, the more energy you give the racer, but you also increase the cross sectional area. This means the racer must push more air out of the way to move forward. This creates a problem, how can you balance making the balloon bigger for more energy without increasing the drag? There are many solutions to this problem. If you come up with one, let us know. One that a visitor found recently was to put one balloon inside another. This made the balloon more difficult to inflate and increased the elastic potential energy without increasing drag. Can you find others?

Footnotes

• 1. Jump up ↑ These are the stiff, foam trays supporting meat (or sometimes vegetables) at the grocery store. Most grocers are happy to give you a few unused ones for doing experiments like this.