A stretched rubber band is a great source of elastic potential energy. When released, that energy is converted to kinetic (motion) energy as the rubber band snaps back to its original size and shape. How can we tap into this energy source? Let's try using it to power a small car! (Adult supervision required.)

Build a Rubber Band Car

What You Need:

Buy our Balloon and Rubber Band Racecar Kit to get everything you need to build a rubber band car and a balloon car.

Or, find the following materials:

  • Car body: sturdy cardboard, foam board, or balsa wood
  • Straws
  • Axles: wooden dowel that fits inside the straws
  • Wheels: use plastic bottle caps, film canister caps, foam board, toy wheels such as K'nex, wooden wheels from a craft store, etc.
  • Rubber bands
  • 2 small cup hooks

What You Do:

the underside of the car

  1. Cut a six-inch length of balsa wood to be the car body, or chassis. Cut a 1×1-inch notch out of one end. Your rear axle will be accessible through this notch.
  2. Cut two lengths of straw the same width as the chassis and glue or tape them across the chassis near each end. Trim away the center of the rear piece where it stretches across the notch in the chassis. Make sure the straws are lined up straight, or else the car won't roll straight. (The picture to the right shows how it should look underneath the car.)
  3. Cut the dowel into two pieces, each one inch longer than the width of the chassis. Thread the dowels through the straws to create your axles. Slide a cup hook onto the rear axle so that it fits tightly in the center of the notch in the chassis. The screw will stick up to catch the rubber band. (If you don't have the right-size cup hook, you can make a catch by wrapping an unfolded paperclip around the axle and securing it with hot glue.)
  4. Attach your wheels to the axles. The wheels need to be a tight fit on the axle; if they aren't, you may want to use hot glue to hold them in place.
  5. Screw a small cup hook into the chassis just behind the front axle.
  6. Choose a large rubber band to power the car. Loop one end on the hook at the front of the car, and loop the other end over the catch on the rear axle.
  7. Your car is ready to roll! Turn the rear axle several times to wind the rubber band around it, set the car on a smooth hard surface, and let go! (Note: if your wheels are smooth, you might need more traction for the car to operate properly. Wrap rubber bands or loops cut from a small balloon around the rear wheels to add traction.)

What Happened:

The more you twisted the rubber band around the axle, the more potential energy you built up. When you let go, the rubber band snapped back to its original form, spinning the axle in the process. The potential energy in the stretched band was converted into kinetic energy propelling the car forward!

There are many ways you could change your car design to make it go faster or farther. Experiment with different types of wheels. Will the car go farther if you use bigger wheels, or wheels with more or less friction? What if you use bigger wheels in the back and smaller in the front? Or a 3-wheeled design? Try building a car with CDs for wheels. Does the weight of your car affect how it travels? Try adding a load like coins or washers to the car and see how it changes the distance or speed. What happens if you make your chassis longer? If you give your car a ramp to start on, how much further will it travel?

Try building two cars with different features and race them against each other! Keep reading to see how you can build another type of vehicle that demonstrates principles of physics!

balloon hovercraftBalloon Hovercraft

Try this very simple project to create a floating disc that skims across a surface similar to the way an air hockey puck or hovercraft does.

What You Need:

  • Blank CD or CD you don't want any more.
  • Pop-top cap from a water bottle or dish soap bottle
  • Balloon
  • Hot glue gun

What You Do:

  1. Use the hot glue gun to carefully glue the bottle cap over the center hole of the CD and let it set. Make sure the edges are fully sealed.
  2. Push the pop-top cap closed. Blow up the balloon, then hold it so that no air escapes, but don't tie it off. Stretch the mouth of the balloon over the bottle cap (you may need an assistant to help you do this so that you don't lose any air from the balloon). Now adjust the balloon so that it stands up straight and centered.
  3. Set the hovercraft on a hard, smooth table and open the pop-top; then nudge the device along and see what happens.

What Happened:

A hovercraft works by forcing air out beneath it, creating a cushion of air to float on. Hovercrafts usually have a “skirt” that surrounds the base to contain the air; in this project the CD is light enough that it doesn't need a large cushion, so no skirt is necessary. The balloon acts as a pressurized gas chamber. When you open the cap, the balloon forces air out through the cap, creating a thin cushion of air beneath the CD.

As you nudged your hovercraft around, you may have noticed that it zipped along the surface like an air hockey puck. That's because air hockey uses the same principle, with the puck floating on a layer of air. In the case of an air hockey table, the air is forced out from the table below rather than a source above like a hovercraft. Try pushing a plain CD across the table, and then your hovercraft. Do the two move differently? That's because the thin cushion of air from the hovercraft reduces the friction between the CD and the table. Because of the reduced friction, hovercrafts can reach higher speeds.

Experiment ideas:

  • Test to see if your hovercraft works differently if you open the cap only part way instead of all the way.
  • Try different sizes of balloons. Does the hovercraft run longer on a larger balloon?
  • Hovercrafts work best on smooth surfaces so the air can spread evenly, but experiment with yours on several different surfaces to see how it behaves. Does it work on a sidewalk or carpet?

More Physics Projects: