Bottle Racer

1. Drill a 1/4-inch (1.25-cm) hole in the center of the cap. Remove the cap for now. You’ll need it again in Step 6.
2. Drill a 1/4-inch (1.25-cm) hole in the center of the bottom of the bottle. If there’s a thick blob of plastic in the way (often remaining from the manufacturing process), insert a piece of PVC pipe into the bottle to help keep it in place when it’s upside down (click to enlarge the photo below) and use the hammer and nail to make a starter hole. Then you can use the drill to finish the hole. Ideally, the hole should be in the center of the bottom, but slightly off-center is okay.   
   
3. Bend one jumbo paper clip to make a hook as in the photo below and set it aside. You’ll need it in a moment.
   
4. Loop the three rubber bands together as shown in the photo below, pulling them tight to form a three-band chain. Then thread the end of the rubber-band chain onto the other paper clip.
   
5. Push the free end of the rubber-band chain up through the hole in the bottom of the bottle, and then feed the rest of the chain into the bottle. If necessary, use the straight end of the paper-clip hook to help push the rubber bands through the hole.
6. Pull the top end of the rubber-band chain out the mouth of the bottle, either by reaching in with a finger (if possible), or by using the paper-clip hook. When the end of the rubber-band chain emerges from the top of the bottle, grab it with your fingers and thread the end loop through the bottom of the bottle cap and then through the washer (see photo below).
   
7. Screw the cap back onto the bottle while still holding the end loop, and then push about 3 inches (7 cm) of the straw through the loop. The rubber band will hold the straw in place against the washer and bottle (see photo below).
   
8. Check that the paper clip attached to the rubber band at the other end is held flush and centered against the bottom of the bottle, and doesn’t stick out past the edge of the bottom (see photo below).
   

Use your finger to wind up the rubber band—try starting with around 25 turns. Put the bottle on the floor, release it—and watch it go! (If it doesn't go, try winding a few more turns.)

If you wind the rubber band tighter, does the bottle go farther? (You might try counting the number of turns you wind the rubber band, so that you can compare.)

The bottle may tend to slip on some slick surfaces. If this happens, try wrapping masking tape around the bottle near the top and bottom, or putting a rubber band around the bottle near the top and bottom to improve traction.

Pick up the bottle, wind the straw, and then release the straw while holding the bottle. What happens? Pick up the bottle and wind the straw again, but this time release the bottle while holding the straw. Now what happens?

Keep playing and experimenting. For more ideas on things to do, see the Going Further section below.

Your Bottle Racer may be simple, but it shares an important similarity with larger, more complicated vehicles: It converts stored potential energy into kinetic energy, the energy of motion.

As you wind up the rubber band with the straw, you do mechanical work, applying a force over a distance. The mechanical work done in winding up the rubber band is stored in the rubber band as spring potential energy. When you let the bottle go, the potential energy is released as kinetic energy, the energy of motion.

Here, energy gets stored in a stretched spring, but energy can be stored in plenty of other ways. Lifting an object against the force of gravity stores gravitational potential energy. To release this stored energy, all you have to do is let go and watch the object fall. Conventional cars get their “go” from chemical potential energy—energy stored in chemical bonds—in the form of gasoline, lithium-ion batteries, or even hydrogen.

Friction is an important and necessary part of getting this or any other car to move. For a car to move forward, it must push backward on the road (or floor) beneath it—so says Newton’s Third Law of Motion: For every action, there is an equal and opposite reaction. The frictional force between the tires (or bottle) and the road (or floor) is the action, and the reaction is the road pushing forward on the tires. Just as you couldn’t take a step forward on perfectly frictionless ice, your car can’t move forward without friction.

There are lots of way to experiment with your Bottle Racer. For instance, try to optimize your racer’s performance for either distance or speed and then test it with a race. If you have friends who also want to build racers, you can set up a competition to see whose racer will travel the farthest, or travel a given distance in the shortest time.

You can also try making racers with different diameters. A larger-diameter bottle exerts a smaller tangential force on the ground, but its larger circumference means that it travels farther in one revolution. This is like high gear. Conversely, a smaller bottle exerts a larger force, but one revolution doesn’t get it very far. This is like low gear. Compare the speed and hill-climbing abilities of the differently “geared” racers.

Can you design a bottle to go in a straight line or a circle? Distance (total length of travel) and displacement (how far from its starting point the bottle ends up) are two distinct concepts in physics. A bottle that travels completely around a large circle has traveled a long distance, but has zero displacement! If you’re trying to maximize the displacement of your bottle, it’s important that you make it travel in a straight line. If the bottle travels in a curve rather than a straight line, how can you correct for this?

You can also run the bottle on different surfaces (wood, carpet, ice, or concrete, for instance). Does the surface affect the performance?