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.