Background Essay: Designing Balloon Cars

Although none of us travel around behind the wheel of a balloon car, all vehicles, including balloon cars, rely on one of the most important laws of physics for their forward motion. Newton's third law of motion states that every action has an equal and opposite reaction. This means, for example, that when you push against a wall, the wall, as inanimate as it is, pushes back on you with an equal amount of force. If you're doubtful of a wall's ability to push you, try leaning against one while standing on a skateboard.

Balloon cars use the principle of Newton's third law in the same way that rocket- and jet-propelled vehicles do. Before it is inflated, a balloon exerts no force on the relatively few molecules of air it contains. As it is inflated, however, more and more air molecules crowd into it, increasing the balloon's internal pressure and causing it to expand. As the rubber of the balloon stretches, it applies an increasing amount of force on the air inside. When the balloon is released, the air escaping from the balloon pushes against the air just outside the balloon. As the third law predicts, the outside air pushes back on the escaping air, propelling the balloon car forward.

Just as all vehicles rely on Newton's third law to propel them forward, the forward motion they create (or harness, in the case of wind-propelled vehicles) must counteract the forces that resist forward motion, namely friction and drag. Although these forces cannot be eliminated, at least not on Earth, intelligent vehicle designs can reduce them considerably. Wheels, for example, are probably the simplest way to reduce friction on land. The more easily and smoothly they roll, the more of a vehicle's force will be applied to forward motion and the faster and farther it will travel. Likewise, the more streamline a vehicle, the more easily it will cut through air or water and the more efficient it will be. Engineers who design cars, boats, trains, and planes go to great lengths to create vehicles that maximize the forward-acting force they produce and minimize the forces that act against this forward motion.