If you were to build a vehicle powered by your breath, what characteristics would increase or decrease its performance the most? In this video segment adapted from ZOOM, four cast members design, construct, and test a variety of puff mobiles.
In order for a vehicle of any type to move, it must produce or harness a force strong enough to overcome the resistance of friction or drag. Most vehicles rely on internal power sources to convert energy into a force that propels them forward. However, some vehicles, including sailboats, use the wind's energy to generate the force that makes them move.
Wind can move a vehicle forward if, and only if, the force it applies to the vehicle is enough to overcome forces like friction and drag that resist forward motion. To make this possible, wind-vehicle designers can try to increase the force that moves the vehicle forward, or reduce the resistant force. Often they do both.
For example, when traveling with the wind, sailors typically use sails large enough to capture as much wind as possible, but not so large that they make the boats they're attached to unstable. If large enough, a sail can capture enough wind to propel a boat forward nearly as fast as the wind itself. Ironically, sailors can travel even faster than the prevailing wind speed by sailing at angles that oppose the wind somewhat. In so doing, they use not only the force of the wind on the sails but also the force of the water on the boat's hull and either its keel or center board (depending on the type of boat) to propel the boat forward.
Reducing friction across the ground or reducing drag through water or air is another way to increase the performance of vehicles. Wheels are probably the best way to minimize friction on land. Rather than dragging and scraping across the ground, automobiles roll on wheels -- so easily, in fact, that a person can push a car to a gas station, if necessary. Likewise, the hull of a boat and the fuselage of an airplane are designed to cut through water and air with a minimum of drag.