A wheel is a disk-like mechanical device mediating between rotary and linear motion, widely used to transmit power, store energy (see flywheel), and to facilitate the movement of heavy objects. More than 5,000 years ago sections of tree trunks were cut to form the first wheels for carts. Spoked wheels were introduced several hundred years later. Eventually, the wheel was used in simple machines, such as the water wheel and potter's wheel.
Wheels may be solid or spoked, flanged or unflanged, with or without tires. Most usefully, they are attached to an axle through the center. Indeed, the wheel and axle is one of the classic simple machines, exemplified in the capstan, winch, and transmission gears. The force ratio, or mechanical advantage of a wheel and axle is the ratio of the radius of the wheel to that of the axle.