Rolling is a combination of
rotation(of a radially symmetric object) and translation of that object with respect to a surface (either one or the other moves), such that the two are in contact with each other without sliding. This is achieved by a rotational speed at the cylinder or circle of contact which is equal to the translational speed. Rolling of a round object typically requires less energy than sliding, therefore such an object will more easily move, if it experiences a force with a component along the surface, for instance gravity on a tilted surface; wind; pushing; pulling; an engine. Objects with corners, such as dice, roll by successive rotations about the edge or corner which is in contact with the surface.
One of the most practical applications of rolling objects is the use of
ball bearingsin rotating devices. Made of a smooth metal substance, the spherical bearings are usually encased between two rings that can rotate independently of each other. In most mechanisms, the inner ring is attached to a stationary shaft (or axle). Thus, while the inner ring is stationary, the outer ring is free to move with very little friction. This is the basis for which almost all motors (such as those found in ceiling fans, cars, drills, etc) rely on to operate. The amount of friction on the mechanism's parts depends on the quality of the ball bearings and how much lubrication is in the mechanism.
Rolling objects are also frequently used as
toolsfor transportation. One of the most basic ways is by placing a (usually flat) object on a series of lined-up rollers, or wheels. The object on the wheels can be moved along them in a straight line, as long as the wheels are continuously replaced in the front (see history of bearings). This method of primitive transportation is efficient when no other machinery is available. Today, the most practical application of objects on wheels are cars, trains, and other human transportation vehicles.
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