lever (redirected from third-class lever)

lever

[lē′vər, lev′ər]

Etymology: L, levare, to lift up

any one of the numerous bones and associated joints of the body that act as a simple machine so that force applied to one end of the bone tends to rotate the bone in the direction opposite from that of the applied force. The muscles of the body produce the forces that move the levers. The basic components of a lever are the fulcrum, the force arm, and the weight arm. A first-class lever, such as the joint between the base of the skull and the first cervical vertebra, has a fulcrum between the weight and the applied force. The body contains few second-class levers, which have the weight between the fulcrum and the force. A third-class lever, such as the forearm and elbow, has the force between the fulcrum and the weight. The body uses its third-class levers for speed and its first-class levers for either force or speed, depending on the force applied to the weight arm.

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lever (lev´ur),

n a bar or rigid body that is capable of turning about one joint or axis and in which are two or more other points where forces are applied. There are three classes of levers, and each has its own most effective use.

lever leverage

n the mechanical advantage gained by the use of a lever. A factor in the magnification of stresses generated by an extension-base partial denture.

lever, second-class,

n a lever in which the force arm is longer than the work-producing arm; thus the work produced is always greater than the energy used, with a resultant high efficiency.

lever, third-class,

n a lever in which the axis is at one end, the load at the other end, and the effort is exerted in between, as in a treadle.