A lever is a simple machine usually consisting of a rigid bar that is free to rotate about a fixed point, called the fulcrum, under the action of two or more forces often termed the applied and resisting forces. Its primary purpose is to multiply power and motion. Although the lever cannot save work, it can make the work easier to perform. Familiar examples include the crowbar, pliers, and the bones of the arms and legs.

The multiplication of effort obtained from a lever, or the ratio of the output force to the applied force, is termed mechanical advantage. For example, if a force of 100 pounds enables one to move a load of 1,000 pounds, the mechanical advantage is 10. In this calculation, friction has been ignored, for although friction is always present, it is generally so small that it does not significantly affect the result. However, the weight of the lever itself must often be taken into account. Depending on the position of the center of gravity (the point where all its weight can be considered to be located) the mechanical advantage may be increased, unchanged, or decreased.

The origin of the lever is unknown, but it was almost certainly used in prehistoric times in the form of digging sticks and other simple tools. By 1550 B.C. the lever was being used in Egypt and India to raise water from shallow wells with a well sweep (swape), for moving heavy objects in construction, and in weighing machines such as the lever-balance. Later the Greeks used the lever in war machines such as catapults and in the lever-press for squeezing grape juice.

The mathematical principle of the lever ("law of the lever") was known to Aristotle but was not rigorously proved until the next century by the Greek mathematician Archimedes (287-212 B.C.). The law states that a lever is in equilibrium when the product of the applied force and the distance from the point of application to the fulcrum equals the product of the resisting force and the distance from its point of application to the fulcrum.

Types of Levers

According to the position of the fulcrum and the applied and resisting forces, levers can be divided into three classes.

First-class levers are those in which the fulcrum is between the applied and resisting forces. A scissors and a claw hammer are examples.

Second-class levers are those in which the resisting force is between the fulcrum and the applied force. The wheelbarrow and the nutcracker fall into this class.

Third-class levers are those in which the applied force is between the fulcrum and the resisting force. Lifting a weight by bending the arm at the elbow and closing a door by pushing it close to the hinge exemplify the action of third-class levers.

The primary advantage in using levers of the first and second classes is that a heavy load can be moved or lifted with a small effort. Third-class levers increase the speed with which a load can be moved. They also allow the load to be moved a greater distance than is possible with first- and second-class levers.

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