consists of a rod or plank that is free at both ends, and some steady object on which the plank can rest. The braced, or fixed, part is called the fulcrum. The distance from the load to the fulcrum is the load arm. The distance between the fulcrum and the lifting force is the effort arm.

In its simplest form, the crowbar, the lever is a device that magnifies the effect of a small effort to move a large weight. But inverting the effort and the fulcrum changes the relationship; gripping a fishing pole and flipping a trout out of the water requires much more effort than the fish weighs, but the extra effort buys the longer distance the fish travels.

The lever is the most basic of all machines, and the moving parts of virtually every mechanical device can be reduced to levers -- in structure or in principle.

How and Why a Lever Works

Levers work according to a simple formula known as the Law of Equilibrium. This law is illustrated by the diagram below: the effort multiplied by its distance from the fulcrum (effort arm) equals the weight multiplied by its distance from the fulcrum (load arm). Thus, two pounds of effort exerted at a distance of four feet from the fulcrum will raise eight pounds located one foot from the fulcrum.

Classes of Levers

	First-Class Levers have the fulcrum placed between the load and the effort, as in a seesaw, crowbar, and balance scale. If the two arms of the lever are of equal length, the effort must be equal to the load. To lift 10 pounds, an effort of 10 pounds must be used. If the effort arm is longer than the load arm, as in the crowbar, the effort travels farther than the load and is less than the load. A pair of scissors is a double lever of the first class.
	Second-Class Levers have the load between the effort and the fulcrum. The best example of a second-class lever is the wheelbarrow. The wheel is the fulcrum, the handles take the effort, and the load is placed between them. The effort always travels a greater distance and is less than the load. A nutcracker is a double lever of this class.
	Third-Class Levers have the effort placed between the load and the fulcrum. The effort always travels a shorter distance and must be greater than the load. The forearm is a third-class lever. The hand holding the weight is lifted by the biceps muscle of the upper arm which is attached to the forearm near the elbow. The elbow joint is the fulcrum.
	Compound Levers combine two or more levers, usually to decrease the effort. By applying the principle of the compound lever, a person could use the weight of one hand to balance a load weighing a ton.

Questions or comments about this page?

Robert O'Brien. Machines. New York:Time Incorporated, 1964.
World Book Encyclopedia. Chicago:World Book-Childcraft International, 1979.

THE ROBINSON LIBRARY
Technology.--Mechanical Engineering and Machinery.--Machines and Mechanics (General).

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