A lever is a rigid bar that moves in an arc about a pivot point. A load is attached to a point on the lever and moved by applying force to another point of the lever.

That may sound complicated, but a lever is really a simple device. The crowbar in the picture is a perfect example of a lever. The worker is using the crowbar to pry against the brick to loosen the board.

Four parts of this scenario are important:

1. The crowbar itself, which is the lever.
2. The force of the worker as he pushes down on the crowbar is known as the effort.
3. The brick, which the worker is prying against, is the fulcrum. You can also think of the fulcrum as the pivot point.
4. The object being moved, the board in this case, is known as the load.

To accomplish work, the lever, fulcrum, and force must work together to move the load. Also, the lever must be rigid: you could not pry the board loose with a limp piece of rope!

There are three classes of levers, Class 1, Class 2, and Class 3. The lever classes are determined by the relative positions of the effort, fulcrum, and load with respect to the lever.