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A lever is a simple machine that magnifies force. Levers are comprised of a rigid bar (lever arm) a pivot point (fulcrum), a load force, and an effort force. The effort force creates a torque around the fulcrum. The magnitude of this torque is dependant on the magnitude of the force and its distance from the fulcrum. This torque must be balanced by the torque created by the load force. Changing he distance from the fulcrum to the load force changes the amount of force magnification.
There are three main types of levers: first class, second class, and third class. A first class lever has the fulcrum located between the effort force and the load force on the lever arm. An example of this type of lever is a pair of pliers. A second class lever has the load force located between the effort force and the fulcrum on the lever arm. An example of a second class lever is a pair of nutcrackers. A third class lever has the effort force located between the fulcrum and the load force on the lever arm. An example of this type of lever is a pair of tongs.
For more information on levers from www.britannica.com: Click Here