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In physics, a lever (from French lever, "to raise", c.f. a levant) is a rigid object that is used with an appropriate fulcrum or pivot point to multiply the mechanical force (effort) that can be applied to another object (load). This leverage is also termed mechanical advantage, and is one example of the principle of moments. A lever is one of the six simple machines.
Contents
Early
The earliest remaining writings regarding levers date from the 3rd century BC and were provided by Archimedes. "Give me a place to stand, and I shall move the earth with a lever" is a remark of Archimedes who formally stated the correct mathematical principle of levers (quoted by Pappus of Alexandria).^{[1]}
It is assumed that in ancient Egypt, constructors used the lever to move and uplift obelisks weighting more than 100 tons ^{[2]}.
Force and levers
The force applied (at end points of the lever) is proportional to the ratio of the length of the lever arm measured between the fulcrum (pivoting point) and application point of the force applied at each end of the lever.
Mathematically, this is expressed by M = Fd, where F is the force, d is the distance between the force and the fulcrum, and M is the turning force known as the moment or torque.
Classes
There are three classes of levers representing variations in the relative locations of the fulcrum, the load and the force:^{[3]}
 Class 1: The fulcrum is located between the applied force and the load, for example, a crowbar or a pair of scissors or a seesaw.
 Class 2: The load is situated between the fulcrum and the force, for example, a wheelbarrow or a nutcracker.
 Class 3: The force is applied between the fulcrum and the load, for example, a pair of tweezers or the human mandible.
In the real world
For the classical mechanics formulas to work, or to be a good approximation of real world applications, the lever must be made from a combination of rigid bodies, (i.e., a beam) and a rigid fulcrum. Any bending or other deformation must be negligible.
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