The principle of moments is derived from Archimedes' discovery of the operating principle of the lever. In the lever one applies a force, in his day most often human muscle, to an "arm", a beam of some sort. Archimedes noted that the amount of force applied to the object, the "moment of force", is defined as "M = rF", where "F" is the applied force, and "r" is the distance from the applied force to object.