levier

Levier is the French word for lever, a rigid bar that rotates around a pivot called a fulcrum. As one of the classical simple machines, the lever enables a small input force to raise or move a larger load by trading force for distance. The operating principle is the lever or moment: the torque produced by the input force equals the torque required to balance the load about the fulcrum. The mechanical advantage depends on the relative distances from the fulcrum to the points where effort and load are applied; longer effort arm yields greater advantage.

Levers are classified into three main types: first class with the fulcrum between effort and load; second class with the load between fulcrum and effort; and third class with the effort between fulcrum and load. In each case, the ratio of lever arms determines the output force. Many tools and devices use levers, such as crowbars and seesaws (class I); wheelbarrows (class II); and the human forearm (class III). Scissors are a compound lever system.

In modern usage, "levier" also denotes a factor or mechanism that enables change, for example levers of growth or political levers used to influence outcomes.

Etymology: levier derives from Old French lever, to raise, from Latin levare.