leviers

Levers, or levier in French, are simple machines consisting of a rigid bar that rotates around a fixed point called the fulcrum. The bar carries a load on one side and an effort on the other. When the lever is used, the user applies a force (the effort) at a given distance from the fulcrum, raising or moving a load at another distance. The relationship between forces is governed by torque: F_e × d_e = F_l × d_l, where d_e and d_l are the distances from the fulcrum to the points of effort and load. The lever’s mechanical advantage equals d_e/d_l, assuming negligible friction.

They are classified into three main classes. Class I levers have the fulcrum between the effort and load, as in a seesaw or crowbar. Class II levers have the load between the fulcrum and the effort, as in a wheelbarrow or nutcracker. Class III levers have the effort between the fulcrum and the load, as in most human forearms and in tweezers. Each class offers different opportunities for changing force and distance.

History and significance: Levers are among the oldest known simple machines. Their understanding dates to ancient Greece, with Archimedes describing the principle of moments and the ability to move heavy objects with a long lever. They remain fundamental in engineering, biology, and everyday tools.

Applications and design: Levers are chosen for desired trade-offs between force, speed, and travel distance. Real-world levers are affected by friction and material strength, so actual performance deviates from ideal equations. Modern machinery often combines levers with other mechanisms to achieve practical tasks, including lifting, gripping, and position control. In biology, bones and muscles act as levers to move joints.