lever

A lever is the simplest of the classical simple machines: a rigid rod or bar that pivots about a fixed point called the fulcrum. It works by the principle that two torques balance if they are equal in magnitude and opposite in sense. If a small force is applied at a long moment arm on one side of the fulcrum, it can balance a much larger force at a short moment arm on the other side. The ratio of the two arms is the mechanical advantage of the lever.

Levers are classified into three classes depending on the relative positions of load, effort, and fulcrum. Class 1 (fulcrum between effort and load): seesaws, crowbars, claw hammers pulling nails, scissors. Class 2 (load between fulcrum and effort): wheelbarrows, bottle openers, nutcrackers. Class 3 (effort between fulcrum and load): human forearms (biceps pulling near the elbow to lift weight at the hand), tweezers, fishing rods.

The law of the lever was first proved rigorously by Archimedes around 250 BCE in On the Equilibrium of Planes. His famous boast — "Give me a place to stand and I will move the Earth" — was a literal statement about the arithmetic of levers: with a long enough lever and a fulcrum, any force, however small, can move any weight, however large. The amount of distance one has to push is another matter — mechanical advantage is a trade, not a gift. The lever is the earliest known device in which humans beat brute-force strength by using geometry.