fulcra

Fulcra are the plural of fulcrum, referring to pivot points or supports around which lever systems rotate or balance. In physics and engineering, a fulcrum is the fixed point about which a lever pivots to transform input forces into output forces.

In a lever, the forces obey 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, respectively. The mechanical advantage is F_l/F_e = d_e/d_l. The fulcrum’s position relative to the load and effort determines the lever’s class: first class has the fulcrum between effort and load; second class places the load between the fulcrum and effort; third class places the effort between the fulcrum and load. Examples include a seesaw (first class), a wheelbarrow (second class), and many sports implements or tools that concentrate effort over a short distance (third class).

Beyond simple machines, fulcra can be used metaphorically to describe pivotal points in more complex systems, such as anatomical joints or organizational processes. The Latin plural fulcra is standard in formal writing, though fulcrums is commonly used in everyday English. Both forms appear in modern technical literature, with preference often determined by stylistic conventions or the historical lineage of the source.

See also: lever, pivot point, mechanical advantage.