impetus

Impetus, impetus, m., is fourth declension, from in plus peto - a going at something. Genitive singular is impetus, spelled like the nominative, and the dative and ablative plural impetibus barely appear.

Two idioms do most of the work in the historians: impetum facere in with the accusative, "to charge at", and impetum sustinere or ferre, "to take the charge". Outside war it is the force of a river, a wind or a crowd, and the drive behind a feeling. English kept only the abstract half of that, so "impetus" now means momentum in a lecture rather than cavalry at the gallop; when Caesar writes impetus he means horses. The related noun petitio and the verb impetro, "obtain by asking", show how far the root travels.