^^Principia. Corollario 1. "Regola del parallelogramma".

In Newton’s Principia, the first corollary (Corollary I to the Second Axiom) that appears after the three laws shows how forces are to be composed:

Corpus viribus conjunctis diagonalem parallelogrammi eodem tempore describere, quo latera separatis.


A body acted on by [two] forces acting jointly describes the diagonal of a parallelogram in the same time in which it would describe the sides if the forces were acting separately (transl. by Cohen/Whitman 1999, p. 417).


Un corpo a [2] forze

congiunte descrive la diagonale del parallelogramma

nello stesso tempo che a separate [descrive] i lati.   (tradotto da Occa)

Some pages later Newton applied the parallelogram composition of an inertial motion and a centripetal attraction for the problem of circular motion and centripetal acceleration.

credits: https://historyofwomenphilosophers.org/chapter-12-of-composite-motion/

ref: Vettori. Storia.