French mathematician, encyclopedist, and theoretical physicist. In association with Denis

Diderot, he helped plan the great

Encyclopédie, for which he also wrote the "Discours préliminaire" (1751). He framed several theorems and principles – notably

d'Alembert's principle –
in dynamics and celestial mechanics, and devised the theory of partial differential equations. The principle that now bears his name was first published in his

Traité de
dynamique (1743), and was an extension of the third of Isaac

Newton's laws of motion. D'Alembert maintained that the law was valid not merely for a static body,
but also for mobile bodies. Within a year he had found a means of applying the principle to the theory of equilibrium and the motion of fluids. Using also the theory of partial differential
equations, he studied th...

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