D'Alembert's principle (dăl´əmbârz´), in mechanics, principle permitting the reduction of a problem in dynamics to one in statics. This is accomplished by introducing a fictitious force equal in magnitude to the product of the mass of the body and its acceleration, and directed opposite to the acceleration. The result is a condition of kinetic equilibrium. Jean le Rond d'Alembert, a French mathematician, introduced the principle in 1742 and established it the next year in his Traité de dynamique. The principle shows that Newton's third law of motion applies to bodies free to move as well as to stationary bodies.