A formulation of Newtonian mechanics that reduces dynamics to an equilibrium condition like statics by extending Newton’s third law to the case of forces acting on bodies rather than just bodies in static equilibrium, i.e. it postulates that bodies are always in equilibrium even when they are acted on by a force because the force applied to the body minus the rate of change of the momentum with respect to time is always zero. It was proposed by the French mathematician Jean le Rond D’Alembert (1717–83) in 1743.