# D'Alembert's principle

D'Alembert's principle, also known as the Lagrange–d'Alembert principle, is a statement of the fundamental classical laws of motion. It is named after its discoverer, the French physicist and mathematician Jean le Rond d'Alembert. It is an extension of the principle of virtual work from static to dynamical systems. D'Alembert separates the total forces acting on a system to forces of inertia (due to the motion of a non-inertial reference frame, now known as fictitious forces) and impressed (all other) forces. Although d'Alembert's principle is formulated in many different ways, in essence it means that any system of forces is in equilibrium if impressed forces are added to inertial forces.[1] The principle does not apply for irreversible displacements, such as sliding friction, and more general specification of the irreversibility is required.[2] D'Alembert's principle is more general than Hamilton's principle as it is not restricted to holonomic constraints that depend only on coordinates and time but not on velocities.[3]