Consider a dynamical system with generalized coordinates q1, q2,…, qn, solely acted on by conservative forces, with Lagrangian L. The action as the system moves from position at time t1 to a position at time t2 equals
The principle of least action states that a trajectory of the dynamical system is an actual motion of the system if and only if the value of S is a stationary value. See also calculus of variations.