Action (physics)

In Lagrangian mechanics, the trajectory of an object is derived by finding the path which minimizes the action integral which is the integral of the Lagrangian over time; this being the kinetic energy minus the potential energy.

This, of course, is only for classical mechanics in a very restrictive sense of the term. More generally, an action is a functional from the configuration space to the real numbers and in general, it needn't even necessarily be an integral because nonlocal actions are possible. The configuration space needn't even necessarily be a functional space because we could have things like noncommutative geometry.

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