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===Derivatives of action===
In classical physics, the derivatives of [[action (physics)|action]] are conjugate variables to the quantity with respect to which one is differentiating. In quantum mechanics, these same pairs of variables are related by the Heisenberg [[uncertainty principle]].
* The ''[[energy]]'' of a particle at a certain [[event (relativity)|event]] is the negative of the derivative of the action along a trajectory of that particle ending at that event with respect to the ''[[time]]'' of the event.
* The ''[[linear momentum]]'' of a particle is the derivative of its action with respect to its ''[[position (vector)|position]]''.
* The ''[[angular momentum]]'' of a particle is the derivative of its action with respect to its ''[[angle]]'' (angular position).
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