Most American dictionaries do not distinguish between dynamic and dynamical. There is good reason to do so, however. Dynamic means changing. Dynamical is what concerns change. The position of an artificial satellite changes rather rapidly by earthbound standards, so it is very dynamic. The orbital energy of the satellite changes very little - not at all to a certain level of approximation - so the orbital energy is a dynamical property.

Analyzing complicated systems need not require great mathematical sophistication or intellectual brilliance. But it does require clear thinking, the kind of thinking that distinguishes sharply between the dynamics of change and the unchanging dynamical principles needed to analyze change.

- Foster Morrison, The Art of Modeling Dynamic Systems, 2, 2008

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