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This link suggests that they are basically the same thing, although I'd never heard of 'dynamical' until I saw your post. I certainly wouldn't use it in normal speech, either. Perhaps someone could suggest a context where it might be used, e.g. in the sciences?
I would have sworn that dynamical is not actually a word, but 17.8 million Google hits suggests that it has indeed found its way into the English language. An examination of those Google hits suggests that dynamical is normally used only among scientific folks. I do not find dynamical in my college dictionary, which was published fairly recently. I would avoid dynamical and always use dynamic unless you do learn that there is a technical difference.
I realize this thread is a bit dated but I have had the same question. Dynamical is a real word is common in mathematics and physics. The best answer I've found for this has been posted on Answers.com:
dynamic: characterized by action or forcefulness or force of personality; "a dynamic market"; "a dynamic speaker"; "the dynamic president of the republic" dynamical: refers to specific systems that change over time or dimension A dynamical systems is a mathematical formalization for any fixed "rule" which describes the time dependence of a point's position in its ambient space. Examples include the mathematical models that describe the swinging of a clock pendulum, the flow of water in a pipe, and the number of fish each spring in a lake.