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Pretty much any system where the state of one thing effects the rate of another, and the state of that other thing effects the rate of the first. Surely you can think of a few of those. One examp...
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#1: Initial revision
Pretty much any system where the state of one thing effects the rate of another, and the state of that other thing effects the rate of the first. Surely you can think of a few of those. One example is a capacitor and inductor wired in parallel. The voltage on the cap effects the rate of current change in the inductor. The current thru the inductor effects the rate of voltage change on the capacitor.