Resonance is an example of self-development we see in the lifeless world around us. Why self-development, because the new arrangements there happen based on the old arrangements, in the same object, plus the outside forces of course. This makes it unique. What happens i think, everything tends to move the way how it's the easiest to go. So when like, there is an overunity, this will give additional energy in case of resonance, because everything moves the most easily the way how they need the least energy for that. And thus everything arranges in that way.

And where the overunity in the resonance may come from? Again may well be, from the reason which keeps the electrons orbiting the nucleus. Which some say comes from the zero point energy. This may provide an additional force in the electromagnetic interaction. So again, the exact same reason that the possible reason for overunity in permanent magnets, induction, and maybe also in the electrolysis, movement of liquids, and maybe in other cases as well.

Just a thought.

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And where the overunity in the resonance may come from?
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When we see this overunity experimentally and without a doubt, the question will become interesting.
Until now, a resonance is the result of stored energy that passes from one form to another at a constant frequency.
The energy stored in a system is obtained initially from a source, often by accumulating step by step small amounts of energy up to a maximum level depending on the Q factor of the resonant system.
Then, a small amount of energy is sufficient to maintain the oscillating system by compensating for losses.