The expression perpetual motion arose historically in connection with the quest for a mechanism which, once set in motion, would continue to do useful work without an external source of energy or which would produce more energy than it absorbed in a cycle of operation.
Most scientists and scholars agree that perpetual motion violates either the first or second law of thermodynamics or in fact breaks both of them. Some devices which don't break these laws but get their energy from an obscure source have been called perpetual motion machines, but really aren't.
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