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The expression “perpetual motion” arose in connection to the quest for a mechanism that, once set in motion, would continue to do useful work without an external source of energy. A mechanism using this type of motion, now called perpetual motion of the first kind, would clearly violate the now firmly established principle of conservation of energy. Still, this fact has not deterred inventors throughout history from proposing hypothetical perpetual motion machines. What are some of them? Discuss
Source: The Free Dictionary