perpetual-motion machine, device that would be able to operate continuously and supply useful work, in violation of the laws of thermodynamics. A machine that would produce more energy in the form of work than is supplied to it in the form of heat would violate the first law of thermodynamics, which is a special case of the law of conservation of energy (see conservation laws, in physics), and is known as a perpetual-motion machine of the first kind. A machine that would completely convert heat from a warm body into work, without letting any heat flow into a cooler body, would violate the second law of thermodynamics, which is concerned with entropy changes, and is known as a perpetual-motion machine of the second kind. There were a number of early attempts to design and construct various types of perpetual-motion machines; however, since the 19th cent., when the laws of thermodynamics became understood, most such attempts have been abandoned.
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per·pet·u·al mo·tion • n. a state in which movement or action is or appears to be continuous and unceasing. ∎ the motion of a hypothetical machine that, once activated, would run forever unless subject to an external force or to wear.
"perpetual motion." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. (February 22, 2018). http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/perpetual-motion-0
"perpetual motion." The Oxford Pocket Dictionary of Current English. . Retrieved February 22, 2018 from Encyclopedia.com: http://www.encyclopedia.com/humanities/dictionaries-thesauruses-pictures-and-press-releases/perpetual-motion-0