perpetual motion

noun Mechanics.
the motion of a theoretical mechanism that, without any losses due to friction or other forms of dissipation of energy, would continue to operate indefinitely at the same rate without any external energy being applied to it.

Origin:
1585–95

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World English Dictionary
perpetual motion
 
n
1.  Also called: perpetual motion of the first kind motion of a hypothetical mechanism that continues indefinitely without any external source of energy. It is impossible in practice because of friction
2.  Also called: perpetual motion of the second kind motion of a hypothetical mechanism that derives its energy from a source at a lower temperature. It is impossible in practice because of the second law of thermodynamics

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Encyclopedia Britannica
Encyclopedia

perpetual motion

the action of a device that, once set in motion, would continue in motion forever, with no additional energy required to maintain it. Such devices are impossible on grounds stated by the first and second laws of thermodynamics.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
She's exploring the external power sources that propel the perpetual motion of
  oceans.
My initial reaction to this article was to think of perpetual motion machines.
The energy balance does not work out, its similar to creating a perpetual
  motion machine.
Perpetual motion machines will solve the energy problem too.
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