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One type of a perpetual motion machine would continue to perform work without new energy, thereby serving as an unlimited energy source.  This is prohibited by the [[First Law of Thermodynamics]].

One type of a perpetual motion machine would continue to perform work without new energy, thereby serving as an unlimited energy source.  This is prohibited by the [[First Law of Thermodynamics]].

Another type of perpetual motion machine simply uses its initial energy to remain in motion perpetually.  It is more difficult to prove that this is impossible, but it might violate the [[Second Law of Thermodynamics]].  If entropy is always increasing, even in closed (and isolated) systems, then indefinite motion is impossible because an increase in the disorder of the system will inevitably disrupt the motion.   

Another type of perpetual motion machine simply uses its initial energy to remain in motion perpetually.  If entropy is always increasing, even in closed (and isolated) systems, then indefinite motion is impossible because an increase in the disorder of the system will inevitably disrupt the motion.  The [[Second Law of Thermodynamics]] predicts that a perpetual motion machine in this sense is impossible, and none has ever been created.

 

This argument is the only evidence against this type of machine.