Editing Perpetual motion (section)

=== Classification ===
One classification of perpetual motion machines refers to the particular law of thermodynamics the machines purport to violate:<ref>{{Cite book |last=Rao |first=Y. V. C. |date=2004 |title=An Introduction to Thermodynamics |url=https://books.google.com/books?id=iYWiCXziWsEC |location=Hyderabad, India |publisher=Universities Press (India) Private |isbn=978-81-7371-461-0 |access-date=1 August 2010}}</ref>
* A '''perpetual motion machine of the first kind''' produces [[Work (thermodynamics)|work]] without the input of [[energy]]. It thus violates the [[law of conservation of energy]].
* A '''perpetual motion machine of the second kind''' is a machine that spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However, it does violate the more subtle [[second law of thermodynamics]] in a cyclic process (see also [[entropy]]). The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, according to the second law of thermodynamics.
*A '''perpetual motion machine of the third kind''' is defined as one that completely eliminates friction and other dissipative forces, to maintain motion forever due to its mass inertia (''third'' in this case refers solely to the position in the above classification scheme, not the [[third law of thermodynamics]]). It is impossible to make such a machine,<ref name="wong">{{Cite book |last=Wong |first=Kau-Fui Vincent |date=2000 |title=Thermodynamics for Engineers |url=https://books.google.com/books?id=rEOMi-85v64C |publisher=CRC Press |page=154 |isbn=978-0-84-930232-9}}</ref><ref name="phi">{{cite book |last1=Akshoy |first1=Ranjan Paul |last2=Sanchayan |first2=Mukherjee |last3=Pijush |first3=Roy |date=2005 |title=Mechanical Sciences: Engineering Thermodynamics and Fluid Mechanics |url=https://books.google.com/books?id=m07QzMlX47wC |publisher=Prentice-Hall India |page=51 |isbn=978-8-12-032727-6}}</ref> as dissipation can never be completely eliminated in a mechanical system, no matter how close a system gets to this ideal (see examples at {{section link|#Low friction}} below).