->"This 'perpetual motion' machine that she made today is a joke -- it just keeps going faster and faster."
-->-- '''Homer Simpson''', ''WesternAnimation/TheSimpsons'', "The PTA Disbands"

A PerpetualMotionMachine (also Perpetuum Mobile, Latin for "forever moving") is an old dream of mankind: A machine that creates more energy than it receives from the outside. (A weaker version just keeps moving on and on, without creating new energy you could put in use.) "Free Energy Device" is another name for it.

Obviously, a PerpetualMotionMachine contradicts the First Law of Thermodynamics, which states, roughly, "[[EquivalentExchange You can't get something from nothing.]]" Of course, this didn't stop some people from believing in a PMM - [[IRejectYourReality they just insist that obviously, the First Law has to be wrong.]]

NoConservationOfEnergy applied to machines.

A perpetual motion machine of the second kind is a machine violating the second law of thermodynamics -- that isolated system evolves toward maximum entropy. This involves transferring heat from colder to hotter objects without spending additional energy[[note]]the required energy is proportional to entropy and temperature, by the way[[/note]].

Subtrope of AppliedPhlebotinum. A MadScientist may work on this.

A subtrope of this: HyperDestructiveBouncingBall. See also PerpetualMotionMonster. Not to be confused with EternalEngine, which is a type of VideoGameSettings.

* A Toyota commercial demonstrates a car with regenerative braking, which attempts to recapture a portion of the energy lost as the car brakes. The actor in the commercial imagines applying this same technology to a roller coaster to create a [[http://www.youtube.com/watch?v=r8iFqz1Ogig "self-sustaining amusement park."]] Unfortunately, he is talking about creating a perpetual motion machine. No matter how perfect the machine, heat, friction, gravity, and air resistance guarantee that this is impossible. The flaw in his idea comes from the fact that the energy expended to cause the roller coaster to start will always be greater than the energy regained from the brakes, in much the same way that hybrid cars need gas.

* ComicBook/GastonLagaffe once invented one of the "weak" type. It doesn't do much, it just hops around (and gets on his co-workers' nerves).

* In ''Film/TheAbsentMindedProfessor'' and its remake ''Film/{{Flubber}}'', Flubber is a perpetual motion substance.
* In ''Film/BattlefieldEarth'', the planet Psychlo has an atmosphere that spontaneously ignites in the presence of radiation. This means radioactive decay does not naturally occur on the planet, meaning the planet ignores the second law of thermodynamics and is effectively a perpetual motion machine.
** In the book, Psychlo's atmosphere has the same implausible property. But, there, it's stated that the Psychlos are actually from a different universe with different physical laws.
* The action-comedy ''Film/KnightAndDay'' revolves around three forces of people trying to secure a prototype Perpetual Energy battery.

* In the second ''Literature/JimButton'' book by Creator/MichaelEnde, the protagonists invent it. Essentially, their version is based on a magnet which you can switch on and off, which pulls their locomotive.
** Violating the Law of Conservation of Momentum as well.
* Discussed in the Literature/VorkosiganSaga novel ''Komarr''. One of the physicists Miles calls in to consult determines that the device he's asking her about ''looks'' like a perpetual motion machine. Since she's a competent physicist who doesn't believe in such things, she concludes that it must be drawing energy from the deep structure of the wormholes it gets pointed at -- because there's nowhere else it could be coming from.
* In ''Literature/AtlasShrugged'', John Galt's engine relies on perpetual motion.
* The main focus of the first episode of Literature/TheChroniclesOfProfessorJackBaling is what happens to an engineering professor when he encounters a working perpetual motion machine. Specifically, an overbalanced wheel [[http://www.lhup.edu/~dsimanek/museum/machines/machines.htm]].
* An 8th grade student in Yevgeni Veltistov's ''The New Adventures of Elektronic'' claims to have created a small device that causes a little light bulb to light up and never go off after you wind the crank once. When the titular android character is asked if such a thing is possible, he simply says that he doesn't know, but that the device doesn't have any moving parts (except, obviously, the crank). The device is put on a shelf in the classroom and forgotten. It's mentioned that it worked without anyone touching it for weeks, but the little light bulb burned out shortly after. Of course, this is the same book where another student from the same class proved FermatsLastTheorem, only to rip up his proof a week later, as he did not want recognition.
* Creator/IsaacAsimov's short story "The Billiard Ball" (reprinted in ''Asimov's Mysteries'') is about a zero-gravity device which, when the zero-g field is established, the field becomes a brightly-glowing cylinder of hard vacuum -- because any air molecules in it lose all proper mass, and thus become incapable of movement at other than the speed of light, so they smash their way out of the field. It's explained by the main scientist character that they get the energy to do this (from nowhere) because in abolishing gravity, the field ''repeals the law of conservation of energy''.
* In Creator/RobertReed's ''An Exaltation of Larks'', time travelers from the [[TheStarsAreGoingOut heat death of the universe]] have been steadily making their way back to the Big Bang (at 15 month intervals) in order to tweak the laws of physics to make the ''entire universe'' a perpetual motion machine - rather than slowly succumbing to entropy, the universe will periodically [[ApocalypseHow collapse]] and then expand again.
* Creator/LeoTolstoy tells a folk story about a Russian peasant who tried to invent this, but failed. The man was quite capable of building mills and claimed to even have repaired mills where professional engineers failed, but [[BookDumb lacked education]], and so wouldn't know about the laws of thermodynamics.
* ''{{Literature/Discworld}}'': Alluded to at the end of ''{{Discworld/The Fifth Elephant}}''. One of the [[{{Magitek}} valuable artifacts]] recovered is a pair of 1cm cubes that rotate in opposite directions about once a minute, ''no matter what.'' It's explained that ancient dwarven civilizations used this minimalist PMM in conjunction with a massive gear and pulley system to power ''absolutely everything.''
* ''Literature/TheDreamOfPerpetualMotion''. When Prospero Taligent diverts his company's resources to building a perpetual motion machine, his stock plummets as it's regarded as a sign he's become a literal MadScientist. Ten years later he claims to have succeeded, installing his only model in a CoolAirship designed never to land. The protagonist realises however that the airship is slowly losing power and will eventually crash.
* The titular ''[[Literature/{{Orthogonal}} Eternal Flame]]'' from the Creator/GregEgan novel is a hypothetical chemical reaction that never exhausts itself, or at least one that continues for an absurdly long period of time, AND can be controlled and put to practical use, such as powering the GenerationShip. [[spoiler:They spend an entire book trying to harness the explosive properties of [[{{Antimatter}} orthogonal matter]] as fuel, only to develop an utterly unrelated type of engine that is just insanely efficient and runs on light. All of the (near-)perpetual motion, without the risk of catastrophic explosion]].
* In the Webcomic/GirlGenius novel ''Literature/AgathaHAndTheClockworkPrincess'', it's mentioned in passing that one of the minor Sparks with the circus exhibited a perpetual motion machine, and was utterly mortified when Agatha proved that it needed a small push every ten years to keep going.
* In the ThursdayNext series, Thursday's uncle Mycroft invents the "Nextahedron", a solid shape that is perpetually unbalanced. When unrestricted on a flat surface, the Nextahedron never stops wobbling, falling over onto a different face, wobbling again, and so on.

* In ''VideoGame/DwarfFortress'' the mechanical energy needed to pump water up one story is only one tenth the amount generated when the water comes back down and powers a water wheel. Power the pump with the water wheel, prime it once with manual labor, and it will endlessly generate power.
* ''VideoGame/{{Portal}}'' fans have proposed several ideas for perpetual motion machines using the portal technology. Most of them revolve around the fact that if one portal is on the ceiling and the other is on the floor, any object thrown in would fall indefinitely.
* The Reapers in ''Franchise/MassEffect'' somehow work without fuel. In ''VideoGame/MassEffect3'' Codex it is outright stated how [[SufficientlyAdvancedAlien strange and impossible this should be]], as well as the fact that without need for resources and capable of replenishing their foot-soldiers from enemy ranks, the Reapers need absolutely no supply lines in war.
* In ''VideoGame/TheIncredibleMachine'', a perpetual motion machine can be easily constructed. A power generator produces electricity from rotation, while an electric engine produces rotation from electricity. Place a generator, plug an engine into it, connect the wheels of the generator and the engine, add an initial source of energy (e.g. bellows and a windmill) - voilą! Perpetual motion.
* There is one in the museum in ''VideoGame/UltimaVI''. It's a set of gears connected by shafts; each gear transmits motion to the next, and the last one transmits it back to the first one. It has no relevance in the story, though.

* In ''IlivaisX'', the titular mech is powered by one of these. How it works isn't explained, all we know is that it's so expensive that the Aztecs will throw as much military force as possible at recapturing it instead of just making another one, and that the limitless energy is the only reason the protagonist was capable of escaping in the first place. It's hinted that it may not be the energy they want, but rather something to do with the sheer fact that it shouldn't be possible, as with one rule of possibility broken, all the others can be as well.
* There's a whole ''gallery'' of plans for these machines -- Donald Simanek's [[http://www.lhup.edu/~dsimanek/museum/unwork.htm Museum of Unworkable Devices]]. Some visitors to this site misinterpret Simanek's motivation as "seeking to eliminate the flaws of such machines, so that they can be made workable"; of course, his actual motivation is to educate the public that those flaws are inherent and can ''never'' be eliminated.
* The [[http://www.scp-wiki.net/system:page-tags/tag/ectoentropic#pages ectoentropic SCPs]] of the Wiki/SCPFoundation all ''technically'' count as Perpetual Motion Machines, though the ones that merely create matter from out of nothing don't really fit under this trope[[note]]since matter is equivalent to energy, creating matter out of nothing is technically creating energy out of nothing[[/note]].

* ''WesternAnimation/TheSimpsons'', "The PTA Disbands": Lisa is going crazy while the teachers are on strike and creates a perpetual motion machine. Homer later told Lisa that no physics law should be broken in his home.
-->'''Homer:''' ''Lisa, get in here! In this house, we obey the laws of thermodynamics!''
* In one episode of ''WesternAnimation/ChipNDaleRescueRangers'', [[GadgeteerGenius Gadget]] claims she once found a perpetual motion machine in the garbage can after a school science fair; of course, by then, [[SubvertedTrope it had stopped moving]].
* In one episode of ''WesternAnimation/{{Recess}}'', Gretchen invented one for a school project, and it was immediately confiscated by the government.
* Subverted in the [[WesternAnimation/GadgetBoyAndHeather Gadget Boy's Adventures in History]] episode "These are a Few of my Favorite Flying Things". The villain Spydra succeeds in stealing Leonardo da Vinci's perpetual motion machine prototype. Leonardo takes the theft well because his machine was supposed to prove that perpetual motion is impossible. Cue Spydra yet again suffering a humiliating defeat when her flying machine crashes.

* In the frictionless vacuum of outer space it is possible to have an object that will move forever through inertia, as in planetary orbits (which do slow down, but by a negligible amount). Technically, due to conservation of Angular momentum, if you spun a planet in empty space and no force or friction acts against it at all, it would spin forever. However, this is just perpetual ''storage'' of an initial amount of energy. If you hook a generator onto your frictionlessly moving object to achieve the ''production'' of energy that's the whole idea of perpetual-motion machines, all it'll do is tap that initial amount of energy. The object will then slow down, and eventually stop.
* Some people claim that [[http://en.wikipedia.org/wiki/Zero_point_energy#Claims_in_pseudoscience Zero Point Energy]] can be used to generate power out of nothing.
* People attempt to patent Perpetual Motion Machines. Most nations' patent offices can and will reject any at face value, but a few get patented if they are labelled as something else.
* There have been a few cases of seemingly successful perpetual motion machines, though careful examination has always shown either a lack of credible documentation, or an external power source. In fairness to the creators, some of these cases used energy sources not well understood at the time. One in particular, [[http://en.wikipedia.org/wiki/Johann_Bessler Orffyreus' Wheel]], is still the subject of controversy as to its functioning; however, it is believed to be a deliberate fraud.