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"Lisa! In this house, we obey the laws of thermodynamics!"
Homer Simpson, The Simpsons, "The PTA Disbands"

A Perpetual Motion Machine (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 Sub-Trope of You Fail Physics Forever: a Perpetual Motion Machine contradicts the First Law of Thermodynamics, which states, roughly, "You can't get something from nothing." Of course, this didn't stop some people from believing in a PMM - they just insist that obviously, the First Law has to be wrong.

No Conservation of Energy applied to machines.

Subtrope of Applied Phlebotinum. A Mad Scientist may work on this.

A subtrope of this: Hyper-Destructive Bouncing Ball. See also Perpetual Motion Monster. Not to be confused with Eternal Engine, which is a type of Video Game Settings.

Examples of Perpetual Motion Machine include:


  • 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 "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 breaks, in much the same way that hybrid cars need gas.

Comic Books

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


  • In The Absent Minded Professor and its remake Flubber, Flubber is a perpetual motion substance.
  • In Battlefield Earth, 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 implied that the Psychlos are actually from a different universe with different physical laws.


  • In the second Jim Button book by Michael Ende, the protagonists invent it. Essentially, their version is based on a magnet which you can switch on and off, which pulls their locomotive.
  • Discussed in the Vorkosigan Saga 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 Atlas Shrugged, John Galt's engine relies on perpetual motion.
  • The main focus of the first episode of The Chronicles of Professor Jack Baling is what happens to an engineering professor when he encounters a working perpetual motion machine. Specifically, an overbalanced wheel [1].

Real Life

  • Some people claim that Zero Point Energy can be used to generate power out of nothing.
  • People attempt to patent Perpetual Motion Machines. Most nation patent offices can and will reject any on face value, but a few to get patented if they are labelled as something else.

Video Games

  • In Dwarf Fortress 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.
  • Portal fans 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 Mass Effect somehow work without fuel. In Mass Effect 3 Codex it is outright stated how 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.

Web Original

  • In Ilivais X, 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.
  • A lot of Troll Physics go to this category.
  • A couple of SCP's are noted they could be used as perpetual motion machines, although they would either A. produce power very slowly or B. to dangerous to use.

Western Animation

  • The Simpsons, "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: This "perpetual motion" machine that she made today is a joke -- it just keeps going faster and faster.

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