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Can a perpetuum mobile work in a vacuum? I think when it is in vacuum it won’t stop working because of outside forces, because there is no outside forces. Tell me if I’m wrong.

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    $\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$
    – ZaellixA
    Oct 24, 2023 at 7:39
  • $\begingroup$ I was thinking about motion who can make energy from nothing. Then I remembered about the perpetual motion. But internet sites said that the perpetual motion is incredible to be build because the outside forces. There is no outside forces in a vacuum if we don’t accept the gravity. If I create perfect vacuum, can the perpetual motion work in it forever? $\endgroup$ Oct 24, 2023 at 10:29

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If there are no external forces acting on a system that is in motion then it will continue in motion. This is a consequence of the conservation of energy. The laws of thermodynamics say that a machine cannot do work indefinitely without an external source of energy - but if there are no external forces then the system is not doing work, so it can continue in motion forever.

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Perpetual motion would also require 0 loss of energy due to internal friction in the object itself. This would not only require a perfect vacuum, but also, among others, perfect bearings and perfect parts. No energy lost to friction and deformations of the object.

So not only external forces on the object will play a part, also internal forces that will lead to a loss of energy.

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Perpetual motion machines of the third kind are not possible because there is always some friction (or friction-like force) that makes them dissipate their energy. Air resistance certainly causes a drag that can be reduced by making the machine run in vacuum, but now other sources of friction matter like the bearings, or even induced currents and electromagnetic radiation from magnetic suspensions. Even two gravitationally bound objects in the depths of space with no outside force whatsoever will lose energy due to gravitational wave emission.

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  • $\begingroup$ Hmm gravitational waves are only emitted if a mass has a quadrupole moment. $\endgroup$
    – Cerise
    Oct 24, 2023 at 11:49
  • $\begingroup$ @Cerise - Two gravitationally bound objects tend to have a quadrupole moment. A single rotating object will have one too, if the mass is not perfectly balanced. But in these cases the slowdown will be absurdly small. $\endgroup$ Oct 24, 2023 at 16:56

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