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Hi - What is perpetual motion?

  1. Sep 24, 2006 #1
    What is perpetual motion? Will it work in space?
  2. jcsd
  3. Sep 24, 2006 #2
    The term "perpetual motion" is usually used to refer to a class of imaginary machines that are supposed to operate perpetually without any imput of energy: no fuel. As concieved of by misguided inventors, the point is always to get the machine to both 1.) do some kind of useful work, and 2.) also do the work of operating itself.

    It is very easy to think such a machine up, and even draw a design on paper, but none of them has ever worked when some physical version of the machine was actually built. A beginning level course in physics is all it takes to gain enough understanding to realize that such a thing is just not physically possible.

    The notion of continuing, non-stop motion is quite a bit different and does, in fact, exist. If you put something into motion it will stay in motion unless something interacts with it to stop it. This is clearly evident in space. Here on earth the forces of gravity and friction are constantly exerted on everything and work to eventually stop any motion that gets started.

    Newton's first law is: "A body in motion or at rest will tend to stay in motion or at rest, unless acted on by an outside force." Here on earth that "outside force" is everywhere and nothing stays in motion for very long.

    In space things will stay in motion forever unless they hit something else.

    Even in space, though, there is no possibility of a machine that can do work and also operate itself. As soon as you make it interact with something to do work on it you have subjected it to that "outside force" which will bring it to a halt.
  4. Sep 24, 2006 #3


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    Excellent response, Zoob. I'll just add that here on Earth, the closest thing to perpetual motion would probably be the current in a superconductor ring. Even then, though, a lot of energy must be expended to keep the temperature in the superconduction range.
  5. Sep 24, 2006 #4
    Thank you, sir.
  6. Sep 24, 2006 #5
    A perpetual motion machine won't work in space. No matter how far you get away form a Mass gravity will always effect it(but the strength decreases) the machine and eventually you'll probably encounter another mass. So it will slow down but it will keep moving.
  7. Sep 24, 2006 #6


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    The laws of thermodynamics forbid motion [at least above quantum scales] without paying taxes. On earth, objects in motion pay frictional taxes. In space, objects in motion pay gravitational taxes [e.g., binary neutron star systems].
  8. Sep 24, 2006 #7
    And just what is the government doing with all our tax calories?
  9. Sep 26, 2006 #8


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    Securing oil supplies last time I checked.
  10. Sep 26, 2006 #9
    Isn't every atom in the universe a perfect example of perpetual motion?
  11. Sep 26, 2006 #10
    And how about the Earth moving through space? Or the stars, et cetera.
  12. Sep 26, 2006 #11
    Not quite, like any other perpetual "machine", it would have to never come into contact with another atom (or interact at all).
  13. Sep 27, 2006 #12
    I'm not sure I understand your response.
  14. Sep 27, 2006 #13
    These cases are covered in Chronos' answer:

    Which I take to mean that anything moving where it is influenced by the gravitation field of anything else is being acted on by an outside force and experiences a corresponding change in its state of motion.

    This means that what I said in my first post:

    is not, strictly speaking, true, since gravity extends throughout space, however much attenuated by distance, and would have some tiny effect of anything, no matter how far it it from anything else.
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