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Perpetual motion and Newton's first law

  1. May 17, 2012 #1
    This might be a very stupid question. But let me still ask. :)

    Perpetual motion is not possible. But by Newton's first law, a body remains in its state of motion unless an external force acts on it.

    Now, I wonder if the statement "perpetual motion is not possible" is true only in a practical context, or even theoretically? I remember reading one of the initial chapters on Feynman lectures, that to me, sounded like perpetual motion is not possible even in theory.

    So assuming an ideal world of no friction, I roll a ball with some force. And then stop the force. It should keep moving at the same velocity, right? Isn't that perpetual motion?

    I know I have some notion that is completely wrong. But can't figure out what. I appreciate any help in clarifying the same.

    Last edited: May 17, 2012
  2. jcsd
  3. May 17, 2012 #2


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  4. May 18, 2012 #3


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    To clarify moving forever and perpetual motion machines (AKA free energy) are two different things. If you had a hypothetically infinite vaccum tunnel and pushed a ball down it it would travel indefinately. The moment you try to get any energy out of it though you will slow it down and eventually stop it.
  5. May 18, 2012 #4


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    The "non-existance" of perpetual motion machines is not a consequence of Newton's laws but of the laws of thermodynamics.

    The non-existance of "perpetual motion machines of the first kind", from which one can draw power, is a consequence of the first law of thermodynamics, the conservation of energy. The machine has only a finite amount of energy to begin with. If you are drawing energy out of it, eventually, it will have no more energy.

    The non-existance of "perpetual motion machines of the second kind", from which one does NOT draw power, is a consequence of the second law of thermodynamics, the fact that there always exist friction or resistance so that even if we do not draw power, there is still a loss of energy resulting in the machiner eventually running out of energy.

    You can consider the first to be a "theoretical" matter and the second "practical".
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