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Newton's Pendulum in space

  1. Aug 7, 2011 #1
    I was wondering what will happen if we had a newton's pendulum (cradle ) in space out of any significant gravitational orbit, although one with only two balls where the string attached to each balls would only be tied to the same center.

    This pendulum would allow the first ball to hit the second one which in his turn by the transfer of energy due to kinetic would orbit itself around the center to then come back hitting the first ball to repeat the cycle.

    My question is; by the law of motion and conservation of energy wouldn't the energy be conserved and only kept transferring from one ball to the other over and over again by kinetic with out resistance since there is no gravity? Would the pendulum be kept in perpetual motion?
  2. jcsd
  3. Aug 7, 2011 #2
    I think so, as long as the centre is fixed in space.
  4. Aug 7, 2011 #3
    Someone told me that due to thermodynamic, the perpetual motion will come to an end. Is it correct?

    If so, how could the energy be released at all? It is not as if it would be emitting heat or radiation. At least not significantly no?
  5. Aug 7, 2011 #4
    Well assuming the collisions are elastic, it would continue forever. You understand we're talking in terms of ideals, far out in space where there is no gravity (no such place exists), a frictionless central rod that has the perfect amount of force applied to it so that the tension due to the string leaves it unmoved...

    Either the motion decays or it doesn't. It's completely dependant on your choice of assumptions. All I'm saying is, as far as I can see, the idealised system breaks no conservation laws.
  6. Aug 7, 2011 #5
    I don't understand why you put it in space, as it needs gravity to function. However, to have almost no resistance it should be in high vacuum.

    Then the motion will still be damped by the few remaining gas molecules and by friction in the strings, at the attachment points to the cradle. And I suspect that even the collisions aren't 100% elastic, I imagine that every time a little (very little) heat is generated. And as Micky indicated, some of the motion may be transmitted to the cradle+support, so you have to consider possible energy absorption there as well.

  7. Aug 7, 2011 #6
    Thank you, both of you confirmed what I suspected and needed to know as for in ideal situation as first describe and also in not so ideal situation.

    As for "why in space?" it is because I wanted to take gravity out of the equation. You are right that a normal newton pendulum would need gravity, though the hypothetically one I describe would not be needing one due to the fact that it would be build to be totally circular and to hit back by doing a 360 degree orbit travel instead of a left to right swing.
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