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Closed System and Center of Mass

  1. Sep 17, 2010 #1
    Hello,

    Can a closed system change the position of its center of mass if no external force is exerted on it?
     
  2. jcsd
  3. Sep 17, 2010 #2

    Doc Al

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    Position with respect to what? Is it moving?
     
  4. Sep 17, 2010 #3
    hmmmmm
    I'd better have said "...change the velocity of its center of mass..."
     
  5. Sep 17, 2010 #4

    Doc Al

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    That's better. If there's no external force on a system, its center of mass will move with constant velocity.
     
  6. Sep 17, 2010 #5
    Yes. BUT, an internal force must be destructive in effect in order to work.
    This violates nothing, as energy is required and expended.
    Think of a rocket with side thrusters.
    There is no external force. The center of mass changes.
     
  7. Sep 17, 2010 #6

    Doc Al

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    No. The center of mass of the system--including the exhausted fuel--remains moving with constant velocity.
     
  8. Sep 17, 2010 #7
    True, of course.
    However, exhausted fuel is often conventionally regarded as not being a current part of the system at some point.
    "Collectively" definitely yes, but with separation, other, effective aspects arise, including COM changes in the primary mass system.
     
  9. Sep 17, 2010 #8

    Doc Al

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    If you regard the fuel as not being part of the system, then the force it exerts is an external force.
     
  10. Sep 17, 2010 #9
    Doc, you're missing the point.
    Internal explosion CAN create off-centered mass.
    That the "total system" is centered is NOT the issue here.
    What is the issue is that the primary mass system changes.
    That's the point.
     
  11. Sep 17, 2010 #10

    Doc Al

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    Reread the question that started this thread:
    Seems pretty clear to me that the 'total system' is exactly the issue.

    Obviously, if a 'system' breaks apart so that the parts push against each other, those parts can change their motion. That's hardly a closed system.
     
  12. Sep 17, 2010 #11
    OK, so what is physically accepted as a "closed system"?
    I suppose we should start with that.
    COM changes can and do occur in violent internal explosions.
     
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