Studying physics, I learned that a barycenter of a binary star system is fixed. Then how about the barycenter of multiple objects? Is it also always fixed? (Ignoring all the external forces)

To clarify my question, is there always an inertal frame of reference where the barycenter stays still?

  • $\begingroup$ That would depend on whether you are measuring its position in a coordinate system that is rigidly attached to the barycenter, or... to something else. That, in its turn, would depend on what problem you were trying to solve. If the problem did not concern any other objects besides the ones whose barycenter you are talking about, then the solution might be a whole lot easier if you choose a coordinate system that was fixed with respect to the barycenter. $\endgroup$ Aug 26, 2020 at 12:27
  • $\begingroup$ @SolomonSlow Check out my edit $\endgroup$
    – DH K
    Aug 26, 2020 at 12:30
  • $\begingroup$ Re, "is there always an inertal frame of reference where the barycenter stays still?" That would depend on whether any external force was acting on any of the objects. (i.e., any force acting between any of the objects in question and some other object that is not a contributor to the barycenter that you're talking about.) In other words, if the group as a whole is freely falling, then it's barycenter can be fixed in an "inertial" coordinate frame. Otherwise, no. $\endgroup$ Aug 26, 2020 at 12:32

1 Answer 1


No. Assuming no external forces, barycenter always stays at same coordinates. As in this 3 body example :

enter image description here

  • 1
    $\begingroup$ A better way to put this would be that there always exists a galileian frame in which the barycentre stays at rest. $\endgroup$
    – yukelid
    Aug 26, 2020 at 12:59

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