How is the Barycenter Determined in a Two-Body System?

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Discussion Overview

The discussion revolves around the determination of the barycenter in a two-body system, particularly focusing on the mathematical formulation and geometric interpretation of the center of mass. Participants explore the implications of the equations involved and seek clarification on related concepts in celestial mechanics.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Conceptual clarification

Main Points Raised

  • One participant questions why the barycentric center can be computed easily and whether this can be shown geometrically or through the concept of zero forces.
  • Another participant confirms the formula for the center of mass and discusses the double derivative of this formula, suggesting it is a standard vector formula.
  • There is a mention of the terminology used, with a participant noting that "barycentre" may be a British term and expressing a personal connection to the topic due to a long absence from physics.
  • Further questions arise regarding the differentiation of vector equations, specifically the expression involving the dot product of vectors and its implications for understanding the derivation presented in a celestial mechanics text.
  • One participant attempts to clarify the differentiation process of the dot product of vectors, providing a mathematical expression to illustrate their point.

Areas of Agreement / Disagreement

Participants express varying levels of familiarity with the concepts discussed, and while some mathematical formulations are agreed upon, there is no consensus on the geometric interpretation or the clarity of the derivative derivation. The discussion remains unresolved regarding the deeper understanding of these concepts.

Contextual Notes

Participants indicate a need for further review of foundational concepts related to center of mass and gravity, suggesting that some assumptions may be missing or that the discussion is dependent on prior knowledge of the subject matter.

solarblast
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I'm looking at <http://en.wikipedia.org/wiki/Two-body_problem>. I'm
looking not too far down the page in the section:

Center of mass motion (1st one-body problem)

He computes easily R as the barycentric center. Why must
this be so? Can it be shown geometrically, or perhaps by forces that are zero there?
 
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hi solarblast! :smile:

(btw, if you just type a url, the pf software will auomatically make a link out of it, eg http://en.wikipedia.org/wiki/Two-body_problem#Center_of_mass_motion_.281st_one-body_problem.29 :wink:)
solarblast said:
He computes easily R as the barycentric center. Why must
this be so? Can it be shown geometrically, or perhaps by forces that are zero there?

you mean R'' = (m1x1'' + m2x2'')/(m1 + m2) ?

that's just the double derivative of the standard vector formula for centre of mass …

R = (m1x1 + m2x2)/(m1 + m2) :smile:

(btw, i don't think anyone actually calls it "barycentre" :rolleyes:)
 
tiny-tim said:
hi solarblast! :smile:

(btw, if you just type a url, the pf software will auomatically make a link out of it, eg http://en.wikipedia.org/wiki/Two-body_problem#Center_of_mass_motion_.281st_one-body_problem.29 :wink:)


you mean R'' = (m1x1'' + m2x2'')/(m1 + m2) ?

that's just the double derivative of the standard vector formula for centre of mass …

R = (m1x1 + m2x2)/(m1 + m2) :smile:

(btw, i don't think anyone actually calls it "barycentre" :rolleyes:)


Thanks. I'll buy that with the caveats below about what I'm doing.

I think the writer of the page is likely British. They still cling to centre.

It's been a very long time since I've done anything substantive in physics. I'm grasping what I can in a book on celestial mechanics to get to the part I'm interested in, orbits. I skipped the preceding chapter which is really about center of mass and gravity, so maybe I might review what I need there.

While I'm at it, I'll ask another question on derivatives of vectors from an early chapter that I've skimmed through. He mentions r dot r = r**2, where the left side r's are vectors. (maybe I need some meta cmds to express these items as you have done. Where would I get them?) He differentiates that with respect to time and gets
r dot r-dot = rr-dot. r dot r = rr*cos(theta) = r**2 sort of gets me there, but I must be missing something. The derivative derivation he gives doesn't jump off the page to me.
 
hi solarblast! :smile:
solarblast said:
… He mentions r dot r = r**2, where the left side r's are vectors. (maybe I need some meta cmds to express these items as you have done. Where would I get them?) He differentiates that with respect to time and gets
r dot r-dot = rr-dot. r dot r = rr*cos(theta) = r**2 sort of gets me there, but I must be missing something. The derivative derivation he gives doesn't jump off the page to me.

the LHS is (r.r)' = 2(r.r') = 2|r||r'|cosθ

(for bold, use the B button just above the reply box, or use [noparse][/noparse] :wink:)
 

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