Star with quadrupole in a binary system violates Newton's 3rd Law?

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Discussion Overview

The discussion revolves around the implications of a non-zero quadrupole moment in one star of a binary system and its effects on the gravitational forces experienced by both stars. Participants explore the potential violation of Newton's third law due to the asymmetrical influence of the quadrupole force.

Discussion Character

  • Debate/contested

Main Points Raised

  • One participant posits that if only one star in a binary system has a non-zero quadrupole moment, it experiences an additional force from the quadrupole potential, leading to an imbalance in action and reaction as per Newton's third law.
  • Another participant challenges the initial premise, suggesting that the claim of imbalance is based on a misunderstanding of gravitational interactions, asserting that both stars will feel the effects of the quadrupole force due to their mutual gravitational influence.
  • A further response emphasizes that when calculating gravitational forces, the contributions from each star's mass distribution must be considered, which ensures compliance with Newton's third law.

Areas of Agreement / Disagreement

Participants express disagreement regarding the interpretation of Newton's third law in the context of quadrupole forces, with some asserting that the law holds true while others question its applicability in this specific scenario.

Contextual Notes

The discussion involves assumptions about the nature of gravitational forces in binary systems and the specific conditions under which quadrupole moments affect these forces. The implications of these assumptions on the validity of Newton's third law remain unresolved.

andromeda123
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TL;DR
If in binary system, one of the two stars has a non-zero quadrupole moment, then the other star feels an additional force. On the other hand, the second star feel only the usual gravity.
Assume that, in a binary system, one (and only one) of the two stars has a non-zero quadrupole moment. Then the other star feels the usual gravity force $F_g$ plus an additional force $F_q$ coming from the quadrupole potential. On the other hand, the first star feels only the usual gravity force $-F_g$. Applying Newton's third principle, however, results in an imbalance between action and reaction on the two stars, since only one of them feels a quadrupole force. How is it possible?
 
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phinds said:
??? Your "reference" is a link to where you can post a new thread. It has nothing in it.
Sorry, there is no reference, I have just deleted the link
 
andromeda123 said:
there is no reference
As far as I can tell, you are asking us to explain why something is true, but that something isn't true to begin with.
 
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Yeah, you can be sure Newton's third law supercedes your intuition. If you go to calculate the gravitational forces, you will take every point in one star and apply its force to every point in the other star. Since you do that for both the total forces, they always obey Newton's third law. So both stars will feel an additional quadrupole term, one because of what you have, its quadrupole moment produces additional gravity on the other star, and the other because its own quadrupole mass distribution receives an additional force from the other star.
 

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