Two-body Kepler problem where the Sun is at rest in a coordinate system orbited by another body

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Discussion Overview

The discussion revolves around the two-body Kepler problem with the Sun at rest in a coordinate system orbited by another body. Participants explore whether this coordinate system can be considered inertial and the implications of the forces acting on the Sun and the other body involved.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant questions if the coordinate system is inertial, seeking elaboration on the principles involved.
  • Another participant notes that if the Sun is at rest, the coordinate system is based on the Sun's rest frame and asks if the Sun experiences any forces that could cause it to accelerate.
  • A hypothetical scenario is presented where the other body has the same mass as the Sun, prompting a discussion on whether either body would be at rest in an inertial coordinate system.
  • One participant argues that since the Sun is pulled by the other mass with an equal force, the coordinate system must be non-inertial, referencing Newton's third law and the definition of inertial systems.
  • Another participant introduces the concept of the Center of Mass frame of reference, suggesting it can simplify the two-body problem.
  • A later reply confirms the reasoning about the barycentre being an inertial frame and its relation to the orbits of both bodies.
  • One participant expresses gratitude for the confirmation and the opportunity to discuss the topic.
  • Another participant critiques the initial framing of the problem, stating that it is an approximation for disparate masses and emphasizes the importance of using reduced mass and relative coordinates.

Areas of Agreement / Disagreement

Participants express differing views on the nature of the coordinate system, with some asserting it is non-inertial while others suggest the barycentric frame is inertial. The discussion remains unresolved regarding the implications of these perspectives.

Contextual Notes

There are limitations in the assumptions made regarding the masses involved and the applicability of the inertial frame definitions. The discussion also highlights the complexity of the two-body problem and the need for careful consideration of the forces at play.

DrToby
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The two-body Kepler problem where the Sun is at rest in a coordinate system orbited by another body: is the coordinate system an inertial reference system or not? Please no yes/no answers. A bit of elaboration is appreciated towards why and which principles apply.
 
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If the Sun is at rest, you are using the Sun's rest frame. Is the Sun affected by any forces that might make it accelerate?
 
Suppose the other body has the same mass as the sun, as in e.g. a binary star system. Would you expect either body to be at rest in an inertial coordinate system?
 
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The Sun is orbited by another mass. The origin of a coordinate system is placed at the center of the Sun. The Sun pulls on the mass with a given force. According to Newtons third law the Sun is pulled towards the mass with an equal force. Hence the coordinate system is accelerated and hence it must be a non-inertial coordinate system. An inertial coordinate system is defined as a system for which Newtons 1. law is valid. Is all this reasoning correct?
 
DrToby said:
An inertial coordinate system
Called the Center Of Mass frame of reference. The same trick is used to reduce many two body problems to modified "one body" ones
 
DrToby said:
Is all this reasoning correct?
Yes.

The barycentre is the point about which both Sun and planet orbit. The barycentric frame is inertial, and the barycentre is one of the foci of the ellipses of both bodies' orbits.
 
Thank you for confirmation. Nice to find people to discuss stuff with.
 
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The problem you describe is not what you want . It is an approximation for very disparate masses. The usual exact solution involves reduced mass and relative coordinates, and the sun is not at rest in the chosen coordinate system. It would be good to read carefully all previous answers as well as any good freshman text.
 

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