Periodic Orbits: The Role of the Coulomb Potential in Producing Periodic Motion

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Discussion Overview

The discussion revolves around the conditions under which periodic motion occurs for a test particle in a central field, specifically examining the role of the Coulomb potential and other potential forms in producing periodic orbits.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Michel questions whether the Coulomb potential is the only potential that produces periodic motion and asks about conditions for periodicity.
  • One participant suggests that while many potentials can lead to periodic orbits, not all will result in closed orbits, introducing the concept of "effective potential" to simplify the analysis.
  • Another participant references Landau-Lifchitz, noting that it claims only two potentials, 1/r and r², yield closed trajectories, but expresses uncertainty about the underlying reasons for this result.
  • A later reply indicates a lack of deeper insight into the fundamental reasons behind the mathematical results discussed.

Areas of Agreement / Disagreement

Participants express differing views on the nature of periodic orbits, with some agreeing on the existence of multiple potentials that can lead to periodic motion, while the specifics of closed orbits remain contested and unresolved.

Contextual Notes

The discussion highlights limitations in understanding the fundamental reasons behind the conditions for closed orbits, as well as the dependence on specific definitions of periodicity and the mathematical framework used.

lalbatros
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Let's consider the motion of a test particle in a central field.
Is the Coulomb potential, 1/r, the only one that produces a periodic motion?
If no, is there a condition for periodicity to occur?

Thanks,

Michel
 
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Most potentials will have periodic oribts, but many of them won't be closed periodic orbits. You can use the idea of an "effective potential" to make the problem one-dimensional, the period of the orbit would then be the time interval between apocenteron or pericentron. But if the angle covered wasn't 2*pi radians, the orbit will be periodic, but not closed.

There's a list in Goldstein "Classical Mechanics" of the force-laws that give closed orbits, IIRC.
 
Thanks a lot for your useful comment.

You gave me the idea to check in Landau-Lifchitz. (I don't have Goldstein unfortunately).
He states that there are only two potentials that result in closed trajectories: 1/r and r² . That's already good to know. However, I don't see where this magic comes from. The algebra is simple and clear, but it does not indicate some more "fundamental" reason.

Michel

PS: In other words, can a property of an integral be understood in another way?
 
I've never had any deeper insight than the mathematical demonstration you've already seen, sorry.
 

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