How Do Orbital Dynamics Change Under Different Force Laws?

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Discussion Overview

The discussion revolves around the effects of different force laws, specifically "1/r^3" and "1/r^2" forces, on the orbital dynamics of a particle. Participants explore the implications of slight nudges to energy and angular momentum on the resulting orbits, considering both theoretical and conceptual aspects of orbital mechanics.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant proposes that a particle under a "1/r^3" force in a circular orbit, when nudged, will exhibit behavior similar to a particle on a flat effective potential, suggesting a constant radial velocity until impact.
  • Another participant states that a nudge from a circular orbit under an inverse-square force (1/r^2) will result in an elliptical orbit, indicating a change in eccentricity.
  • A participant expresses uncertainty about the outcome of the first question regarding the "1/r^3" force, indicating that they had previously known the answer but have forgotten it.
  • One participant references previous threads on the inverse cube force law for additional context and information.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the outcome of the first question regarding the "1/r^3" force. There is agreement on the effect of a nudge in the case of the "1/r^2" force, but uncertainty remains about the implications of the "1/r^3" force.

Contextual Notes

The discussion includes references to effective potential and conservation of angular momentum, but lacks detailed mathematical derivations or definitions that could clarify the assumptions involved.

Odyssey
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Suppose a particle is under the influence of a "1/r^3" force. The particle travels in a circular orbit, but its nudged so that its energy and angular momentum changes slightly. What would happen?

What if the force varies with distance as 1/r^2 (like Earth)? (Earth's orbit is almost circular) What would happen?:confused:
 
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Answer to the second question: a nudge off inverse-square central force orbiting body from circular orbit would make its orbit elliptical. (Of course a circle is an ellipse, but I mean an ellipse with some eccentricity.) I used to know the answer to the first question, but have forgotten. Someone here will know.
 
Janitor said:
Answer to the second question: a nudge off inverse-square central force orbiting body from circular orbit would make its orbit elliptical. (Of course a circle is an ellipse, but I mean an ellipse with some eccentricity.) I used to know the answer to the first question, but have forgotten. Someone here will know.

Thank you Janitor. :smile:
 
Odyssey said:
Suppose a particle is under the influence of a "1/r^3" force. The particle travels in a circular orbit, but its nudged so that its energy and angular momentum changes slightly. What would happen?

What if the force varies with distance as 1/r^2 (like Earth)? (Earth's orbit is almost circular) What would happen?:confused:

Look for the other thread (two threads) on the inverse cube force law.

https://www.physicsforums.com/showthread.php?t=45277

https://www.physicsforums.com/showthread.php?t=45330

The effective potential for an particle in a circular orbit is flat (zero). So the behavior of a particle given a small radial nudge will be the same as a particle on a flat line given a nudge, i.e. dr/dt will be positive or negative and constant. I'd assume this continues on blissfully until the particle impacts the surface, at which point it will be moving quite rapidly in because of the conservation of angular momentum.

If you give the particle a nudge that increases its angular momentum, the effective potential won't be zero anymore, making the problem a little more difficult, so I'll refer you to the first thread above.
 

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