Potentials of conservative forces

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Discussion Overview

The discussion revolves around the nature of conservative forces and the conditions under which a potential is considered conservative. Participants explore the definitions and implications of conservative potentials, particularly in the context of time dependence and the mathematical formulation of forces.

Discussion Character

  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant questions the interpretation of conservative potentials, suggesting that only potentials of the form ##V(r)## should be considered conservative, while ##V(r,\dot{r})## may not fit this definition.
  • Another participant references a discussion on whether the magnetic force is conservative, indicating that this topic is contentious and has been debated in various sources.
  • Some participants propose that if a conservative force is defined as one for which the closed loop integral is zero, then ##V(r,\dot{r})## would not be considered conservative based on that definition.
  • A participant expresses uncertainty about the work of Goldstein, indicating a lack of familiarity with his definitions and context.
  • One participant raises a semantic issue regarding the classification of ##V(\vec{x},\dot{\vec{x}})## as a potential, suggesting that the mathematical formulation and invariance under time translations are more critical than the semantic classification.

Areas of Agreement / Disagreement

Participants do not appear to reach a consensus on the definitions and implications of conservative forces and potentials. Multiple competing views remain, particularly regarding the role of time dependence and the specific forms of potentials.

Contextual Notes

There are limitations in the discussion regarding the assumptions made about the definitions of conservative forces and potentials, as well as the implications of time dependence in the context of Lagrangian mechanics.

Kashmir
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Goldstein writes

"only if ##V## is not an explicit function of time is the system conservative"That means ##V(r,\dot{r})## is a conservative potential, however I think that only potentials of the form ##V(r)## are conservative potentials.

Could you please tell me where I'm going wrong.

Thank you.
 
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PeroK said:
There's a discussion here about whether the magnetic force is conservative or not.

https://en.wikipedia.org/wiki/Conservative_force

And, there is plenty of further discussion online.
If we define a conservative force those whose closed loop integral is zero, then ##V(r,\dot{r})## isn't conservative?
That's the definition the author began with.
 
Kashmir said:
If we define a conservative force those whose closed loop integral is zero, then ##V(r,\dot{r})## isn't conservative?
That's the definition the author began with.
I'm not familiar with Goldstein, so I'm not sure what he's up to!
 
I'd not call ##V(\vec{x},\dot{\vec{x}})## a potential, but that's only a semantic question.

The math simply is that the first variation of the action is invariant under time translations if the Lagrangian is not explicitly time dependent, and then the Hamilotonian,
$$H=\dot{q}^k p_k - L=\text{const}$$
along the solutions of the Euler-Lagrange equations of motion, where the canonical momenta are defined by
$$p_k=\frac{\partial L}{\partial \dot{q}^k}.$$
 

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