Product Rule for Derivatives of Theta and Time Functions

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Discussion Overview

The discussion revolves around the application of the product rule in the context of derivatives involving functions of theta and time, specifically focusing on the expression \(\frac{\delta}{\delta \theta}(\dot{\theta} \cos\theta)\). The scope includes calculus of variations and the interpretation of the derivative operator.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that the product rule is applicable to the expression involving \(\dot{\theta}\) and \(\cos\theta\).
  • One participant questions whether \(\frac{\delta}{\delta \theta}\) should be interpreted as a derivative or as "total variation," suggesting a different approach to the problem.
  • Another participant expresses concern that \(\dot{\theta}\) may not be independent of \(\theta\), indicating a potential complication in applying the product rule straightforwardly.
  • A participant mentions that the total variation approach is part of a Jacobian, implying a connection to derivatives but expressing uncertainty about its application in this context.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the interpretation of the operator \(\frac{\delta}{\delta \theta}\) or the applicability of the product rule, indicating multiple competing views and unresolved questions.

Contextual Notes

There are limitations regarding the assumptions about the independence of \(\dot{\theta}\) from \(\theta\) and the interpretation of the derivative operator, which remain unresolved in the discussion.

Ultimâ
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This is a bit weird, both are functions of theta (and time) ... so I assume the operator is on both ... is it a case of applying the product rule?

<br /> \frac{\delta}{\delta \theta}(\dot{\theta} cos\theta)<br />
 
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I would say product rule looks good.
 
Thanks for the input, will give it a go
 
Are you assuming here that \delta()/\delta is a derivative? I would be more inclined to think it means "total variation".
 
Ultimâ said:
This is a bit weird, both are functions of theta (and time) ... so I assume the operator is on both ... is it a case of applying the product rule?

<br /> \frac{\delta}{\delta \theta}(\dot{\theta} cos\theta)<br />

Hi Ultimâ! :smile:

If this is calculus of variations, doesn't \frac{\delta}{\delta \theta} mean that you assume that \dot{\theta} is independent of \theta, so that:

\frac{\delta}{\delta \theta}(\dot{\theta} cos\theta)\,=\,-\dot{\theta} sin\theta ? :confused:
 
It would be nice to think it was that simple tiny-tim, but I fear \dot{\theta} is related to changes in \theta even for small variations. HallsofIvy, I'm not familiar with using the total variation approach, but it is part of a Jacobian so I would assume it was a derivative.

I've just posted the full problem here:
https://www.physicsforums.com/showthread.php?p=1731700#post1731700

(I know cross posting is frowned upon, but I was trying split the problem down into simpler components)
 

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