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More derivative fun

  1. May 26, 2008 #1
    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?

    \frac{\delta}{\delta \theta}(\dot{\theta} cos\theta)
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  3. May 26, 2008 #2


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    I would say product rule looks good.
  4. May 27, 2008 #3
    Thanks for the input, will give it a go
  5. May 27, 2008 #4


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    Are you assuming here that [itex]\delta()/\delta[/itex] is a derivative? I would be more inclined to think it means "total variation".
  6. May 27, 2008 #5


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    Hi Ultimâ! :smile:

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

    [tex]\frac{\delta}{\delta \theta}(\dot{\theta} cos\theta)\,=\,-\dot{\theta} sin\theta[/tex] ? :confused:
  7. Jun 24, 2008 #6
    It would be nice to think it was that simple tiny-tim, but I fear [tex] \dot{\theta}[/tex] is related to changes in [tex] \theta[/tex] 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:

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