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Homework Help: Lagrangian: How did they get from this step to the other?

  1. Feb 24, 2013 #1
    1. The problem statement, all variables and given/known data
    wi7wg9.png


    2. Relevant equations



    3. The attempt at a solution
    I don't understand where the third term from the first equation of 5.192 come about.. as clearly L doesn't depend on x at all, so ∂L/∂x should be zero.

    sc75p1.png
     
    Last edited: Feb 24, 2013
  2. jcsd
  3. Feb 24, 2013 #2
    The second and third terms both come from differentiating the [itex]m\ell\dot{\theta}\cos{\theta}[/itex] term...you get two terms because there are two [itex]\theta[/itex]'s in it.
     
    Last edited: Feb 24, 2013
  4. Feb 24, 2013 #3
    ah that makes sense! thanks alot!
     
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