Hey I have a question about an expression in the textbook of Born. There a pendulum of variable is considered. Then it is stated that for shortening a work given by \begin{equation} -ml \int \dot{\phi}^2 dl \end{equation} is done. What I want to ask is how to express this in cartesian coordinates. I mean $\dot{\phi} = \frac{v}{l}$ usually, but $l$ is written outside of the integral. Is there a second $l$ in the integral?


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