Question: Why the derivative of r_ work in ?examples,physics (Lagrangian for a pendulum) but not in my document


I have copied codes from the example of the pendulum to use for a simple case of the motion of a particle in the gravitational field. But when it is time to find v_, I get 0 while in the examples worksheet, it works? I try it in Maple 2022 and 2023 so it is not a question of a version of Maple. It seems that I am missing something. Some help would be appreciated.

Here is my

Thank you.

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