Hi, I was trying to implement some functionality for PyDy for this year's GSoC, and was looking for some advice. In dynamics problems, you usually have time-varying quantities, like generalized coordinates, speeds, and accelerations. Often, you want to take the partial derivative of an expression with respect to the time derivative of one of these quantities. This come up when using Lagrange's Method (or Kane's Method). It's described to some degree here: http://en.wikipedia.org/wiki/Lagrangian_mechanics https://gist.github.com/1005937 In Lagrange's Method, you end up taking the partial derivative of the energy with respect to the time derivative of a generalized coordinate. I'm trying to figure out a way to make this work in PyDy/SymPy. Derivative won't take in anything but a Symbol. The only idea I have come up with is to extend Symbol and write my own .diff() method for it which returns a new symbol representing the time differentiation of the original extended Symbol. Once my new object is inside a Mul or Add sympy object, then my .diff() method is no longer called. Can anyone give some insight into how I could get this desired behavior, taking the derivative of an expression wrt a time-differentiated symbol, to work in a way consistent with existing SymPy behavior? Thanks.
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