# Symbolic derivative of function using diff()

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Oscar Sandoval el 23 de Mayo de 2014
Editada: Walter Roberson el 13 de Jun. de 2015
For those familiar, I am writing a function to find the EOMs using the lagrangian.
I have defined all variables as syms: x(t), y(t), z(t)...
Afterward, I define eqns KE and PE as functions of x,y, and z; and L = KE - PE.
When I call diff(L,x(t)) matlab returns the correct answer, but when attempting to take a second derivative with respect to time [diff(diff(L,x(t)),t)], Matlab calls the incorrect form of diff() and yields: "Error using sym/diff, [secondary] arguments must not be symbolic." Is there a way to force this command such that it yields a result? Happy to provide specific code on request, thank you.
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### Respuestas (2)

Mahdi el 23 de Mayo de 2014
Why don't you just tell diff to take the second derivative immediately? Look under the Higher-Order Derivatives subsection for diff.
In this case, you can do:
diff(L,x(t),2) % Second derivative
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Oscar Sandoval el 23 de Mayo de 2014
Mahdi,
x(t) and y(t) are two of six coordinates defining position and orientation of the Center of Mass of a dynamic system. These variables are measured inputs of the system. I am attempting to solve the function symbolically to arrive at the EoMs of the system, which will then be evaluated at the measured values of x and y, amongst other inputs, in order to design a real-time controller. Are you familiar with the Euler-Lagrange equations?
Mahdi el 26 de Mayo de 2014
I am afraid that I am not. I would suggest posting the question again and adding a bit more detail. Try searching the forums because I have run into this problem into the past and found a solution online.

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Mischa Kim el 13 de Jun. de 2015
Check out the Euler-Lagrange tool on File Exchange.
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