Question: Differentiate with respect to a derivative.

Dear all,

I'm trying to derive the equation of motion for a mechanical system using lagrange equation

I do have u := sum(sum(U[2*i, n](t)*sin((2*I)*pi*x/a)*sin(n*pi*y/b), n = 1 .. 1), i = 1 .. 1); so u is a function of (x,y,t) but U is function of t only.

T:=subs(x = (1/2)*a, diff(u, t))^2;

Therefore T is function of (x,y,t)  and it has terms containing U(t) and d/dt(U(t))

now I need to differentiate T with respect to d/dt(U(t)) .

d T /d U_dot(t)

I tried diff(T,diff(U(t),t)) but it always returns a zero.

Would you please help asap.

Bast Regards,

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