@Christopher2222 Yes. So by adding (t) in the initialization of the components the vector becames function of time. Thank you, Christopher.

The second part of the post is related to the manupulation itself. I need now to create even a matrix dependent from time. Maybe

T:=Matrix(3,3,[[T1(t),T2(t),T3(t)],[T4(t),T5(t),T6(t)],[T7(t),T8(t),T9(t)]])

And then compute the derivative with Diff(r1,t).

I tried. The computation works but everything is in an explicit form. So actually I can not do a substitution. I don't know if there is a way to report the formula in a more implicit form.

Thanks