To determine response using ode45 for a time dependent forcing at specific nodes

function [qdot] = deriv(t,q,A,B)

F(9,1) = 100*sin(100*t);          % There are total 7 nodes (4 dof at each node). I want to apply these 2 forces
F(10,1) = 100*cos(100*t);         % at 3rd node in x and y direction 
qdot = A*q +B*F ;                 % state space eqn - A - 56x56 | q - 56x1 | B - 56x28 | F - 28x1 (all zeroes except 9th/10th row)
                                  % How to write this time dependent force vector the way I have
                                  % written is not correct. In simple
                                  % system we used to directly write
                                  % the force eqn. But in this
                                  % assembled system I dont know how
                                  % to proceed the way I have written
                                  % is not showing any error but it is
                                  % incorrect as I am getting weird
                                  % response.
                               
% All the matrix A B are pre defined and are constant. Force vector is a
% function of time (28x1) with only two values in 9th and 10th row. How to incorporate this in the state space eqn ?
options = odeset;
[time,q]=ode45(@deriv,Time0,zeros(2*ndof,1),options,A,B);

time = time';
response = C*q';

return

I am doing FEM formulation of a rotordynamics problem. Please help me regarding this.

[Jan]

You forgot to mention, what the problem is.

Currently you call deriv from inside itself. This recursion is not wanted.