%  FILE: dyn310.m
%  WHAT: function for use with ODE45 for simulating 2-DOF undamped 
%        unforced spring-mass system
%        This function accepts: t (scalar) and y (4x1) as arguments
%        This function returns: ydot = A * y     (ydot is 4x1)
%
%        2-DOF COUPLED UNDAMPED FORCED RESPONSE
%
%  WHO:  LLW
%  WHEN: 1 DEC 1996

% function heading
function ydot = dyn310(t, y)

%---define k and m---
k1 = 8;
k2 = 0;
k3 = 0;

m1 = 2;
m2 = 2;

c1 = 1;
c2 = 0;
c3 = 0;


%---define the A matrix---
M = [    m1    ,   0 ;
          0    ,  m2 ];

MINV = inv(M);
      
K = [ k1+k2   ,  (-k2) ;
       (-k2)  ,  k2+k3 ];

C = [ c1+c2   ,  (-c2) ;
       (-c2)  ,  c2+c3 ];

        
IDENT = [ 1       0 ;
          0       1 ];

ZERO  = [ 0       0 ;
          0       0 ];
      
A = [     ZERO     IDENT;
      -MINV*K    -MINV*C  ];

%---define the forcing function
fbar = [ 0; 0; (MINV * [1;0])];
omega = 2;

%---define the forcing function----
B = fbar * cos( omega * t);

if t == (-9999)
  % for debugging and testing
  ydot = A;
else
  %---compute ydot--- 
  ydot = A * y + B;
end