%  FILE: dyn200.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)
%
%        SUMULATES AN UNDAMPED 2-DOF SYSTEM
%
%  WHO:  LLW
%  WHEN: 1 DEC 1996

% function heading
function ydot = dyn200(t, y)

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

m1 = 2;
m2 = 2;

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


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

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;
       -inv(M)*K    -inv(M)*C  ];

% ---compute ydot--- 
ydot = A * y;

% Return the A matrix only for debugging and testing
if t == (-9999)
  ydot = A;
end