function x_dot = f_3springs(t,x)
%simple masses w/ spring dampers: 
global m b k l0 f_ext;

%shortcuts
x1 = x(1);
x2 = x(2);
x3 = x(3);
x4 = x(4);
x5 = x(5);
x6 = x(6);

input = 0.5*f_ext(t)/m;

x_dot = [x2; 
	 -(b/m )*x2-(k/m)*(x1-l0)+(k/m)*(x3-x1-l0); 
	 x4;
	 -(0.5*k/m)*(x3-x1-l0) + (0.5*k/m)*(x5-x3-l0);
	 x6;
	-(0.5*k/m)*(x5-x3-l0)+input];