function x_dot = f_onespring(t,x)
%simple mass-spring damper: 
global m b k l0 f_ext;

%shortcuts
x1 = x(1);
x2 = x(2);
input = f_ext(t)/m;
x_dot = [x2; -(b/m )*x2-(k/m)*(x1-l0)+input];