function dxdts = sec_ord(t,x,params)
% dxdts is the column vector of the first order differential equations
% We pass in the parameters for z and wn in the vector params
z = params(1);
wn = params(2);
dxdts = zeros(2,1);
dxdts(1) = x(2);
dxdts(2) = -(2*z*wn*x(2) + wn^2*x(1));