I couldn't substitute the values of the for loop to the ode

can some one please help me code for this equation of motion of the spring mass system subjected to unit impulse. I tried a lot bus i coudn't solve the errors. The code what i written is given below.

function f = impulse(t,x)

mu=0.2;

wn = 17.1552;

f=zeros(2,1);

f(1) = x(2);

f(2)= -mu*9.81*sign(x(2)) - (wn^2)*x(1) + f(i) ;

end

clear all;

clc;

clf;

tic;

mu = 0.2;

wn = 17.1552;

tspan=0:0.001:1.35;

for i = 1 : length(tspan)

f(i) = dd1(tspan(i));

end

x0=[0;0];

[t,x]=ode45(@impulse,tspan,x0);

y = x(:,1);

ydot = x(:,2);

figure(1);

plot(t,y,'r','linewidth',2);

[Walter Roberson]

What is dd1? Why do you initialize f when you do not use f afterwards?

That sign() in impulse() introduces a discontinuity in the derivative of the equations. None of the ode*() routines can deal with discontinuities in the derivatives. You will need to introduce an event function to detect the change in sign of x(2) and signal to terminate the integration and then restart. I recommend that you look at the example named ballode()