function f = spring_mass_eq(m,b,k)

close all

n = [1];
d = [m b k];

[A,B,C,D] = tf2ss(n,d);
sys = ss(A,B,C,D);

t = 0:0.15:10;

u = zeros(1,length(t));

xo = 0.5*[10 0; 0 10; -10 0; 0 -10];

p1 = subplot(121);
plot(0,0,'o','MarkerSize',2,'MarkerFaceColor','b')
hold on
axis([-10 10 -10 10])
axis square
set(gca,'FontSize',12,'FontWeight','b')
xlabel('Position')
ylabel('Velocity')
title('Phase Portrait')
p2 = subplot(122);
plot(0,0);
hold on
axis([0 10 -15 15])
set(gca,'FontSize',12,'FontWeight','b')
xlabel('Time (s)')
ylabel('Position')
title('Zero Input Response')

disp('hit any key to continue')
pause

for j = 1:length(xo)
    [Y, T, X] = lsim(sys,u,t,xo(j,:));

    for i = 1:length(X)
        axes(p1)
        %plot(X(i,1),X(i,2),'ro','MarkerFaceColor','r')
        %hold on
        pph(j) = plot(X(1:i,1),X(1:i,2),'-','LineWidth',2);
        %set(gca,'Xlim',[-10 10],'Ylim',[-10 10])
        %axis square
        drawnow
        %hold off
        
        axes(p2)
        zirh(j) = plot(T(1:i),Y(1:i),'-','LineWidth',2);
        drawnow
        %pause(0.01)
    end
    set(pph(j),'Color',[0.8 0.8 0.8]);
    set(zirh(j),'Color',[0.8 0.8 0.8]);
end