clf
disp('For second order scalar DEs that do not depend on t')
disp('one may consider the phase portrait in the (y1,y2)-')
disp('plane, that is in the (y,y''), or position/speed plane')
pause
[y1,y2]=meshgrid(-4:0.5:4,-3:0.4:3);
v1=y2;
v2=-3*sin(y1)-0.2*y2;
quiver(y1,y2,v1,v2)
pause
[t,y]=ode23('damped',[0 10],[-2,0])
pause
hold on
plot(y(:,1),y(:,2),'r-')