global gg beta vt;
x0=[0.;0.;-10;10];
gg=10;beta=0.05;vt=20.;
options = odeset('RelTol',1e-8,'AbsTol',1e-8,'MaxStep',0.01);
[t,x]=ode45(@ahf2,[0,50],x0,options);
axis equal
grid on
hold on
n=size(t);
for i=1:1:n
    if (x(i,2)>0)
plot(x(i,1),x(i,2)); 
    end
    end
line([0,-10],[0,0]);
hold off
