r0=[0;6400];
v0=[9.3;0];
y0=[r0;v0];
rz=6400;
fi=linspace(0,2*pi,100);
clf;
hold on
plot(rz*cos(fi),rz*sin(fi));
axis equal
[y,t]=rk4('gravitacija',y0,0,20e4,1e3,0);
plot(y(1,:),y(2,:),'r')