In the code, I calculate r=(x[sup]2[/sup]+y[sup]2[/sup])[sup]1/2[/sup]

,the force was calculated as F=-GMm/r[sup]2[/sup].

When I want to have x-component of the force: I use Fx=F*x/r

Similar for y-component of the force: Fy=F*y/r.

Because x/r =cos(theta), y/r=sin(theta).