I think i've got it

if I use F=ma(r) I can get
a(r) = (k*q1*q2)/(m*r**2)

then if i use euler/RK I can get velocity and then distance at time t :D

actually, no I don't know
because still, doing this... itsays it would get to the moon in a few seconds - and I'm not sure that's right....