Really? Than why can't it compute the outer orbits of galaxies?
You're confused. You're thinking of F=G(m1*m2)/(d^2) which is Newton's "law" of gravitation. F=ma, concerns the relationship between mass, acceleration and force, and seems to work everywhere.
Really? Than why can't it compute the outer orbits of galaxies?
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That's a different problem. To compute the orbit of heavenly bodies you need to take into account their mutual gravitational pull, along with F=ma, etc. It turns out to be a pretty involved equation because all the variables depend on the other ones. There is no closed form solution for any n-body problem where n>2.
Our only solution is to simulate their dances but of course this simulation is highly dependent on the time step size you pick, how accurate you measure their initial positions, cause any error will get componded over time.