Sorry for late, I don't get on forum regularly.
I use physics in the beginning, and the force is not as same as what it suppose to act like Magnetic force.
In order to make greater control, I use Circular motion formula:
F = v^2 / r
But few weeks later, I found that physics system is not as accurate as I think, and performance is not as great as I expect, so I redesign it again.
Finally I use bullet behavior, but with same circular motion formula.
Playing with physics and math is quite fun :)