Mechanics problem

a ball is connected to a string with length L and is swung around a horizontal axis. On top of the ball sits a fly. At the bottom of the path, the speed of the ball is v, and I need to find what force should be exerted by the ball on the fly to keep it from falling. This is confusing to me because the only forces on the fly are downward, and I'm not sure how to combine the knowledge I have about the velocity with the forces. I also know the masses of the ball and fly(ball=M, fly=2M)
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I think it has to do with the fact that acceleration in a circular path is v^2/R, but I'm not sure how

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