# How to find frequency in centripetal motion?

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A 2.0 x 10^2 g mass is tied to the end of a 1.6 m long string and spun in a vertical circle. What is the minimum frequency of rotation required to keep the mass moving in a circle?

I believe I need to use the equation Fc = m4(pi)^2rf^2 however I don’t know what Fc is so I’m stuck.
The question tries to find the minimum speed at the top of the circle so that the mass fall off a circular path. At the top, with the minimum speed the gravitational force would provide the centripetal acceleration.   $$mg=mv^2/r$$
Therefore, $v=\sqrt{gr}$.
Then convert this minimum speed to minimum $\omega$ by using $\omega r=v$.