Circular motion question – finding rpm

1. The problem statement, all variables and given/known data

A 0.60 kg sphere rotates around a vertical shaft supported by two strings, as shown. If the tension in the upper string is 18 N. Calculate
the tension in the lower string?
the rotation rate (in rev/min) of the system?

2. Relevant equations

Fr= mv^2/r
ω = v/R

3. The attempt at a solution

I believe I have the first part correct, my question is really about the second part. For the second part of the question: I calculated the x components of each tension force to find the centripetal force = 20.96N. I then found the velocity by rearranging the equation: Fr = mv^2/r to get 3.74m/s. I used the equation ω = v/R to get 9.35 rads/sec and converted it to 89.3 rpm by multiplying 9.35 by 60s and dividing it by 2(pi). Am I on the right track?? Just seems like a small answer…

Attached Images
File Type: gif lesson09_image1_2014jan08.gif (14.7 KB)

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