I don't know what I'm doing wrong with this physics problem, please help!?

Question

A small block with mass 0.300 kg is attached to a string passing through a hole in a frictionless, horizontal surface. The block is originally revolving in a circle with a radius of 0.750 m about the hole with a tangential speed of 4.50 m/s. The string is then pulled slowly from below, shortening the radius of the circle in which the block revolves. The breaking strength of the string is 30.0 N.

What is the radius of the circle when the string breaks?

Thank you in advance!

Answer 1

The tension in the string must provide the centripetal force to keep the mass going in a circle. Centripetal force is m*v^2/r.