A string under a tension of 68 N is used to whirl a rock in a horizontal circle of radius 3.7 m at a speed of 16.53 m/s. The string is pulled in, and the speed of the rock increases. When the string is 0.896 m long and the speed of the rock is 71.5 m/s, the string breaks.

What is the breaking strength of the string? Answer in units of N