A string is wrapped around a disk of mass m = 2.3 kg and radius R = 0. Starting from rest, you pull the string with a constant force F = 8 N along a...

A string is wrapped around a disk of mass m = 2.3 kg and radius R = 0.12 m. Starting from rest, you pull the string with a constant force F = 8 N along a nearly frictionless surface. At the instant when the center of the disk has moved a distance x = 0.12 m, a length d = 0.27 m of string has unwound off the disk.what is krot? I know v=.9137. I simplified krot=(1/4)mv^2=.480037 but my webassign says that is incorrect.