A string is wrapped around a disk of mass $2$ kg and radius $0\mathrm{.}1$ m$.$ Starting from rest the string is pulled with a constant force of $10$ N on a nearly frictionless surface for a time $0\mathrm{.}5$ second. During this time a length of string of $1\mathrm{.}25$ m unwinds from the disk without slipping $.$ What is the rotational kinetic energy of the disk? $***$Please explain the process, I already know the correct answer is $12\mathrm{.}5$ J$)$