$7\mathrm{.}4\mathrm{.}31.[$Level $2]$ A simplified disk brake assembly is shown. A force $F=30lb$ is applied to the lower end of the massless brake lever at a distance $d$ from the lever's pivot $O.$ The brake pad is located $L=0\mathrm{.}2ft$ from $O$ at the upper end of the lever, and the coefficient of friction between the brake pad and disk is $=0\mathrm{.}7.$ The disk has a weight and radius of $W=120lb$ and $R=0\mathrm{.}25ft,$ respectively, and it is initially spinning at a constant ${}_o=1000rpm.$ What should $d$ be if the disk is t come to a stop in $t=1s?$