$7\mathrm{.}64$ A disk $\backslash ($ D $\backslash )$ rolls around in a circle with its plane vertical and its center traveling at constant speed $\backslash ($ v$\_\{\backslash$mathrm$\{$c$\}\}\backslash ).$ Find the tension in the string, and the friction force exerted on $\backslash ($ D $\backslash )$ by the floor. $($See Figure P$7\mathrm{.}64.)$