A uniform solid sphere of mass $\mathrm{M}$ and radius $\mathrm{R}$ is lying on a rough horizontal plane. A constant force $F=\mathrm{4 }Mg$ acts vertically downwards at point $\mathrm{P}$ such that the line $\mathrm{OP}$ makes an angle of $60°$ with the horizontal as shown in the figure. The minimum value of the coefficient of friction $\mu$ so that sphere performs pure rolling, is