A 0.5 kg mass is in contact against the inner wall of a cylindrical drum of radius 4 m rotating about its vertical axis. The minimum rotational speed of the drum to enable the mass to remain stuck to the wall (without falling) is $5 \mathrm{rad} / \mathrm{s}$. The coefficient of friction between the drum's inner wall surface and mass is $\_\_\_\_$ . (Take $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2$ )