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…

A $\displaystyle 0.5$ kg mass is in contact against the inner wall of a cylindrical drum of radius $\displaystyle 4$ 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 $\displaystyle 5$ rad/s. The coefficient of friction between the drum's inner wall surface and mass is $\displaystyle \_\_\_\_$. (Take $\displaystyle \mathrm{g}=10 \mathrm{~m} / \mathrm{s}^2$ )