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