Q. A solid cylinder of mass M and radius R is kept inside a hollow cylinder of same inner radius but having length 'h' more than the solid cylinder. The empty space is filled with liquid η and open end is closed with cap. Now solid cylinder is given initial angular velocity Wo about ZZ' axis passing through centre of the cylinder. Find the time after which it's angular velocity reduces to half the initial value. Neglect friction acting between the walls of cylinder.