A solid cylinder of mass m = 5 kg and radius R = 0.4 m is given an initial angular velocity ω0=40rads−1 about a horizontal axis through its centre and gently placed on a plank of mass m = 5 kg. The coefficient of friction between the cylinder and plank is μ=0.2. The plank rests on a smooth horizontal surface. The time after which pure rolling motion of cylinder will start relative to the plank is  (Take g=10ms−2 )