A small block of mass ‘m’ is rigidly attached at ‘P to a ring of mass ‘3m’ and radius ‘r’. The system is released from rest at θ = 900 and rolls without sliding. The angular acceleration of hoop just after release (about the centre of the ring) is
∑F=ma⇒f=4 ma..(1)About the center τ=I=α⇒(mg−f)r=(3mr2+mr2)a or mg−f=4ma...(2)From(1) and(2) 8ma=mg a=gg a=g8r