An insulating uniform rod of mass m,length l and having a uniform distribution of charge q is hinged at one of its ends on a vertical wall having sigma surface charge density of the same sign,such that it is free to move in a vertical plane perpendicular to wall .Find the charge per unit length of the rod if it makes an angle of 60 w.r.t the vertical in its eq. state.Ans=(under root 3)mg(epsilon not)/(sigma l)