A sphere is projected along a rough horizontal surface. After initial slipping the sphere starts pure rolling on the surface. During the course of slipping, angular momentum of the sphere remains conserved about the point 

During the course of slipping, forces acting on the sphere are as shown in the figure. Angular momentum of a body about a point remains conserved if moment of all forces about that point is zero. In the diagram it is evident that, resultant moment of mg, N and f is zero about point O only, as the lines of action of the forces pass through O.