Among the following items, for a ring, the centre of mass lies outside the body. Centre of mass (COM) is just a point where the entire mass of the body can be considered to be concentrated, while the actual concentration of mass may be at some other place. It is not necessary that it will always lie within the body. It may lie outside the body also. For a body like a uniform ring or a hollow cylindrical object, we find that the COM does not lie on the body. Instead it is found to lie outside the body itself. For a ring, its COM lies at the centre of the circular shape made by it.