centre of mass

and when no torque acts on a system ,u cna conserve angular momentum

plse tell were pure rolling will occur nd werer not???

Well think it intuitively, see when we say a body is rolling purley on a surface we say that for that body v_cm=rω this is because:

imagine a sphere rolling now think abt the surface in contact if the velocity of that point with respect to the ground is zero then we can say no friction is acting(because at that point forward vlocity is rω and the backward velocity is v_cm henc net = v_cm+rω=0 ) hence the rolling is pure(no force is acting).
now if rω≠v_cm then friction acts and will make it to roll purely..

For bodies such as rod or ring the force of gravity cannot be considered to be acting through the c.m. What to do in such a case??