The centre of mass of a system of particles is a specific point at which, for many purposes, the system's mass behaves as if it were concentrated. The centre of mass is a function only of the positions and masses of the particles that comprise the system. In the case of a rigid body, the position of its centre of mass is fixed in relation to the object (but not necessarily in contact with it). In the case of a loose distribution of masses in free space, such as, say, shot from a shotgun, the position of the centre of mass is a point in space among them that may not correspond to the position of any individual mass. In the context of an entirely uniform gravitational field, the center of mass is often called the centre of gravity — the point where gravity can be said to act. from: Wikipedia
Centre of Gravity vs Centre of Mass. In a uniform gravitational field the centre of gravity is identical to the centre of mass , a term preferred by physicists. The two do not always coincide, however. For example, the Moon's centre of mass is very close to its geometric centre (it is not exact because the Moon is not a perfect uniform sphere), but its centre of gravity is slightly displaced toward the Earth because of the stronger gravitational force on the Moon’s near side. from: http://www.britannica.com
CG of pendulum bob.
The centre of gravity of the pendulum bob is in the centre of the bob.
where is CG now (plasticine on top)?
The centre of gravity has moved higher up. It is not possible to know the exact spot as it is no longer a regular shape.
where is CG now (plasticine wrapped around bob/roti-prata method)?
Provided the plasticine is wrapped around the bob with equal weight all around, the centre of gravity would still be in the centre. If the plasticine is placed unevenly, the centre of gravity would be near the centre but not quite there.
CG difference when there is a hole drilled into the centre.
I don't really know. I guess it would still be in the centre.
By the way, all the pictures were drawn by me on Paint.
