What is the difference between Centre of gravity and Centre of mass of a system?
The centre of gravity of a body is defined as the point where the whole weight of the body can be supposed to act.
The centre of mass of a body is the point the where the whole mass of the of the body can be supposed to be concentrated.
For regular bodies, the C.G and C.M. coincide but, for a body of large dimension. The value of g is different for its different parts, the C.M may not coincide with C.G. At a gravity free space, C.G. makes no sense but C.M. has still meaning.