The position of centre of mass of extended bodies can not be easily calculated because a very large number of particles constituting the body have to be considered. The fact that all the particles of a rigid body have same mass and are uniformly distributed makes things somewhat simpler. If the body is regular in shape and possesses some symmetry (cylindrical or spherical), the calculation is a little bit simplified.

Points to note about the centre of mass:

  1. It may be outside the body as in case of a ring.
  2. When two bodies revolve around each other, they actually revolve around their common centre of mass.