Suppose that particles P1,…, Pn, with corresponding masses m1,…, mn, have position vectors r1,…, rn, respectively. The centre of mass (or centroid) is the point with position vector rC, given by
m being the total mass of the particles.
For a rigid body, the corresponding definitions involve integrals. In vector form, the position vector rC of the centre of mass is given by
where ρ(r) is the density at the point with position vector r, R is the region occupied by the body and m is the total mass of the body.