The centre of mass, G, of a system of particles (masses mr at Pr) can be defined by the relation, O being an arbitrary point,

This is equivalent to finding the point Q1 which divides P1P2 in the ratio m2 : m1 ; then the point Q2 which divides Q1P3 in the ratio m3 : (m1 + m2) ; then the point Q3 which divides Q2P4 in the ratio m4 : (m1 + m2 + m3), and so on until finally G is reached.