The linear and angular momentum of a collection of particles can be simplified by measuring the position and velocity of the particles relative to the center of mass. Let the system of particles P i, i = 1,..., n of masses m i be located at the coordinates r i with velocities v i. Select a reference point R and compute the relative position and velocity vectors, r i = ( r i − − R ) + R , v i = d d t ( r i − − R ) + v .