# General Mechanics/Many Particles

The uneven dumbbell consisted of just two particles. These results can be extended to cover systems of many particles, and continuous media.

Suppose we have *N* particles, masses *m*_{1} to *m*_{N}, with total mass *M*. Then the centre of mass is

Note that the summation convention only applies to numbers indexing axes, not to *n* which indexes particles.

We again define

The kinetic energy splits into

and the angular momentum into

It is not useful to go onto moments of inertia unless the system is approximately rigid but this is still a useful split, letting us separate the overall motion of the system from the internal motions of its part.