Angular momentum

The angular momentum of a point mass about a point is defined as $\vec H = \vec r \times \vec p$ where r is the position vector of the point mass with respect to the point of reference and p is the linear momentum vector of the point mass.
The derivative of angular momentum with respect to time is equal to the sum of the external moments applied to the system. This relation is shown by the equation $\vec {\dot H} = \vec r \times \vec F$ From this, it can be concluded that in the absence of an external moment, angular momentum must be conserved.