Angular momentum

From Glossary of Meteorology

angular momentum

In Newtonian mechanics the angular momentum (or moment of momentum) L about a point O of a body with linear momentum p is the vector cross product
where r is the position vector of the body relative to O.

In the absence of a net torque, angular momentum is conserved. But angular momentum is a more fundamental quantity than that defined by this equation. For example, photons have intrinsic angular momenta (spin), which can be transferred to objects (as evidenced by radiation torque), and yet the photon has zero rest mass. Thus, angular momentum is best looked upon as a single entity, complete in itself, governed by the dynamical law
where N is the torque acting on the body with angular momentum L. In meteorology, it is conventional to deal with angular momentum per unit volume, which is given by the product r × ρV, where ρ is the density and V the velocity.
Compare momentum, relative angular momentum, absolute angular momentum.