Angular momentum is defined asor
where
is the angle as shown below.
All bodies in motion have angular momentum about some point, even if the the particle is travelling in a straight line. As the particle moves from A to B above the quantitystays constant. Angular momentum may be more usefully visualised the particles moving in a circle.
For a particle moving in a circleand
are at right angles, and
are at right angles to both
and
This means that
Kepler's Laws of Motion are a direct consequence of the Law of Conservation of Angular Momentum. The Law applies on all scales, including the quantum mechanical scale, and is responsible for the stability of atoms.