Angular momentum is defined as
or
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 quantity
stays constant. Angular momentum may be more usefully visualised the particles moving in a circle.
For a particle moving in a circle
and
are at right angles, and
are at right angles to bothand
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.