Conservation of Angular Momentum

Angular momentum is defined asorwhereis 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 circleandare at right angles, andare at right angles to bothandThis 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.