Specific relative angular momentum

In celestial mechanics the specific relative angular momentum ${\displaystyle {\vec {h}}}$ plays a pivotal role in the analysis of the two-body problem. One can show that it is a constant vector for a given orbit under ideal conditions. This essentially proves Kepler's second law.

It's called specific angular momentum because it's not the actual angular momentum ${\displaystyle {\vec {L}}}$, but the angular momentum per mass. Thus, the word "" in this term is short for "mass-specific" or divided-by-mass:

Thus the SI unit is: m²·s⁻¹. ${\displaystyle m}$ denotes the reduced mass ${\displaystyle {\frac {1}{m}}={\frac {1}{m_{1}}}+{\frac {1}{m_{2}}}}$.

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