Vis-viva equation
= Vis-viva equation
{wiki}
For a <Kepler orbit> with <semi-major axis> $a$, the <vis-viva equation> is
$$
v^2=\mu\left(\frac2r-\frac1a\right).
$$
For the convention that a <hyperbolic Kepler orbit> has positive semi-major-axis magnitude $a$, its final term instead has a plus sign.