 Laplace–Runge–Lenz vector

For the signed inverse-square force $\mathbf{F}=-\kappa \mathbf{r}/r^3$ on a particle of mass $m$, with Hamiltonian $H=p^2/(2m)-\kappa /r$, the Laplace-Runge-Lenz vector $\mathbf{A}=\mathbf{p}\times \mathbf{L}-m\kappa \hat{\mathbf{r}}$ is conserved. Attraction has $\kappa >0$ and repulsion has $\kappa <0$. It obeys $\mathbf{A}\cdot \mathbf{L}=0$ and $A^2=m^2{\kappa }^2+2mH{L}^2$. For unit mass in a Kepler orbit, $\kappa =GM$.

The Laplace–Runge–Lenz (LRL) vector is a fundamental concept in celestial mechanics and classical mechanics, particularly in the study of central force problems, such as the motion of planets and satellites around a central body (like the Sun). ### Definition The LRL vector \( \mathbf{A} \) is defined in the context of the motion of a particle under a central force, such as gravity.