Defines a Kepler two-body problem in 2D (inverse-square central force). Unit mass: r'' = -μr / |r|^3. State: [x, y, vx, vy].
Optional
Gravitational parameter μ = G(M+m)
Defines a Kepler two-body problem in 2D (inverse-square central force). Unit mass: r'' = -μr / |r|^3. State: [x, y, vx, vy].