# How to derive the true spatial paths (orbits) from the Jacobi-Maupertuis condition?

How can differential equations describing a physical object's true spatial paths (orbits) be derived from the time-independent Jacobi-Maupertuis principle of least action? According to this, it is possible.

The time-independent version of Maupertuis' principle yields (Lanczos 1970, Landau and Lifshitz 1969) corresponding differential equations for the true spatial paths (orbits).

I guess I am looking for equations analogous to the Euler-Lagrange conditions, but for true spatial paths (orbits) instead of true trajectories as a function of time.

This PSE post is relevant, but is concerned with the derivation of equations of motion, whereas here I am concerned with the derivation of true spatial paths (orbits).

• Comment to the question (v1): It seems the question formulation is mixing the following non-standard terminology used in the linked Scholarpedia article: 1. a true path = a parametrized on-shell curve, i.e. a function of time; and 2. a true orbit = an un-parametrized on-shell curve, i.e. not a function of time. – Qmechanic Dec 15 '15 at 13:59
• @Qmechanic I have tried to address your comment with an edit. – ben Dec 15 '15 at 16:42

1. Recall first of all, that in the principle of stationary action, the initial and final times, $$t_i$$ and $$t_f$$, are fixed; while in the abbreviated action principle, the initial and final times, $$t_i$$ and $$t_f$$, are free, but the initial and final positions, $$q^j_i$$ and $$q^j_f$$, together with the energy $$E$$, are fixed.
2. There exist reparametrization-invariant formalisms. However, let us here just outline an elementary pedestrian approach. This consists in picking one of the position coordinates (which possibly after a relabelling we call) $$q^1\equiv \lambda$$ to play the role of the curve parameter $$\lambda$$. The other position coordinates $$q^2, \ldots, q^n$$ are now viewed as functions of $$q^1$$.
4. The good news is that $$q^1\equiv \lambda$$ is naturally fixed at the initial and final position, $$q^1_i$$ and $$q^1_f$$.
5. Now let there be given one of the Maupertuis-like action functional, say $$A$$. After some straightforward manipulations, $$A$$ becomes of the form $$A[q^2, \ldots, q^n]~=~ \int_{q^1_i}^{q^1_f} d\lambda~ f\left(E,\lambda, q, \frac{dq}{d\lambda}\right) .$$ It should be straightforward to see which equations describe the stationary points of $$A$$!