Kepler problem

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This article is about a special case of the two-body problem in classical mechanics. For the problem of finding the densest packing of spheres in three-dimensional Euclidean space, see Kepler conjecture.

In classical mechanics, Kepler’s problem is a special case of the two-body problem, in which the two bodies interact by a central force F that varies in strength as the inverse square of the distance r between them

\mathbf{F} = \frac{k}{r^{2}} \mathbf{\hat{r}}

where k is a constant and \mathbf{\hat{r}} represents the unit vector along the line between them.[1] The force may be either attractive (k<0) or repulsive (k>0). This corresponds to a potential energy of the form

V(r) = - \frac{k}{r}

The Kepler problem is named after Johannes Kepler, who proposed Kepler's laws of planetary motion and investigated the types of forces that would result in orbits obeying those laws (called Kepler's inverse problem).[2]

The Kepler problem arises in many contexts. The Kepler problem is important in celestial mechanics, since Newtonian gravity obeys an inverse square law. Examples include a satellite moving about a planet, a planet about its sun, or two binary stars about each other. The Kepler problem is also important in the motion of two charged particles, since Coulomb’s law of electrostatics also obeys an inverse square law. Examples include the hydrogen atom, positronium and muonium, which have all played important roles as model systems for testing physical theories and measuring constants of nature.

The Kepler problem and the simple harmonic oscillator problem are the two most fundamental problems in classical mechanics. They are the only two problems that have closed orbits, i.e., return to their starting point with the same velocity (Bertrand's theorem). The Kepler problem has often been used to develop new methods in classical mechanics, such as Lagrangian mechanics, Hamiltonian mechanics, the Hamilton-Jacobi equation, and action-angle coordinates. The Kepler problem also conserves the Laplace-Runge-Lenz vector, which has since been generalized to include other interactions. The solution of the Kepler problem allowed scientists to show that planetary motion could be explained entirely by classical mechanics and Newton’s law of gravity; the scientific explanation of planetary motion played an important role in ushering in the Enlightenment.

[edit] Solution of the Kepler problem

All attractive central forces can produce circular orbits, which are naturally closed orbits. The only requirement is that the central force exactly equal the centripetal force requirement, which determines the required angular velocity for a given circular radius. Non-central forces (i.e., those that depend on the angular variables as well as the radius) are ignored here, since they do not produce circular orbits in general.

The equation of motion for the radius r of a particle of mass m moving in a central potential V(r) is given by Lagrange's equations

m\frac{d^{2}r}{dt^{2}} - mr \omega^{2} = m\frac{d^{2}r}{dt^{2}} - \frac{L^{2}}{mr^{3}} = -\frac{dV}{dr}

where \omega \equiv \frac{d\theta}{dt} and the angular momentum L = mr2ω is conserved. For illustration, the first term on the left-hand side is zero for circular orbits, and the applied inwards force \frac{dV}{dr} equals the centripetal force requirement mrω2, as expected.

The definition of angular momentum allows a change of independent variable from t to θ

\frac{d}{dt} = \frac{L}{mr^{2}} \frac{d}{d\theta}

giving the new equation of motion that is independent of time

\frac{L}{r^{2}} \frac{d}{d\theta} \left( \frac{L}{mr^{2}} \frac{dr}{d\theta} \right)- \frac{L^{2}}{mr^{3}} = -\frac{dV}{dr}

This equation becomes quasilinear on making the change of variables u \equiv \frac{1}{r} and multiplying both sides by \frac{mr^{2}}{L^{2}}

\frac{d^{2}u}{d\theta^{2}} + u = -\frac{m}{L^{2}} \frac{d}{du} V(1/u)

For an inverse-square force law such as the gravitational or electrostatic potential, the potential can be written

V(\mathbf{r}) = \frac{-k}{r} = -ku

The orbit u(θ) can be derived from the general equation

\frac{d^{2}u}{d\theta^{2}} + u = -\frac{m}{L^{2}} \frac{d}{du} V(1/u) = \frac{km}{L^{2}}

whose solution is the constant \frac{km}{L^{2}} plus a simple sinusoid

u \equiv \frac{1}{r} = \frac{km}{L^{2}} \left[ 1 + e \cos \left( \theta - \theta_{0}\right) \right]

where e (the eccentricity) and θ0 (the phase offset) are constants of integration.

This is the general formula for a conic section that has one focus at the origin; e = 0 corresponds to a circle, e < 1 corresponds to an ellipse, e = 1 corresponds to a parabola, and e > 1 corresponds to a hyperbola. The eccentricity e is related to the total energy E (cf. the Laplace-Runge-Lenz vector)

e = \sqrt{1 + \frac{2EL^{2}}{k^{2}m}}

Comparing these formulae shows that E < 0 corresponds to an ellipse, E = 0 corresponds to a parabola, and E > 0 corresponds to a hyperbola. In particular, E=-\frac{k^{2}m}{2L^{2}} for perfectly circular orbits.

[edit] See also

[edit] References

  1. ^ Arnold, VI (1989). Mathematical Methods of Classical Mechanics, 2nd ed.. New York: Springer-Verlag, 38. ISBN 0-387-96890-3. 
  2. ^ Goldstein, H. (1980). Classical Mechanics, 2nd edition, Addison Wesley. 
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