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The gravitational two-body model on an elliptic orbit: two simple turning points of the effective potential and
Assuming countable choice for the orbit suppliers, A worked instance inside the two-label gravitational model of the framework's force branch: on the unique global central-force extension of an elliptic relative orbit (, , , eccentricity ), the Kepler effective potential has exactly two simple turning points on the energy window between its minimum and ; the one-dimensional action variable of the radial problem is there with derivative ; and for a noncircular orbit the radial quadrature is the orbital period, so with . The example uses only the model's declared couplings and masses; no measured value of and no empirical premise is used, and no real orbit is claimed to obey the model.
Example
Assume (The Axiom of Countable Choice ()), as required by the conic, ellipse and period suppliers.
Adopt the two-label gravitational model of Adopted two-label gravitational model: the inverse-square pair potential with coupling parameters, the empirical mass identification, and the measured value and uncertainty of the Newtonian constant clause (i) inside the central-force model of Adopted central-force model class: static central pair potentials, the reduced one-label problem, the fixed-centre idealization, reading conventions and exclusions: inertial masses , , reduced mass , gravitational couplings , and the pair potential of Two-body reduction of a static central pair-potential model: the relative coordinate is a central-force trajectory for the reduced mass, the centre of mass moves uniformly, and the reconstruction is a bijection. Let be a motion whose relative coordinate has constant angular momentum with and constant energy . Let be the unique global central-force extension of supplied by Completeness and periodicity of the negative-energy Kepler orbit, with and clause 1 and Kepler's third law for the gravitational two-label model: the relative period is , so under the mass identification, and both labels are periodic relative to the centre of mass clause 1. In all subsequent period and quadrature statements, write , and let be its increasing polar-angle lift. Its restriction to the initial interval is the original relative motion; no global physical time window is assumed. Let , ,
be its conic data (The Kepler orbit equation is a conic: with , Kepler's third law for the gravitational two-label model: the relative period is , so under the mass identification, and both labels are periodic relative to the centre of mass). Write
for the effective potential of Reduction of the central problem to the effective one-dimensional potential: , the energy identity, and the converse reconstruction of the planar trajectory from the radial data (its effective potential ). The example verifies:
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(two simple turning points) is of class on , has the unique critical point with , is strictly decreasing on and strictly increasing on ; hence for every there are exactly two solutions of , with and between them, and the datum with satisfies the hypotheses of The action variable of a two-simple-turning-point family is with derivative , the period quadrature over on the energy window .
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(action derivative) consequently the action variable of The one-dimensional action variable of a family of two-turning-point energy levels is of class on with
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(quadrature equals the orbital period) for a noncircular orbit (equivalently ), the radial coordinate of this global extension solves the one-dimensional problem of clause 1 of Reduction of the central problem to the effective one-dimensional potential: , the energy identity, and the converse reconstruction of the planar trajectory from the radial data with the same energy, and its least positive period is the orbital period of the conic data; the time quadrature of that problem is , so that
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(sharpness of the hypotheses) for the two turning points coalesce at the critical point and the simple-turning-point hypothesis of the action-variable theorem fails; the value is outside this example (no two-sided bound radial motion), so both hypotheses of the construction are genuinely used.
The couplings and the masses are declared model parameters; no measured value of and no empirical premise enters.
Facts & Assumptions
Given: Assume . The gravitational model data , the motion with , , , its unique global relative extension with and polar lift , the conic data , the effective potential and the energy window .
Model and reduction (Adopted two-label gravitational model: the inverse-square pair potential with coupling parameters, the empirical mass identification, and the measured value and uncertainty of the Newtonian constant, Adopted central-force model class: static central pair potentials, the reduced one-label problem, the fixed-centre idealization, reading conventions and exclusions, Two-body reduction of a static central pair-potential model: the relative coordinate is a central-force trajectory for the reduced mass, the centre of mass moves uniformly, and the reconstruction is a bijection, Reduction of the central problem to the effective one-dimensional potential: , the energy identity, and the converse reconstruction of the planar trajectory from the radial data): the relative coordinate of the motion and its global extension are central-force trajectories for with , and its solves the one-dimensional problem of mass in the potential : , with constant; moreover for the polar angle .
Conic geometry (The Kepler orbit equation is a conic: with , Completeness and periodicity of the negative-energy Kepler orbit, with and , Kepler's third law for the gravitational two-label model: the relative period is , so under the mass identification, and both labels are periodic relative to the centre of mass, Polar reduction of a non-radial central-force trajectory: , the effective-potential equation and energy conservation): with and as in the cited conic theorem, and ; consequently is equivalent to , and for the radial coordinate is -periodic, and , with , and the polar angle is a strictly increasing bijection with .
Calculus and integrals (The derivative of at a point that is a limit point of , and differentiability on a set, For a natural the function is differentiable everywhere with derivative ; for it is the constant , with derivative ; for a natural the function is differentiable at every with derivative ; consequently every polynomial function is differentiable at every real, with the derivative computed term by term, Sums, scalar multiples, products and quotients: , , , and when , The chain rule, in one line from Carathéodory: if is differentiable at and is differentiable at , then is differentiable at with , The mean value theorem, as the case of Cauchy's: for continuous on with and differentiable on there is with , Intermediate value theorem, by bisection with a canonical left-half rule: a continuous function on takes every value between and , Continuity of at a point of and on : the - condition, its agreement with at a limit point, and continuity at an isolated point, Euclidean maps and diffeomorphisms, Derivative of an inverse: if is continuous and injective on a nondegenerate interval and differentiable at with , then the inverse is differentiable at with ; and if then is not differentiable at , Substitution: if is differentiable on with integrable and is continuous on an interval containing , then , If is differentiable with integrable then ; and a bounded derivative makes Lipschitz, The lower and upper Darboux integrals of a bounded on as and , Darboux integrability as their equality, and the notation , Intervals of : the nine order-convex forms, nondegeneracy, and length): sums, quotients with nonzero denominator and powers of class are ; the derivative of is and that of is ; the mean-value theorem, the intermediate value theorem and the one-variable inverse-function rule hold on intervals; a continuous integrand on a compact interval is Riemann integrable, substitution is available for strictly monotone changes of variables, and for a function on .
Action-variable theorems (The one-dimensional action variable of a family of two-turning-point energy levels, The action variable of a two-simple-turning-point family is with derivative , the period quadrature over ): for the datum with the two simple turning points on an energy interval and , the action variable is and with the displayed quadrature .
Verification
Proof technique: direct; compute the effective potential's derivative and signs, produce the two simple roots by the intermediate-value and mean-value theorems, apply the one-dimensional action-variable theorem, and identify the time quadrature with the orbital period.
Derivative and critical point: by [L3], which vanishes exactly at and has the sign of ; and . Also , and for any and one has .
Monotonicity and the two simple roots: if , then by [L3] the mean-value theorem applied to on gives ; hence is strictly decreasing on . Similarly, on , the increment is positive, so is strictly increasing on . Now fix . By step 1.1, ; since is continuous, the intermediate-value theorem gives with , and strict monotonicity makes it unique on . Likewise gives a unique with , and monotonicity makes the root unique on . The intermediate values on lie below because the only point where could reach again would be a third root. The roots are simple: because and the sign computation of step 1.1. The assignments therefore have the properties required in [L4] on the energy interval .
Applying the action-variable theorem: by step 2.1 the datum satisfies the hypotheses of [L4] on ; hence is on and with the quadrature , which is claim 2.
The quadrature measures one radial cycle: for , choose a periastron time of the global extension, and let and be the unique times with polar-angle increments and from it, respectively. These exist by the increasing bijection of [L2]. The conic formula of [L2] gives endpoint radii , , the roots of step 2.1 by the energy identity of [L1], and places strictly between them on each open leg. Energy then makes nonzero there. At periastron and , so continuity makes immediately afterwards and throughout , since it cannot change sign without vanishing. At apastron gives on by the same reasoning. On the increasing leg its inverse is differentiable for , with by [L1] and the inverse rule in [L3]. For , FTC on the interior compact interval gives . Letting , continuity of the inverse at its endpoint limits gives on the left, and the convergent improper integral of step 3.1 gives the limit on the right. On the decreasing leg the same interior calculation with gives the identical duration . Thus ; no derivative or compact-interval substitution at a turning endpoint is used.
Least period and the value of the derivative: by [L2] the radial coordinate satisfies with the orbital period of the conic data. Since is a strictly increasing bijection with and with , the periastron value and the apastron value are attained exactly once in every half-open interval of the form . Every positive period must carry a periastron time to another periastron time. The strictly increasing polar angle attains successive periastra at increments of , and shows their successive times differ by ; hence any positive period is at least . Since is a period, it is the least positive radial period. Therefore by step 4.1, and where the last identity uses and : explicitly . Squaring returns . This is claim 3.
Sharpness: if then and , the lower endpoint of ; there is no energy interval containing this circular energy on which two distinct simple turning points exist, so the hypothesis of [L4] fails, in agreement with the circular orbit having a constant radial coordinate and no radial oscillation. The case has no term and no critical point with the stated two-sided conservation, so it is outside the example by hypothesis. This is claim 4.
Claims 1-4 are verified by steps 1.1 and 2.1, by step 3.1, by steps 4.1 and 5.1, and by step 6.1 respectively; the example is verified.
Remarks
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What is checked and what is cited. The elementary computations (the critical point of the effective potential, the two simple roots, the period quadrature) are verified here; the orbit theory that supplies , the energy relation and the period is cited from the mathematical central-force development collected on the page
cm-math-central. The example uses only the model's declared couplings and no measured constant. -
Conventions. here is the radian action of The one-dimensional action variable of a family of two-turning-point energy levels; the corresponding period-one action of Action-angle coordinates of a completely integrable presentation: period-one angles, action coordinates, and linear motion on the tori is its -multiple. The identity is the one-dimensional case of the action-period relation used by the action-angle items of this page.
Depends on
Adopted two-label gravitational model: the inverse-square pair potential with coupling parameters, the empirical mass identification, and the measured value and uncertainty of the Newtonian constant
Adopted central-force model class: static central pair potentials, the reduced one-label problem, the fixed-centre idealization, reading conventions and exclusions
Two-body reduction of a static central pair-potential model: the relative coordinate is a central-force trajectory for the reduced mass, the centre of mass moves uniformly, and the reconstruction is a bijection
Reduction of the central problem to the effective one-dimensional potential: , the energy identity, and the converse reconstruction of the planar trajectory from the radial data
Kepler's third law for the gravitational two-label model: the relative period is , so under the mass identification, and both labels are periodic relative to the centre of mass
The Kepler orbit equation is a conic: with
Completeness and periodicity of the negative-energy Kepler orbit, with and
Polar reduction of a non-radial central-force trajectory: , the effective-potential equation and energy conservation
Central-force trajectories: the central-force equation, angular momentum, energy and the effective potential
The one-dimensional action variable of a family of two-turning-point energy levels
The action variable of a two-simple-turning-point family is with derivative , the period quadrature over
The associated first-order system, motions, maximal motions and the Newtonian flow of a force model
Dimensions and SI unit conventions of the non-relativistic classical mechanics framework
Continuity of at a point of and on : the - condition, its agreement with at a limit point, and continuity at an isolated point
Euclidean maps and diffeomorphisms
The derivative of at a point that is a limit point of , and differentiability on a set
For a natural the function is differentiable everywhere with derivative ; for it is the constant , with derivative ; for a natural the function is differentiable at every with derivative ; consequently every polynomial function is differentiable at every real, with the derivative computed term by term
Sums, scalar multiples, products and quotients: , , , and when
The chain rule, in one line from Carathéodory: if is differentiable at and is differentiable at , then is differentiable at with
The mean value theorem, as the case of Cauchy's: for continuous on with and differentiable on there is with
Intermediate value theorem, by bisection with a canonical left-half rule: a continuous function on takes every value between and
Derivative of an inverse: if is continuous and injective on a nondegenerate interval and differentiable at with , then the inverse is differentiable at with ; and if then is not differentiable at
Substitution: if is differentiable on with integrable and is continuous on an interval containing , then
If is differentiable with integrable then ; and a bounded derivative makes Lipschitz
The lower and upper Darboux integrals of a bounded on as and , Darboux integrability as their equality, and the notation
Intervals of : the nine order-convex forms, nondegeneracy, and length
Action-angle coordinates of a completely integrable presentation: period-one angles, action coordinates, and linear motion on the tori
The Axiom of Countable Choice ()
Dependency tree · 2 levels
Sources
- D. Tong, Classical Dynamics (Cambridge lecture notes, January 2015 printing), Sections 4.5.3 and 4.5.4, printed pages 108-113 (PDF pages 114-119): the one-dimensional frequency identity and the Kepler action variables of the bound orbits
- K. K. Likharev, Essential Graduate Physics, Part CM: Classical Mechanics (title page last edited September 1, 2026), Sections 3.4 and 10.2, chapter pages 7-15 and 5-8 (PDF pages 39-47 and 241-244): the effective potential of the central motion with its minimum, and the action variable of a periodic motion
- I. Stewart, Advanced Classical Mechanics (MIT 8.09/8.309 Fall 2014 course text), Chapter 4, Sections 4.4-4.6, printed pages 74-91 (PDF pages 77-94): the Hamilton-Jacobi equation, the Kepler problem and action-angle variables in the course's local treatment