Central-force effective potential and orbit stability
For a conservative central interaction U(r), relative motion is planar and angular momentum L=μr²θ̇ is conserved.
Key knowledge
For a conservative central interaction U(r), relative motion is planar and angular momentum L=μr²θ̇ is conserved. Radial energy is E=μṙ²/2+U_eff(r), where U_eff=U+L²/(2μr²); turning points satisfy E=U_eff, and a circular orbit is linearly radially stable when U_eff′=0 and U_eff″>0.
Values and equations
U_eff=U(r)+L²/(2μr²); E=μṙ²/2+U_eff; ω_r²=U_eff″(r_c)/μ
- U,U_eff,E
- Potential, effective potential and relative energy J
- L
- Relative angular momentum magnitude J s
- μ
- Reduced mass kg
- r,r_c
- Separation and circular radius m
- ω_r
- Small radial-oscillation angular frequency rad s⁻¹
Conditions and scope
- Newtonian two-body reduction, fixed L, μ>0, smooth central potential and r>0; the energy excludes centre-of-mass translation.
- A stationary effective-potential point is not automatically a minimum; zero curvature requires nonlinear analysis. Orbit closure is not guaranteed for a general central force.
Sources · 3
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Richard Fitzpatrick / University of Texas at Austin2 sources
- Structure and Interpretation of Classical Mechanics: Canonical transformationsGerald Jay Sussman and Jack Wisdom / MIT PressTextbook chapter
Evidence and review
Newtonian Dynamics: General central force: Equations 299–300: conserved specific angular momentum and general radial orbit. Newtonian Dynamics: Nearly circular orbit: Equations 305–311: radial equilibrium and linear stability; equivalent effective potential derived by integrating radial force. Structure and Interpretation of Classical Mechanics: Canonical transformations: Section 5.1 central-field polar Hamiltonian: radial/tangential energy and cyclic angular coordinate.
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