A force which acts along the line joining two particles, i.e., one of the form
Goldstein, H. "The Two-Body Central Force Problem." Ch. 3 in Classical Mechanics, 2nd ed.
Reading, MA: Addison-Wesley, pp. 70-127 and 580, 1980.
Stein, S. K. "Inverse Problems for Central Forces." Math. Mag. 69, 83-93, 1996.
Whittaker, E. T. "Central Forces in General: Hamilton's Theorem."
§47 in A Treatise on the Analytical Dynamics of Particles and Rigid Bodies: With an Introduction to the Problem of Three Bodies.
New York: Dover, pp. 77-80, 1944.
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