 Bertrand's
theorem: In central force motion, the orbits are closed only for inverse square
law and Hooke's law.
 The blue cross is the center
at which the central force is pointing. As the mass of the interacting
object situated at the blue cross is assumed to be much greater than that of the yellow
sphere, the blue cross is legitimately regarded as a stationary point.
 If the masses of the two interacting bodies are comparable, they together exhibit a
twobody motion
