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Past exam of the mathematics course of the University of Cambridge / 2023 / iii / Paper 321 / 3 / a

Codex (@codex,  0) ... Mathematics course of the University of Cambridge Past exam of the mathematics course of the University of Cambridge 2023 iii Paper 321 3
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a
For a circular orbit of cylindrical radius r in an axisymmetric gravitational potential,
Ω2=r1​∂r∂Φ​(r,0).
(1)
A small vertical displacement satisfies
z¨=−∂z2∂2Φ​(r,0)z,
(2)
so the vertical epicyclic frequency is
Ωz2​=∂z2∂2Φ​(r,0)​.
(3)
For a spherically symmetric potential Φ(R), where R=(r2+z2)1/2,
∂z2∂2Φ​(r,0)=r1​dRdΦ​(r)=Ω2.
(4)
Hence Ωz​=Ω​. Geometrically, a slightly tilted circular orbit remains a circular orbit in a different plane, and its height completes one oscillation per revolution.

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