Solution

ID: past-exam-of-the-short-options-paper-shared-by-prelims-and-parts-a-and-b-of-the-physics-course-of-the-university-of-oxford/2018/s7/2/d/i/solution

Put . Then . For it increases from zero at the north pole to infinity as and has no interior minimum. The vertical orientation is stable. Near zero,
For , away from the marginal case , this gives
The locally signed tilt oscillates at angular frequency , while the azimuth precesses approximately at . The nonnegative polar coordinate is , so its successive pole-to-pole excursions have period and angular repetition frequency . Stating this convention avoids confusing a signed small displacement with the polar-angle magnitude. The full axis direction combines that excursion with precession.
Figure 1.
The vertical spinning top changes from a stable minimum to an unstable maximum
. Dimensionless effective potentials are shown for alpha equal to 0.5 and 2. The second panel marks the off-axis minimum and the zero-energy turning angle approached by a near-separatrix excursion. Details.

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