 Solution

ID: past-exam-of-the-physics-course-of-the-year-1-of-the-natural-sciences-course-of-the-university-of-cambridge/2011/paper-1/section-e/e15/ii/solution

A noncircular ellipse with $r_{0}$ at pericentre has $1 < k < 2$ . Its energy is negative. Merely specifying

k < 1

would make the stated closest-approach point an apocentre, so it is excluded by the geometry; the circular case is listed separately.

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