 Solution

ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2009/h/h1/ii/solution

The third particle is force-free, so its torque about the origin vanishes:

\frac{d}{d t} (m_{3} q_{3} \times \dot{q}_{3}) = q_{3} \times m_{3} \ddot{q}_{3} = 0.

Thus this angular momentum is conserved.

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