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Three identical particles A, B and C, each of mass m, travel with the same speed v along the three medians of an equilateral triangle and meet simultaneously at the centroid G. After the collision A stops, while B reverses and moves back with the same speed v. Determine the velocity of C.

1. v directed along the median from G towards C
2. v directed along the median from G towards A
3. 2v directed along the median from G towards C
4. zero

Correct answer: v directed along the median from G towards A

Solution

Initial total momentum is zero by symmetry. After collision A contributes zero and B (which retraced) contributes momentum mv directed from G towards B. For the total to remain zero, C must carry momentum mv directed from G towards A.

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