Three small metal charged balls have equal charges q and masses m, 4m and m. The balls are connected by light non-conducting strings of length d each and placed on a horizontal non-conducting frictionless table. Initially, the balls are at rest and form a straight line as shown. Then, a quick horizontal push gives the central ball a speed v directed perpendicular to the strings connecting the balls. Find the minimum subsequent distance between the balls of mass m.

We received a very large number of correct solutions from all over the world to our December Challenge, The December Descent. We are pleased to recognize the following selected contributors, both students and teachers:

Guidelines for contributors:

Note: as always, we would very much appreciate reader-contributed original Challenges.

Boris Korsunsky, Column Editor

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