A small ball with mass m and charge +q is suspended vertically on a long light insulating string of length l. The location of the ball coincides with the center of a large horizontal non-conducting ring of radius r. The ring has a uniformly distributed charge +Q.

Find the period of small-amplitude oscillations of the ball.

Readers from all over the world contributed their solutions to our December Challenge, That’s the way the ball bounces. We are pleased to recognize the following successful solvers:

Guidelines for contributors

Many thanks to all contributors and we hope to hear from many more of you in the future!

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

Boris Korsunsky, Column Editor

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