A small negatively charged ball of mass m is suspended on a long insulating string. Another small negatively charged ball is slowly moved by a student toward the first one from a large distance. Eventually, the second ball is placed at the original location of the first one. At that moment, the first ball is elevated a small distance h above its original position. How much work is done by the student moving the second ball?

Our December Challenge, Squaring the Circle: Now in 3D! proved to be tough. However, almost all received solutions were correct. We are pleased to recognize the following contributors:

Don Easton (Lacombe, Alberta, Canada)

Fernando Ferreira (Universidade da Beira Interior, Covilhã, Portugal)

Rickard Fors (Södra Latins Gymnasium, Stockholm, Sweden)

Art Hovey (Galvanized Jazz Band, Milford, CT)

José Ignacio Íñiguez de la Torre (Universidad de Salamanca, Salamanca, Spain)

José Costa Leme (High School Lanheses, Viana...

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