A small marble of charge q and mass m can slide without friction along a long, thin vertical rod passing through the center of a horizontal conducting ring of radius r, mounted on an insulating support. What is the magnitude of the minimum charge Q placed on the ring that would allow the marble to oscillate along the rod?

Our December Challenge, Lord of the ring, attracted quite a few solutions from teachers and students alike. Many students, however, did not follow the submission guidelines—we hope and wish they do better in the future.

We are pleased to recognize the following contributors who solved the problem correctly:

  • Philip Blanco (Grossmont College, El Cajon, CA)

  • Phil Cahill (The SI Organization, Inc., Rosemont, PA)

  • Don Easton (Lacombe, Alberta, Canada)

  • Supriyo Ghosh (KolKata, India)

  • Fredrick P. Gram (Cuyahoga Community College, Cleveland, OH)

  • Gerald E. Hite (TAMUG, Galveston, TX)

  • Art Hovey...

Supplementary Material

Supplementary materials- zip file
AAPT members receive access to The Physics Teacher and the American Journal of Physics as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.