A sphere of radius r is rolling horizontally without slipping on two parallel rails placed a distance r/2 apart (in the diagram the sphere is rolling perpendicular to the plane of the page).

Which point(s) of the sphere have the maximum instantaneous velocity? Find that maximum velocity if the center of mass of the sphere has a speed v.

The 2010–2011 season of Challenges had a strong finish, with many readers from all over the world submitting the solutions to our March, April, and May problems.

We are pleased to recognize the following contributors:

Mehmet Nuri Akinsi (Istanbul Technical University, Istanbul, Turkey)

Balamurugan, student (Women's Christian College, Chennai, India)

Hratch Barsoumian (Haigazian University, Beirut, Lebanon)

André Bellemans (Université Libre de Bruxelles, Belgium)

Phil Cahill (Lockheed Martin Corporation, North Yorkshire, United Kingdom)

Bartley L. Cardon (South Portland, ME)

Daniel Cartin (Naval Academy Preparatory School, Middetown, RI)

Gang Chen (Zhejiang...

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