From http://star.tau.ac.il/QUIZ/ [Broken] website: This problem can also be found on page 32 http://kvant.mirror1.mccme.ru/1990/05/resheniya_zadach_f1206_-_f1212.htm [Broken], but it's in Russian so might be hard to read... Can someone explain this solution? Is it always true that F=Fo/2? Don't we need to know some details about the thread? I would think this is just a nonhomogeneous differential equation with a force "F". What do you guys think about the solution? Does it always hold true in general?