Physics Cup – 2017, Problem 1

Problem 1

The best solutoin awards [bonus factor exp(1/3)] goes to all those who managed to find an exact solution:
Siddharth Tiwary
Diogo Netto
Cem Arda Tepe together with Artun Bayer (total points will be divided by 2.) This exact solution deviates from what I had in mind, and is based on a research paper (cited properly). What is noteworthy is that the answer is actually precise and not dependent on the assumption L>>r.

Apart from these, here is a minimalistic solution (non-exact estimate) by Kaarel Hänni (this earns him a bonus factor of 1.1)