IYPT 2014, Задачи на турнира — Задача 14. Rubber motor

Source: Problems for the 27th IYPT 2014 (iypt.org, Aug. 1, 2013)
Selection proposed by the Committee for Problem Selection on July 12; approved and edited at the IOC meeting in National Dong Hwa University on Aug. 1; released on Aug. 1, 2013
Reference Kit for 2014 (first day draft, Aug. 1, 2013) →
Exemplary solutions for 2014 →

When throwing pebbles into water, watch the ripples;
Otherwise throwing the pebbles becomes a futile pastime.
Kozma Prutkov