This puzzle appears in V. I. Arnold's classical text on ODEs, where it's attributed to N.N. Konstantinov.
Puzzle
Suppose there are cities A and B connected to each other by two non-intersecting roads. Furthermore, suppose we know that two cars attached by a rope of length less than 2R are able to travel together on different roads from City A to City B without the rope tearing. Given this, is it possible for two circular wagons, each of radius R, each traveling along its center and each starting in a different city, to travel in opposite directions along different roads without colliding as they pass?
Help
Solution
{{{1}}}
References
Ordinary Differential Equations - V. I. Arnold's book on ODEs.