Elephant walking in railway carriage · Physics Brawl 2014 #FoL.9
Physics · mechanics
Problem
Consider a railway carriage with mass M=20 t, which moves on the tracks without friction. Inside the carriage, there is an elephant with mass m=4 t, initially at rest. Find the distance by which the carriage moves, if the elephant walks a distance of l=12 m and then stops again. (The elephant can move only parallel to the tracks, because the carriage is quite narrow.)