1. The problem statement, all variables and given/known data http://img15.imageshack.us/img15/407/draw.th.png [Broken] Initially m and m' are at rest and at the same height. The planes are perpendicular. Find the minimal distance that separates m and m'. 2. Relevant equations 3. The attempt at a solution Well the distance that separates them is the length of the vector that describes m 's position relative to m'. But then I fail to find any idea on how to get to that vector. Thanks for your help.