The problem of friction in two-dimensional relative motion
  We analyse a mechanical system in two-dimensional relative motion with friction. Although the system is simple, the peculiar interplay between two kinetic friction forces and gravity leads to the wide range of admissible solutions exceeding most intuitive expectations. In particular, the strong qualitative dependence between behaviour of the system, boundary conditions and parameters involved in its description is emphasised. The problem is intended to be discussed in theoretical framework and might be of interest for physics and mechanics students as well as for physics teachers. 
Dariusz Grech (ITP, Univ. of Wroclaw), Zygmunt Mazur (IEP, Univ.   Wroclaw)