1. The problem statement, all variables and given/known data A heavy uniform bar of mass M rests on top of two identical rollers which are continuously turned rapidly in opposite directions, as shown. The centres of rollers are a distance 2l apart. The coefficient of friction between the bar and the roller surface is ##\mu##, a constant independent of the relative speed of the two surfaces. Initially the bar is held at rest with its centre at distance ##x_0## from the midpoint of the rollers. At time t=0 it is released. Find the subsequent motion of the bar. 2. Relevant equations 3. The attempt at a solution The forces acting on the bar are its own weight, normal reactions from the rollers and force of friction due to relative motion between the rollers and the bar. But the problem is how do I determine the direction of friction? I have this dilemma from a long time and I hope to clear it by means of this thread. Any help is appreciated. Thanks!