flow of an ideally granular medium, graphical, rigid-perfectly plastic model, friction law, granular materials, properties of discontinuity lines in velocities, Mechanika stosowana - czasopisma [KABA], technique of constructing the velocity field, velocity distributions, dilatant materials, strong velocity discontinuities, analytical methods, coincidence of slip lines with planes, plane steady incompressible flow, Theory of constitutive functions in solid mechanics, kinematical assumption, sliding on a surface accompanied by an uplift normal to the surface, Soil and rock mechanics, discussion of basic assumptions, Plastic materials, materials of stress-rate and internal-variable type, analysis of the type of governing differential equations