The main hypothesis employed during the development of the Bergmark–Roos formulation, as discussed in Ref. , is that fragments move in straight lines from their resting point to the opening where they are removed continuously. Another important assumption of the model is that the fragments are influenced solely by two opposite forces, namely the gravitational force and the friction force due to the interaction with the surrounding fragments. Defining the angular coordinate y, with respect to the vertical, the gravity force is mg cos y, with m being the mass of a fragment and g the gravity acceleration.