In the Bergmark–Roos derivation, it is also assumed that the acceleration of a single granulate is constant during the entire movement. From the force balance, the acceleration reads, arðyÞ ¼ gðcos y cos yGÞ, for jyjpyG, where yG is the maximum allowed angle for displacement i.e., the angle at which the friction force equals particle weight, mg cos yG ¼ Ff , with Ff being the friction force. With our definitions, the aperture has a width 2D ¼ 2rD sin yG and is located at a distance rD from the origin of the polar coordinates, as indicated in Fig. 2. Notice that with this choice of coordinates, the acceleration is negative, thus arðyÞ ¼ jgðcos y cos yGÞj.