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 [4]. 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.