where Mg is the mass of the granular bed, m(y)=2yA is the mass
of the unbalanced section (striped zone in Fig. 5 A), A is the crosssectional
area of the tube, is the bulk static density and is a
dissipative coefficient included to account for friction losses, and
used as the only free parameter. The right side of Eq. (1) is obtained
assuming that only two forces act on the whole granular material:
the first term is the pull of the unbalanced section due to the
effective acceleration, the second term is intended to model the
dissipative effect of friction5. The effect of the pull of the unbalanced
section of granular material (thatweare modeling as a liquid)
is similar to the effect of gravity in the case depicted in Fig. 5B.
The only difference is that the chain will fall continuously, while
the granular material will move only during the time when it