2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/26937Recent experiments by Baxter et al. showed the existence of density waves in granular material flowing out of a hopper. We show, using Molecular Dynamics Simulations, that this effect is a consequence of static friction and find that these density fluctuations follow a $1/f$ spectrum. The effect is enhanced when the opening angle of the hopper decreases.7 pages, plain TeXCondensed Mattertext