The sliding of contacting surfaces is a formidable problem with impact in many areas, ranging from atomic to tectonic in scale. In several cases it can result in steady sliding or periodic stick-slip motion, but often displays an intermittent and erratic nature, for which effective descriptions are lacking. In order to investigate this kind of dynamics we have set up an experiment in which a rigid plate engages in stick-slip shear motion on a granular bed. In order to describe the resulting motion we have adopted an unusual approach which is not deterministic in nature (as opposed to, e.g. Burridge-Knopoff and Frenkel-Kontorova models), finding that the dynamics can be accurately described in a quantitative way by a simple stochastic equation, in which the resulting force exerted by the medium on the plate performs a random walk. Further work on the statistical properties of friction in solid-on-solid systems and comparisons with other phenomena suggest that a large class of driven instabilities can be described in terms of similar general mechanisms. Preliminary results on variable height and shaken granular bed are presented.
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