CROSSOVER FROM CREEP TO INERTIAL MOTION IN FRICTION DYNAMICS

FRICTION between dry surfaces plays a role in solid dynamics over a wi de range of length scales, from the mechanics of micromachines to eart hquakes. Some common features of frictional dynamics have been observe d for rather different materials, such as rock sliding on rock or meta l on metal1,2. But there is still relatively little understanding of t he way in which frictional motion varies generically with mechanical p arameters. Here we present the results of experiments on frictional sl iding of Bristol board, which show how the characteristics of friction al sliding depend on mass, driving velocity and stiffness of the drivi ng spring constant. In general, there is a bifurcation from stick-slip to steady sliding with increasing velocity. The character of the fric tional motion and of the associated bifurcation changes with velocity: at low speeds creep is dominant, which can be ascribed a characterist ic length, whereas at high speeds the motion can be described as inert ial, with a characteristic timescale. The kinetic friction coefficient decreases with increasing velocity in the former case, and increases with velocity in the latter. We anticipate that these results are like ly to be generic.