HOW TRANSVERSAL FLUCTUATIONS AFFECT THE FRICTION OF A PARTICLE ON A ROUGH INCLINE

We present molecular-dynamics simulations of a sphere moving down an i nclined plane consisting of similar spheres of smaller size. For a cer tain range of inclinations, the sphere moves down the plane with a mea n velocity (v) over bar(x) not equal 0. We investigate the properties of the motion in this steady state and the limits for its existence fo r a certain set of parameters. It is found that the steady-state veloc ity of the particle is independent of material properties and depends only on the geometry of the system. This means that the particle exper iences an effective velocity-dependent friction force, with an effecti ve ''viscosity'' determined only by the geometry. The fluctuations of the motion, however, can depend on the coefficient of restitution e(n) . For example, the diffusion coefficient D-x is influenced by e(n), bu t hardly depends on the roughness of the plane, while for D-y the reve rse is true. The range of the inclination angle and the roughness for which a steady state exists also depends on e(n). We discuss how these results can be understood by considering the details of the motion.