Anomalous frictional behavior in collisions of thin disks

We report on two-dimensional collision experiments with nine thin Delrin di sks with variable axisymmetric mass distributions. The disks floated on an air table, and collided at speeds of about 0.5 to 1.0 m/s with a flat-waile d stationary thick steel plate clamped to the table. The collision angle wa s varied. The observed normal restitution was roughly independent of angle, consistent with other studies. The frictional interaction differed from th at reported for spheres and thick disks, and from predictions of most stand ard rigid-body collision models. For 'sliding'' two-dimensional collisions, most authors assume the ratio of tangential to normal impulse equals mu (f riction coefficient). The observed impulse ratio was appreciably lower roug hly 0.5 mu. slightly into the sliding regime, approaching mu only for nearl y grazing collisions. Separate experiments were conducted to estimate mu; c heck its invariance with force magnitude; and check that the anomalies obse rved are not strongly dependent on velocity magnitude. We speculate that th ese slightly anomalous findings are related to the two-dimensional deformat ion fields in thin disks, and with the disks being only "impulse-response'' rigid and nos "force-response" rigid.