We report collision experiments with Delrin disks and a steel plate. The disks floated on an air table, and collided at low speeds with a thick plate clamped to the table. The collision angle was varied. The observed normal restitution was consistent with other studies. The frictional interaction differed from that reported for spheres and thick disks, and from predictions of standard rigid-body collision models.

For ‘sliding’ 2D collisions, most authors assume the ratio of tangential to normal impulse equals μ (friction coefficient). The observed impulse ratio was appreciably lower: roughly μ/2 slightly into the sliding regime, approaching μ only for grazing collisions. Separate experiments were conducted to estimate μ; check its invariance with force magnitude; and check that the anomalies observed are not from dependence on velocity magnitude.

We connect our findings with the nature of deformation fields in disks, and with ‘force-response’ and ‘impulse-response’ rigidity.

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