  The nature of an instability that controls the transition from static to
dynamical friction is studied in the the context of an array of frictional
disks that are pressed from above on a substrate. In this case the forces are
all explicit and Newtonian dynamics can be employed without any
phenomenological assumptions. We show that an oscillatory instability that had
been discovered recently is responsible for the transition, allowing individual
disks to spontaneously reach the Coulomb limit and slide with dynamic friction.
The transparency of the model allows a full understanding of the phenomenon,
including the speeds of the waves that travel from the trailing to the leading
edge and vice versa.
