A penny of mass 3.1 g rests on a small 20 g block supported by a spinning disk. The coefficients of friction between the block and the disk are 0.75 (static) and 0.64 (kinetic) while those for the penny and the block are 0.520 (static) and 0.450 (kinetic). What is the maximum rate of rotation that the disk can have, without the block or penny sliding on the disk.