in fig. 9-57, a stationary block explodes into two pieces l and r that slide across a frictionless floor and then into regions with friction, where they stop. piece l, with a mass of 2.0 kg, encounters a coefficient of kinetic friction ml 0.40 and slides to a stop in distance dl 0.15 m. piece r encounters a coefficient of kinetic friction mr 0.50 and slides to a stop in distance dr 0.25 m. what was the mass of the block?