A 30-kg block is resting on a flat horizontal table. On top of this block is resting a 15.0-kg block, to which a horizontal spring is attached. The spring constant of the spring is 325 N/m. The coefficient of kinetic friction between the lower block and the table is 0.600, while the coefficient of static friction between the two blocks is 0.900. A horizontal force F is applied to the lower block. This force is increased in such a way as to keep the blocks moving at a constant speed. At the point where the upper block begins to slip on the lower block determine (a) the amount by which the spring is compressed and (b) the magnitude of the force F.