Block A in $\textbf{Fig. E8.24}$ has mass 1.00 kg, and block $B$ has mass 3.00 kg. The blocks are forced together, compressing a spring $S$ between them; then the system is released from rest on a level, frictionless surface. The spring, which has negligible mass, is not fastened to either block and drops to the surface after it has expanded. Block $B$ acquires a speed of 1.20 m/s. (a) What is the final speed of block $A$? (b) How much potential energy was stored in the compressed spring?