Block $A$ in has mass $1\mathrm{.}00$ kg $,$ and block $B$ has mass $3\mathrm{.}00$ 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 $1\mathrm{.}60\frac{m}{s}.$