$59$ H In Fig. $9\mathrm{.}41,$ block $1($mass $2\mathrm{.}0$ kg $)$ is moving rightward at $10\frac{m}{s}$ and block $2($mass $5\mathrm{.}0$ kg $)$ is moving rightward at $3\mathrm{.}0\frac{m}{s}.$ The surface is frictionless, and a spring with a spring constant of $1120\frac{N}{m}$ is fixed to block $2.$ When the blocks collide, the compression of the spring is maximum at the instant the blocks have the same velocity. Find the maximum compression.

Figure $9\mathrm{.}41$ Problem $59.$

The compression is maximum if the two blocks have the same velocity, why?