The force constant of a spring is 600 N/m and the unscratched length is 0.72 m. A 3.2-kg block is suspended from the spring. An external force slowly pulls the block down, until the spring has been stretched to a length of 0.86 m. The external force is then removed, and the block rises. In this situation, when the spring has contracted to a length of 0.72 m, what is the upward velocity of the block?