A block of mass $4\ \text{kg}$ is released from rest at the top of a smooth track shaped like a quarter of a circle of radius $1.8\ \text{m}$. At the bottom, the track joins a smooth horizontal floor, where a block of mass $2\ \text{kg}$ is at rest. The two blocks collide head-on, and the collision is perfectly elastic. Taking $g = 10\ \text{m/s}^2$, the speed of the $2\ \text{kg}$ block after the collision, in m/s, is ____.