A $2.0\ \text{kg}$ ball moving at $3.0\ \text{m/s}$ hits a $1.0\ \text{kg}$ ball at rest. Afterwards the $2.0\ \text{kg}$ ball carries on in the same direction at $1.0\ \text{m/s}$. How fast does the $1.0\ \text{kg}$ ball move?