Answer:
2.0 kg
Explanation:
For a closed system, the momentum is always conserved. Momentum of an object is given by mv where m is the mass and v is the velocity.
Momentum of the system before the collision is given by:
Momentum of the system after the collision is given by:
Mv where M is the combined mass of the 2 blocks as they stick together and v is the velocity.
As the momentum is conserved i.e momentum before collision and after collision is the same: