Answer with Explanation:
since the two blocks move (stick) together, the collision is inelastic, which does not conserve kinetic energy. So do not use kinetic energy consideration.
Fortunately, in such a situation, momentum is still conserved.
Momentum of 1.0 kg block
= 1.0 * 3.0 = 3.0 kg-m/s
Momentum of second block
= 1.0 * 1.0 = 1.0 kg-m/s
Total mass after collision = 1.0+1.0 = 2.0 kg
Common velocity after collision
= total momentum / total mass
= (3.0+1.0)/2.0 = 2.0 m/s
