Collisions Analysis

Masses are equal. Collisions are elastic (velocities exchange).

1. Collision 1 (Box-Wall): Box rebounds. $v_{box} \to v$ (away), $v_{ball} \to v$ (towards).
2. Collision 2 (Ball-Box): Head-on inside box. They exchange velocities. $v_{box} \to v$ (towards), $v_{ball} \to v$ (away).
3. Collision 3 (Box-Wall): Box rebounds again. $v_{box} \to v$ (away).

Now both move away with velocity $v$. Relative velocity is 0.