Answer: The conservation conditions of angular momentum are: the vector sum of the external torque of the system is zero, the external force of the rod is gravity when colliding, and the tension and force of the shaft pass through the rotating shaft, so the torque is zero; The external force on the block is gravity, ground support force and friction force, and the gravity and support force also pass through the rotating shaft, so the torque is zero; The action time of friction is very short, which can be ignored compared with the collision elastic force between them, so the angular momentum of the system is conserved;

The conservation condition of momentum is that the system is free from external force or the sum of external force vectors is zero, while the force on the upper shaft of the rod analyzed above is external force, which has a left component in the horizontal direction, so it does not meet the conservation condition of momentum.