Elastic: A ball of mass much less than the wall collides with the wall. If the collision is elastic the kinetic energy of ball is unchanged. It means that kinetic energy of wall is zero before and after the collision. Therefore velocity of wall HAS NOT CHANGED. When Velocity is Zero momentum is zero. The change of momentum of ball is 2mv while the change of momentum of wall is ZERO. Momentum is not conserved. Inelastic: ball collides with the wall and due to the glue applied on the surface of ball the ball sticks on the wall after collision. The change of momentum is mv. The change of momentum is zero as the wall doesn't move. The kinetic energy is converted into internal energy and sound energy. Momentum is although not conserved again. As mass of wall is much bigger the change in velocity of ball has to be small but not zero. Here it is zero. In a video Professor walter lewin said that in this case the momentum of wall is NOT zero but kinetic energy of wall is zero. He said that it can be mathematically proven it but i don't this it is possible.