Suppose a sticky substance is thrown at wall. The initial momentum of the wall and substance system is only due to velocity of the substance but the final momentum is 0. Why is momentum not conserved?
Answer
You should also consider what the wall is attached to. And obviously it is the Earth. If we assume the Earth's velocity is zero after the substance is thrown, since there is the force that slow down the substance at the moment of impact, there is also the reaction force on Earth with the same magnitude and opposite direction. So Earth will gain velocity and final momentum of combined Earth and substance system will be equal to the intial momentum of the substance.
And also we can look at the situation in a bit different way. When we stand on the floor and throw the substance, there appears a friction force between our feet and the floor and it acts on us in the throw direction. So the friction force on Earth will be opposite to the throw direction and Earth will pick up speed towards the substance, too. And at any moment, Earth plus substance system will have zero momentum. The substance and the Earth will move towards each other and after the impact their speed will be zero.
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