Suppose a ball obliquely strikes a rough horizontal surface then it experiences a frictional impulse and conservation of linear momentum cannot be done on the horizontal direction.
Now consider another setup in which one block is resting on a rough horizontal surface and another block moving towards the 1st block collides with it. Then is the momentum of the system conserved? I think that it should not be conserved because the value of friction acting on the system (combination of the 2 blocks) changes from zero to a non zero value. But in books I have seen that they apply conservation of linear momentum. Why do we consider frictional impulse in 1st setup and neglect frictional impulse in the second one?