In a ballistic pendulum test, the mv is added to the system and that total remains constant over time. This what conservation of Momentum states. But at the end of the swing velocity is zero. Thus no momentum. What is conserved here? As in energy total energy always remains the same. In momentum its nor true even in ordinary motion with resistance, the velocity decreases thus momentum. To me it is just a Law of Union/Separation. After Union/Separation its not use/applicable.