Question

IS LINEAR MOMENTUM OF A SYSTEM ALWAYS CONSERVED

Answer 1

If a system is closed or isolated, that means there are no outside forces and the forces taking place are only internal to the system, then linear momentum is conserved.  

If two objects collide, each object's momentum is changed. But if you consider both objects as parts of a system, and the collision is an internal force, then the total linear momentum (or the momentum of its center of mass) is conserved.  

Energy is only conserved in completely elastic collision. In any inelastic collisions, energy is lost in forms of sound and heat and many others

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