Question

define and derive law of conservation of momentum

Answer 1

The law of conservation of momentum states that for two objects colliding in an isolated system, the total momentum before and after the collision is equal.
Law of conservation of momentum : It states that if a group of objects are interacting with each other, then their total momentumremains conserved before and after the interaction, provided no external force acts on them.
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Answer 2

The law of conservation of momentum states that "If no external force acts on a system of particles, the algebraic sum of the linear momentan of the particles remains conserved."In other words, for a bit isolation system the linear momentum of the system is always conserved.