Question

state the law of conservation of momentum with diagram

Answer 1

if no external force acts on a the system then the linear momentum of the system remains constant irrespective of interactions between the particles
internal forces do not change the l.m of the body
the law of conservation of linear momentum is always can be applied to inertial frame 

Answer 2

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Conservation of momentum, general law of physics according to which the quantity called momentum that characterizes motion never changes in an isolated collection of objects; that is, the total momentum of a system remains constant.