Question

Explain me law of conservation of momentum

Answer 1

This law states that when no external force acts on the system of several interacting particles The total linear momentum of the system is conserved thetotal linear momentum is the vector sum of the linear momentum of all the particles of the system.

Answer 2

this law states that the total momentum of a system before collision is equal to the total momentum after collision.
it can be illustrated with an example as follows.
let a system of balls, ball a and ball b work. let m1, u1 and v1 be the mass, initial velocity, and final velocity of ball a and m2, u2 and v2 be the same for ball b. let u1>u2
according to newton's second law of motion, 
force acting on ball a by ball b:
             f=m1v1-m1u1/t
force acting on ball b by ball a:
             f=m2v2-m2u2/t
according to newton's third law of motion,
             f=-f
therefore,
            m1v1-m1u1/t=-(m2v2-m2u2/t)
or         m1v1-m1u1=-m2v2+m2u2
or         m1v1+m2v2=m2u2+m1u1
or         m1u1+m2u2=m1v1+m2v2
       total momentum=total momentum
       before collision         after collision