Question

I want to know that why we use conservation of momentum

Answer 1

In general, the total momentum of the system is always a constant (i.e) when the impulse due to external forces is zero, the momentum of the system remains constant. This is known as law of conservation of momentum.

The law of momentum conservation can also be stated as follows.
For a collision occurring between object 1 and object 2 in an isolated system, the total momentum of the two objects before the collision is equal to the total momentum of the two objects after the collision. That is, the momentum lost by object 1 is equal to the momentum gained by object 2.

Application of the law of conservation of momentum is important in the solution of collision problems. The operation of rockets exemplifies the conservation of momentum: the increased forward momentum of the rocket is equal but opposite in sign to the momentum of the ejected exhaust gases.
Recoil of gun, bomb explotion also uses the principles of conservation of momentum.
These types of problem cannot be encountered if we can't use law of conservation of momentum. So it is important to use the princples of conservation of momentum.