The law of conservation of momentum states that the sum of momenta of two objects before collision is equal to the sum of momenta after the collision provided that

The law of conservation of momentum explains that unless an external force is introduced, the total momentum of two or more bodies operating on each other in an isolated system remains constant.

$m_1$$u_1$$+m_2$$u_2$ = $m_1$$v_1$$+m_2$$v_2$

As per result, neither we create nor destroy momentum. 

• The idea of momentum conservation must be a direct result of Newton's third law of motion. 
• Newton's third law explains that when object A produces a force on object B, object B responds with a force of the same magnitude but opposite direction. 
• Newton formed the law of conservation of momentum from this concept.