Momentum- Law of conservation of Momentum

MOMENTUM
Quantity of motion of a body is referred to as "MOMENTUM".
Definition
Momentum of a moving body defined as :
"the product of mass and velocity of a body is called MOMENTUM."
Mathematically
Momentum = mass x velocity
It is a vector quantity. Momentum is always directed in the direction of velocity.    The unit of momentum is in S.I system kg .m/s or NS.    Momentum depends upon mass and velocity of body.
LAW OF CONSERVATION OF MOMENTUM.
The law of conservation of momentum states that:
"when some bodies constituting an isolated system act upon one another, the total momentum of the system remains constant." OR "the total momentum of an isolated system of interacting bodies remains constant." OR "Total momentum of an isolated system before collision is always equal to total momentum after collision."
Consider an isolated system of two bodies 'A' and 'B' as shown. The masses of bodies are ma and mb and
MATHEMATICAL REPRESENTATION
Consider two bodies of mass m1 and m2 moving initially with velocities u1 and u2.
Total momentum before collision = m1u1 + m2u2
Let after collision their velocities become v1 and v2.
Total momentum after collision = m1v1 + m2v2
According to the law of conservation of momentum
m1u1 + m2u2 = m1v1 + m2v2