Impulse and Momentum
Thus, the total impulse between time t1 and t2 is the area below the resultant force curve from t1 to t2.
Conservation of linear Momentum:
Since the time rate of change of linear momentum is equal to the resultant force acting on the particle, if there is no resultant force, the linear momentum is constant. This is the principle of conservation of momentum which is valid for the system of particles as well.
Suppose, there are two particles A and B that interact during an interval of time. If the interactive forces F and -F between them are the only unbalanced forces acting on the particles during the interval, it follows that the linear impulse on particle A is the negative of linear impulse on particle B. Therefore, the change in linear momentum 
of the particle A is the negative of the change 
in linear momentum of particle B.
Hence,
