Dear

 

  Sppose two balls comming towards each other collides

 

 

then at there collision point there is pair of newtonian third law impulsive force pair Right ????

 

 

Now if you will chose both the balls as your system (which we generally do to apply conservation of momentum) then these forces will be internal to the system

 

Now suppose take one of the ball as your sdystem then on it there is an impulsive force at the contact point and in this case this will be external to the system note you cant apply conservation of liner momentum taking a sinlge ball as your system can you ????