Consider two particles A and B connected by an intensible string of length L and lying ona smooth table. The distance between A and B is less than L, so the string is not taut.

 

 

 

 

Now if A is projected away from B, it wil move with constant velocity until AB=L. At that instant the string jerks tight and suddenly exerts equal and opposite impulsive tension, J, on A and B.

 

 

 

 

 

 

 

These impulsive tensions cause A and B to experience equal and opposite changes in momentum. Therefore, just as in the case of collision, the total momentum of the system is unchanged by the jerks, i.e. the principle of conservation of linear momentum can be applied.

 

Now the impulse that acts on B jerks B into motion, while the impulse that acts on A gives A a jerk backwards and, because the string is now taut, A and B begin to move on with equal speeds.