Why calculate impulsive tension on only one mass in a two-mass system

[Bucky]

Homework Statement

Two small bodies A and B with masses 1kg and 2kg respectively are placed side-by-side on a smoooth horizontal table and are joined by a light inealstic string. A is projected horizontally away form B with a velocity of 6m/s. i)Determine the speed of B after the string becomes taut,
ii)the impulsive tension in the string,
iii)and the loss in kinetic engergy at this stage.

Homework Equations

The Attempt at a Solution

got all three parts ok, but i don't follow the answer for part ii (and in my opinion not understanding is just as bad as getting the wrong answer).

in my notes i have...


v = 0
u = 2m/s
(from i)


Impulsive Energy:

For B: 2u = T
2(2) = T
T = 4N/s


why is this only calculated for B? Since it's the energy in the string I thought it would be the energy in the whole system? or some sort of balance?

In the lecture notes example it seems the impulse is only calculated for the other particle in the system. why? also what would happen if you had 3+ particles in the system?

 

[PhanthomJay]

The problem statement asks for "impulsive tension" and your solution talks about "impulsive energy", but neither term is correct, and your units are not correct. What is the definition of Impulse? You can calculate it using particle A or B.