There is this paragraph in the solution of the question :
Since ball A is suspended by an inextensible string, therefore, just after collision, it can move along horizontal direction only. Hence, a vertically upward impulse is exerted by thread on the ball A. This means that during collision two impulses act on ball A simultaneously. One is impulse interaction J between the balls and the other is impulsive reaction J’ of the thread.
But, why do we need to include impulse by thread? Can't we just apply concept of conservation of mechanical energy? I mean, it got horizontal velocity and this horizontal velocity will act as tangential velocity and will help ball to complete one vertical circle. But, the book doesn't seem to understand this, or am I missing something?