Two massless string of length 5 m hang from the ceiling very near to each other as shown in the figure. Two balls `A` and `B` of masses `0.25kg` and `0.5kg` are attached to the string. The ball `A` is released from rest at a height `0.45m` as shown in the figure. The collision between two balls is completely elastic. Immediately after the collision, the kinetic energy of ball `B` is `1 J` The velocity of ball `A` just after the collision is