Two identical balls B and C, in contact with each other and at rest on a horizontal friction less table, are hit head-on by another identical ball A moving initially with a speed v as shown in fig. (3). What is observed, ifthe collision is elastic ?

Here we apply the law of conservation of energy. If m be the mass of each ball, then K.E. before collision =12mv2 case (a). After collision, kinetic energy of the system =0+12mv22+12mv22=14mv2  case (b). =0+0+12mv2=12mv2  case (c). =12mv32+12mv32+12mv32 =16mv2  case (d). =0+0+0=0 Hence case (b) is the only possible result.