A clay ball of mass m and speed v strikes another metal ball of the same mass m, which is at rest. They stick together after collision. The kinetic energy of the system after collision is what?

By conservation of linear momentum,

momentum before collision =momentum after collision

                                     mv = 2m v’

                                  v’ =v/2

 Final kinetic energy,         KE_f =(1/2)x(2m)x (v’)² =(mv²) /4