4. A ballot box with mass m = 6.0 kg slides with speed v = 4.0 m/s across a frictionless floor in the positive direction of an x axis. The box explodes into two pieces. One piece, with mass m1 = 2.0 kg, moves in the positive direction of the x axis at  v1  = 8.0 m/s. What is the velocity of the second piece, with mass m2 ? _______m/s.