11， Vt^2 - Vo^2 = 2aS, a = (100-4)/120 = 0.812, 设车长S, 2aS = Vt^2 - Vo^2 = 48aS = 24车中点为S/22a*S/2 = Vt^2 - Vo^2Vt^2 = 24 + 1 = 25Vt = 5m/s
