设在这一秒开始时的速度为v
0.4 = v*1-1/2*a*1^2 = v-1/2*0.4*1 = v-0.2
v = 0.6m/s
这一秒前已运动得路程 x = (v0^2-v^2)/(2a) = (3^2-0.6^2)/(2*0.4) = (9-0.36)/0.8 = 10.8m