v1=1
x1=v1t=1*t=t
a2=0.4
x2=1/2a2t^2=1/2*0.4t^2=0.2t^2
x1=x2
t=0.2t^2
t1=0,t2=5
即出发后5秒追上
间距f(t)=x1-x2=t-0.2t^2=-0.2(t-2.5)^2+1.25
t=2.5时
fmax=1.25
即：出发后2.5秒时最大间距1.25m