v=v0+at
a = (v-v0)/t = (6-8)/1 = -2m/s^2
汽车的加速度-2m/s^2
x =- v0^2/(2a) = 8^2/(2*2) = 16m
汽车开始刹车后继续前进16m
t=-v0/a = 8/2 = 4s
汽车开始刹车4s后停住