先求时间：v2-v1=at，t=(8-10)/(-4)=0.5s在求位移：s=v1t+(1/2)at^2=10*0.5-0.5*4*0.5*0.5=4.5m
10-4*t = 8，求得t = 0.5。所以s = 1/2*4*t^2 = 0.5m。
