平均速度:v=s/t= s/〔(s/2)/v1+(s/2)/v2〕
=2v1v2/(v1+v2)
=2×4×6/(4+6)
=4.8m/s
( s/2 是一半路程;v1=4m/s ,v2=6m/s 总时间t=(s/2)/v1+(s/2)/v2 )