骑自行车的人以5m/s初速度匀减速地上坡,每秒速度减少0.4m/s,斜坡长30m,求骑自行车人通过斜坡需要多少时

Vo=5m/s,a=-0。4m/s^2,S=30mS=Vo*t+(1/2)at^230=5t(0。5)*(-0。4)t^2t^2-25+150=0(t-10)(t-15)=0t1=10st2=15t讨论:速度减至0所需时间T=(0-Vo)/a=(0-5)/(-0。4)=12。5st2>T,不合题意,舍去所求时间为t=10s