汽车的车身长占正桥长的比例很小,
l/s=5.13m/480m=0.01068=1.068%,趋向于0
所以车身长可以忽略不计,即可以认为汽车是一个质点,
t=s/V=480m/100km/h=0.48km/100km/h=0.48x0.01h=0.48x0.01x3600s=17.28s.
答：汽车通过正桥所用时间是17.28s.
如果计算车身长
t'=(s+l)/V=(480m+5.31m)/100km/h=485.13m/100km/h=0.48513km/100km/h=0.48513x0.01h=17.46s
/t-t'//t'=/17.28-17.46//17.46=0.0105=1.05%