题目内容
(14分)汽车从甲地由静止出发,沿直线运动到丙地,乙在甲、丙两地的中点.汽车从甲地匀加速运动到乙地,经过乙地时的速度为60 km/h;接着又从乙地匀加速运动到丙地,到丙地时速度为120 km/h,求汽车从甲地到达丙地的平均速度.
45 km/h
设甲、丙两地距离为2l,汽车通过甲、乙两地的时间为t1,通过乙、丙两地的时间为t2.
甲到乙是匀加速运动,由x=
·t得
t1=
=
.
从乙到丙也是匀加速运动,由x=
·t得
t2=
=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141852127627.png)
所以
甲丙=
=
=45 km/h.
甲到乙是匀加速运动,由x=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141851799506.png)
t1=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141851830737.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141851924473.png)
从乙到丙也是匀加速运动,由x=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141851799506.png)
t2=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141852049792.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141852127627.png)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141852205276.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141852283548.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824141852314861.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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