题目内容
一辆车违规超车,以v1="108" km/h的速度驶入左侧逆行道时,猛然发现正前方L=80m处一辆卡车正以v2=72km/h的速度迎面驶来,两车司机同时刹车,刹车加速度大小都是a=10m/s2,两司机的反应时间(即司机发现险情到实施刹车所经历的时间)都是△t,试问△t为何值时才能保证两车不相撞?
甲同学很快列出以下方程:![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247369851129.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247369851202.jpg)
又因为
所以得到![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247370161198.jpg)
要使两车不相撞,上式无解,即判别式![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247370631130.jpg)
从而解得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737063778.jpg)
乙同学思考一会后发现甲同学解题过程有误,请你指出上面解题过程的失误之处,并给出正确解题过程和结果。
甲同学很快列出以下方程:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247369851129.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247369851202.jpg)
又因为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737001766.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247370161198.jpg)
要使两车不相撞,上式无解,即判别式
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247370631130.jpg)
从而解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737063778.jpg)
乙同学思考一会后发现甲同学解题过程有误,请你指出上面解题过程的失误之处,并给出正确解题过程和结果。
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737079869.jpg)
甲同学的的失误在于两车不是同时停下来,因此两车不应有共同的运动时间。(4分)设轿车行驶的速度为v1,卡车行驶的速度为v2,则![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737126798.jpg)
(2分)
则在反应时间△t内两车行驶的距离分别为
和
(2分)
由公式
得
(2分)
(2分)
为保证两车不相撞,必须有
(2分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247378742460.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737126798.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737126708.jpg)
则在反应时间△t内两车行驶的距离分别为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737141620.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737172500.jpg)
由公式
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737188991.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247372041327.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247372501236.jpg)
为保证两车不相撞,必须有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824124737313342.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241247378742460.jpg)
![](http://thumb2018.1010pic.com/images/loading.gif)
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