题目内容
(16分)如图所示,质量为
的卡车载有质量为
的重物在平直的公路上以速率
前行,重物与车厢前壁距离为
.卡车紧急制动后做匀变速直线运动,车轮与地面间的动摩擦因数为
,重力加速度为
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020239292960.jpg)
(1)若重物与车厢没有相对滑动,从卡车制动开始到完全停止需要多长时间?此期间重物所受的摩擦力为多大?
(2)若重物与车厢有相对滑动,它们之间的动摩擦因数为
,为了避免制动时重物与车厢前壁发生碰撞,卡车制动前的速率最大为多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023851399.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023867337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023867265.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023882280.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023898335.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023913273.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020239292960.jpg)
(1)若重物与车厢没有相对滑动,从卡车制动开始到完全停止需要多长时间?此期间重物所受的摩擦力为多大?
(2)若重物与车厢有相对滑动,它们之间的动摩擦因数为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023945355.png)
(1)
,
(2) ![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020240691657.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023960574.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023976607.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020240691657.png)
试题分析:(1)设卡车运动方向为正,重物与车厢之间的摩擦力大小为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024085304.png)
由牛顿第二定律:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024101973.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024132489.png)
联立可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023960574.png)
重物所受的摩擦力大小为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024194711.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002023867265.png)
(2)设卡车与地面的摩擦力大小为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024225326.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024241333.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024257348.png)
在制动过程中,由题意:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020242721156.png)
设卡车从制动到停止运动的对地位移为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024288283.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024319315.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024335603.png)
设从卡车制动到重物停止运动,重物的对地位移为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024350316.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024366353.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024381344.png)
由题意有
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024506723.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024522572.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024537451.png)
设重物与车厢前壁恰能发生接触 ,则有
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024693476.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024709629.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002024725648.png)
联立可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020240691657.png)
所以卡车制动前的速率最大为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250020240691657.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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