题目内容
(10分)特战队员从悬停在空中离地55m高的直升机上沿绳下滑进行降落训练,某特战队员和他携带的武器质量共为80kg,设特战队员用特制的手套轻握绳子时可获得200N的摩擦阻力,紧握绳子时可获得1000N的摩擦阻力,下滑过程中特战队员至少轻握绳子才能确保安全,(
)。试求:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/2014082500304959915433.png)
(1)特战队员轻握绳子降落时的加速度的大小;
(2)若要求特战队员着地时的速度不大于5m/s,则特战队员在空中下滑的最少时间为多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049568664.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/2014082500304959915433.png)
(1)特战队员轻握绳子降落时的加速度的大小;
(2)若要求特战队员着地时的速度不大于5m/s,则特战队员在空中下滑的最少时间为多少?
(1)
(2)6s
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049615580.png)
试题分析:(1)根据牛顿第二定律可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049677624.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049693934.png)
(2)先轻握绳子下滑,再紧握绳子以最大的摩擦阻力做减速运动,着地时速度为5m/s。这样的下滑过程所需时间最少。
有最大的摩擦阻力时:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049708692.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049724965.png)
设:其中最大速度为v, 有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049740837.png)
解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049771593.png)
所以:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825003049786777.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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