题目内容
(12分)小明站在水平地面上,手握不可伸长的轻绳一端,绳的另一端系有质量为m的小球,甩动手腕,使球在竖直平面内做圆周运动。当球某次运动到最低点时,绳突然断掉,球飞行水平距离d后落地,如右图所示.已知握绳的手离地面高度为d,手与球之间的绳长为
,重力加速度为g。忽略手的运动半径和空气阻力。
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250002543904218.jpg)
(1)求绳断时球的速度大小v
(2)问绳能承受的最大拉力多大?
(3)改变绳长,使球重复上述运动,若绳仍在球运动到最低点时断掉,且绳的最大承受力不变,要使球抛出的水平距离最大,绳长应为多少?最大水平距离为多少?
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254375402.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250002543904218.jpg)
(1)求绳断时球的速度大小v
(2)问绳能承受的最大拉力多大?
(3)改变绳长,使球重复上述运动,若绳仍在球运动到最低点时断掉,且绳的最大承受力不变,要使球抛出的水平距离最大,绳长应为多少?最大水平距离为多少?
(1)
;(2)
; (3)![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254453619.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254421463.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254437567.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254453619.png)
试题分析:设绳断后球飞行时间为t,
由平抛运动规律,有d -
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254484385.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254531338.png)
水平方向d=vt (1分)
联立解得v=
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254421463.png)
(2)设绳能承受的最大拉力大小为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254562364.png)
球做圆周运动的半径为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254577614.png)
由圆周运动向心力公式,有
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254609831.png)
联立解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254624714.png)
(3)设绳长为L,绳断时球的速度大小为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254702302.png)
有
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254780860.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254796727.png)
绳断后球做平抛运动,是竖直位移为d-L,水平位移为x,时间为t1
有
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254874732.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254889473.png)
得
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254905879.png)
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254921542.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825000254952792.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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