题目内容
质量为m的小球悬挂于O点,悬线长为l,如图建立平面直角坐标系xOy,y轴沿悬线竖直向下。现将小球拉到(l,0)点后无初速释放,不计空气阻力和钉子的直径,试计算:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241456102162407.jpg)
(1)如果在(0,l)点钉一枚钉子可以挡住细线,那么细线刚碰到钉子后对小球的拉力是多大?
(2)如果将钉子钉在y=
的水平虚线上某位置,要求细线碰到钉子后能够绕钉子做圆周运动通过最高点,那么钉子所钉的位置的横坐标x应该满足什么条件?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241456102162407.jpg)
(1)如果在(0,l)点钉一枚钉子可以挡住细线,那么细线刚碰到钉子后对小球的拉力是多大?
(2)如果将钉子钉在y=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610668346.png)
(1)F=5mg(2)
<x≤
或
≤x<![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610933465.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610746490.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610809511.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610855487.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610933465.png)
试题分析:(1)(4分)设摆到竖直时速度大小为v,悬线刚碰钉子后对小球的拉力大小为F,则由机械能守恒定律知mgh=½mv2? r=l/2 ?,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241456109963375.jpg)
由向心力公式得 F- mg = mv2/r ?, 联解???得 F=5mg
(2)(9分)设小球恰能通过最高点时绕钉子转动的半径为r′,在最高点的速度大小为v′,
则由向心力公式得 mg = mv′ 2 /r′ ?(1分) 机械能守恒
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611011855.png)
联解??得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611074432.png)
设这种情况下钉子的横坐标为x1,则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611152792.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611230605.png)
由于钉子必定在以O为圆心半径为l的圆内,设直线y=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611292346.png)
则有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611355644.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611417606.png)
所以要让小球能通过最高点做圆周运动,钉子在直线y=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145611292346.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610746490.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610809511.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610855487.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610933465.png)
若未考虑?式限制结果写成“x≤
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610809511.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824145610855487.png)
点评:在分析圆周运动时,一定要弄清楚向心力的来源,以及运动半径的变化
![](http://thumb2018.1010pic.com/images/loading.gif)
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