题目内容
如图所示,质量为m的物体用细绳经过光滑小孔牵引在光滑水平面上做匀速圆周运动,拉力为某个值F时,转动半径为R,当拉力逐渐减小到
时,物体以另一线速度仍做匀速圆周运动,半径为2R,则物体克服外力所做的功是( )
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408242211106682359.jpg)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221109764396.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408242211106682359.jpg)
A.0 | B.![]() | C.![]() | D.![]() |
B
试题分析:设物体两次做匀速圆周运动的速度分别为v1和v2,根据牛顿第二定律可知,当拉力为F时有:F=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221113305610.png)
当拉力为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221109764396.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221109764396.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221116144668.png)
物体在整个运动过程中受重力mg、水平面的支持力N和拉力作用,由于mg和N在竖直方向上,与速度方向始终垂直,因此始终不做功,根据动能定理有:WF=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221117018605.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221117704603.png)
W克=-WF ④
由①②③④式联立解得:W克=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824221110700484.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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