题目内容
质量为m的物体,以g/3 的加速度由静止竖直下落高度h,在此过程中下列说法中正确的是
A.物体的重力势能减少mgh/3 | B.物体的机械能减少2mgh/3 |
C.物体的动能增加mgh/3 | D.重力对物体做功mgh/3 |
BC
试题分析:下落过程中重力做功为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164316959498.png)
因物体的加速度为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164317131432.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164317443604.png)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164317708692.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164318067718.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164318426909.png)
重力做功等于重力势能的改变量,重力做正功,故重力势能减小
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164316959498.png)
机械能的变化量等于除重力以外的力做的功,所以机械能减少
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164319720673.png)
由动能定理可得动能的改变量
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241643200951160.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824164320454636.png)
故选BC
点评:由于部分同学没弄明白功和能的关系,导致在解答中出现问题;应注意重力做功等于重力势能的改变量;而合力的功等于动能的改变量;阻力的功消耗机械能.
![](http://thumb2018.1010pic.com/images/loading.gif)
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