题目内容
(18分) 如图所示,轻质长绳水平地跨过相距
的两个定滑轮A、B上,质量为
的物块悬挂在绳上O点,O与A、B两滑轮的距离相等,在轻绳两端C、D分别施加竖直向下的恒力
,先托住物块,使绳处于水平拉直状态. 从静止释放物块,在物块下落过程中,保持C、D两端的拉力
不变,则:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250025565063141.png)
(1)当物块下落距离
为多大时,物块的加速度为零.
(2)当物块下落上述距离的过程中,克服C端恒力
做的功
为多少?
(3)求物体下落的最大速度
和最大距离
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556428366.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556443337.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556459511.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556490302.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250025565063141.png)
(1)当物块下落距离
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556521313.png)
(2)当物块下落上述距离的过程中,克服C端恒力
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556490302.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556553396.png)
(3)求物体下落的最大速度
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556568367.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556584303.png)
(1)
(2)
(3)
,![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556662544.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556599629.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250025566311047.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556646899.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556662544.png)
试题分析::(1)当物块所受的合外力为零时,加速度为零,此时物块下降距离为
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556521313.png)
因
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556490302.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556490302.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556724297.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556740759.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556755482.png)
由图知:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556787948.png)
(2)物块下落
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556521313.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556833353.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556849688.png)
克服C端恒力F
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556490302.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556880608.png)
由①②③式联立解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250025566311047.png)
(3)在物块下落过程中,共有三个力对物块做功.重力做正功,两端绳子对物块的拉力做负功.两端绳子拉力做的功就等于作用在C、D端的恒力F所做的功.
因物块下降距离
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556521313.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/201408250025569431069.png)
将①②③式代入④式解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556646899.png)
当物块速度减小为零时,物块下落距离达到最大值
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556584303.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002557021331.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002557036717.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002557052660.png)
联立解得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140825/20140825002556662544.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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