题目内容
如图所示,在光滑水平面上,质量为m的小球B连接着一个轻质弹簧,弹簧与小球均处于静止状态.质量为2m的小球A以大小为v0的水平速度向右运动,接触弹簧后逐渐压缩弹簧并使B运动,经过一段时间,A与弹簧分离.
(1)当弹簧压缩至最短时,弹簧的弹性势能EP为多大?
(2)若开始时,在B球的右侧某位置固定一块挡板,在A与弹簧未分离前使B球与挡板发生碰撞,并在碰撞后立即将挡板撤走.设B球与挡板碰撞时间极短,碰后B球的速度大小不变,但方向与原来相反.欲使此后弹簧被压缩到最短时弹簧的弹性势能能达到第(1)问中EP的2.5倍,必须使两球在速度达到多大时与挡板发生碰撞?![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148096766.gif)
(1)当弹簧压缩至最短时,弹簧的弹性势能EP为多大?
(2)若开始时,在B球的右侧某位置固定一块挡板,在A与弹簧未分离前使B球与挡板发生碰撞,并在碰撞后立即将挡板撤走.设B球与挡板碰撞时间极短,碰后B球的速度大小不变,但方向与原来相反.欲使此后弹簧被压缩到最短时弹簧的弹性势能能达到第(1)问中EP的2.5倍,必须使两球在速度达到多大时与挡板发生碰撞?
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148096766.gif)
(1)
(2)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148112510.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148127439.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148143439.gif)
(1)弹簧压缩至最短时,A、B速度均![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148158332.jpg)
为v,选取向右为正,根据动量守恒定律,有:
3分![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148158332.jpg)
根据机械能守恒定律,有:
3分
(2)设B球与挡板碰撞时,A球速度为v1、B球速度为v2(均向右)
根据动量守恒定律:
——① 2分
此时弹簧弹性势
3分
则B球与挡板刚碰后:A球速度为v1、 B球速度为-v2(向左),此后
弹簧压缩至最短时共同速度为v3,则:
——② 2分
此时弹簧弹性势能![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241151482991100.gif)
2分
由题意:
得
——③ 3分
由①②③式可得:
2分
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148158332.jpg)
为v,选取向右为正,根据动量守恒定律,有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148174577.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148190425.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148158332.jpg)
根据机械能守恒定律,有:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148221856.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148112510.gif)
(2)设B球与挡板碰撞时,A球速度为v1、B球速度为v2(均向右)
根据动量守恒定律:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148252572.gif)
此时弹簧弹性势
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148252901.gif)
则B球与挡板刚碰后:A球速度为v1、 B球速度为-v2(向左),此后
弹簧压缩至最短时共同速度为v3,则:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148268663.gif)
此时弹簧弹性势能
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241151482991100.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148330840.gif)
由题意:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148408653.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148424422.gif)
由①②③式可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148127439.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824115148143439.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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