题目内容
如图12所示,用线将金属块M和木块m连在一起浸没在水中,从静止开始以加速度a加速下沉.经过t1时间,线断了,再经过t2时间,木块停止下沉.求此时金属块M的速度vm.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241208447035376.jpg)
图12
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241208447035376.jpg)
图12
vm=![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241208447341663.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241208447341663.gif)
在t1时间内,对M与m整体由牛顿第二定律得?
(M+m)g-(Fm浮-Fm浮)=(M+m)a ①?
在(t1+t2)时间内,对M与m整体由动量定理得?
[(M+m)g-(Fm浮-Fm浮)](t1+t2)=Mvm ②?
由①②联立得![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241208447501755.gif)
(M+m)g-(Fm浮-Fm浮)=(M+m)a ①?
在(t1+t2)时间内,对M与m整体由动量定理得?
[(M+m)g-(Fm浮-Fm浮)](t1+t2)=Mvm ②?
由①②联立得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408241208447501755.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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