7.As shown above,a ball of mass m,suspended on the end of a

来源：学生作业帮编辑：作业帮分类：英语作业时间：2024/04/27 20:55:52

7.As shown above,a ball of mass m,suspended on the end of a wire,is released from height h and collideselastically,when it is at its lowest point,with a block of mass 2m at rest ona frictionless surface.After the collision,the ball rises to a final heightequal to
(A)    1/9h
(B)   1/8h
(C)   1/3h
(D)   1/2h
(E)    2/3h

如果用能量守恒,和动量守恒做的话,mv=mv1+2mv2 1/2mv1^2+1/2mv2^2=mgh=1/2mv^2

得出v1=3/5根号2gh 方向左 v2=4/5根号2gh 方向有相对速度由原来的v变成了 7/5v 哪里做错了

mv=mv1+2mv2 正确
1/2mv1^2+1/2mv2^2=mgh=1/2mv^2 错误,应该是：
1/2mv1^2+1/2(2m)v2^2=mgh
解出的一组解将是v1=√(2gh) v2=0 应略去,另一组解是有效解,且v1一定是负值.

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