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如图所示，光滑绝缘水平桌面上固定一绝缘挡板P，质量分别为mA和mB的小物块A和B（可视为质点）分别带有+QA和+QB的电荷量，两物块由绝缘的轻弹簧相连，一不可伸长的轻绳跨过定滑轮，一端与物块B连接，另一端连接轻质小钩．整个装置处于正交的场强大小为E、方向水平向左的匀强电场和磁感应强度大小为B、方向水平向里的匀强磁场中．物块A，B开始时均静止，已知弹簧的劲度系数为K，不计一切摩擦及AB间的库仑力，物块A、B所带的电荷量不变，B不会碰到滑轮，物块A、B均不离开水平桌面．若在小钩上挂一质量为M的物块C并由静止释放，可使物块A对挡板P的压力为零，但不会离开P，则

（1）求物块C下落的最大距离；

（2）求小物块C下落到最低点的过程中，小物块B的电势能的变化量、弹簧的弹性势能变化量；

（3）若C的质量改为2M，求小物块A刚离开挡板P时小物块B的速度大小以及此时小物块B对水平桌面的压力．

答案

（1）开始时弹簧的形变量为x1，

对物体B由平衡条件可得：kx1=QBE

设A刚离开挡板时，

弹簧的形变量为x2，

对物块B由平衡条件可得：kx2=QAE

故C下降的最大距离为：h=x1+x2=

E
k
(QA+QB)

（2）物块C由静止释放下落h至最低点的过程中，

B的电势能增加量为：

△Ep=QBEh=

E2
k
QB(QA+QB)

由能量守恒定律可知：

物块由静止释放至下落h至最低点的过程中，

c的重力势能减小量等于

B的电势能的增量和弹簧弹性势能的增量

即：Mgh=QBEh+△E弹

解得：△E弹=

E
k
(Mg-QBE)(QA+QB)

故小物块C下落到最低点的过程中，小物块B的电势能的变化量为

E2
k
QB(QA+QB)，弹簧的弹性势能变化量为△E弹=
E
k
(Mg-QBE)(QA+QB)

（3）当C的质量为2M时，

设A刚离开挡板时B的速度为V，

由能量守恒定律可知：2Mgh=QBEh+△E弹+

1
2
(2M+mB)V2

解得A刚离开P时B的速度为：V=

2MgE(QA+QB)
(2M+mB)k

因为物块AB均不离开水平桌面，

设物体B所受支持力为NB1，所以对物块B竖直方向受力平衡：

mBg=NB1+QBvB

由牛顿第三定律得：NB=NB1

解得：NB=mBg-BQB

2MgE(QA+QB)
k(2M+mB)

故小物块A刚离开挡板P时小物块B的速度大小为：V=

2MgE(QA+QB)
(2M+mB)k
，此时小物块B对水平桌面的压力为：NB=mBg-BQB
2MgE(QA+QB)
k(2M+mB)
．

多项选择题

国家发展改革委会同国家环保总局组织开展烟气脱硫特许经营试点，提高脱硫设施的（）。

A.建设质量

B.运行效率

C.运行质量

D.能效
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单项选择题

Regardless of their political affiliation, in all countries women must overcome a host of stumbling blocks that limit their political careers. "Most obstacles to progress consist of (1) of various kinds," says the Inter-Parliamentary Union (IPU), a Geneva-based organization (2) 139 parliaments, including the lack of time, training, information, self-confidence, money, support, motivation, women’s networks and solidarity between women.

In every culture, prejudice and stereotypes (3) hard. The belief still holds (4) that women belong in the kitchen and (5) the children, not at election (6) or in the Speaker’s chair. The media often reinforce traditional images of women, who, upon entering politics, also bear the brunt (正面冲击) of verbal and physical (7) .

In impoverished (贫穷的) countries (8) by civil conflicts and deteriorating economic and social conditions, women are (9) by the tasks of managing everyday life and looking after their families.

The IPU stresses the general lack of child-care facilities—often (10) a privileged few—the (11) of political parties to change the times and running of meetings and the weak backing women receive from their families. That support, which is (12) as well as financial, is (13) vital because women have internalized (14) images of themselves since the (15) of time and often suffer from low self-confidence.

Another obstacle is the lack of financial resources, especially as election campaigns become increasingly expensive. (16) , women encounter more or less open machismo (男子汉的高傲) in the (17) of closed political circles (18) entry to the "second sex. " Lastly, they (19) the lack of solidarity between women, (20) by the fact that the number of available positions is limited.

10（）

A. allowed for

B. put in for

C. reserved for

D. compensated for
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