问题 计算题

如图所示,光滑水平面上有一小车,小车上固定一杆,总质量为M;杆顶系一长为L的轻绳,轻绳另一端系一质量为m的小球.绳被水平拉直处于静止状态(小球处于最左端).将小球由静止释放,小球从最左端摆下并继续摆至最右端的过程中,小车运动的距离是多少?

答案

该题立意新颖,但仔细分析与“人船模型”有相似之处,都是研究两个物体相互作用的过程,都具有水平方向合外力为零的特征,从而水平的动量守恒,初始状态球和车都处于静止状态,因此可借助“人船模型”来处理.

设某时刻小球速度的水平分量为v(方向向右),小车的速度为V(方向向左),取水平向左为正方向,根据动量守恒定律有:

MV-mv ="0   " 即

因为小球在摆动过程中,系统动量在每个时刻都等于零,所以每一时刻小球速度的水平分量与小车的速度都跟它们的质量成反比;从而可知小球从最左端摆至最右端的过程中,小球的水平位移s1与小车的位移s2与它们的质量成反比.即

由图4知  s1+s2=2L

由以上两式解得s2=

上面这题关键是找好球与船的相对位移。

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.

16()

A. To make matters worse

B. In contrast

C. By the same token

D. To sum up

判断题