半径分别为r和2r的两个质量不计的圆盘，共轴固定连结在一起，可以绕水平轴O无摩擦

发布时间：2017-07-13 17:08 │ 来源：www.tikuol.com

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问题：

半径分别为r和2r的两个质量不计的圆盘，共轴固定连结在一起，可以绕水平轴O无摩擦转动，大圆盘的边缘上固定有一个质量为m的质点，小圆盘上绕有细绳。开始时圆盘静止，质点处在水平轴O的正下方位置。现以水平恒力F拉细绳，使两圆盘转动，若两圆盘转过的角度θ＝时，质点m的速度最大，则恒力F＝______________；若圆盘转过的最大角度θ＝，则此时恒力F＝________________。

考点：力矩的平衡机械能守恒定律

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Passage Three

Immigrants are consumers as well as producers, so they create jobs as well as taking them. And the work they do need not be at the expense of native workers. Immigrants often hold jobs that natives are unwilling to accept at any feasible wage.
Also, immigrants sometimes help to keep industries viable (能存活的) that would otherwise disappear altogether, causing employment to fall. This was the conclusion of a study of the Los Angeles garment industry in the 1970s and 1980s. And when immigrants working for low wages do put downward pressure on natives’ wages, they may raise the (real) wages of natives in general by keeping prices lower than they otherwise would be.
In theory, then, the net effect of immigration on native wages is uncertain. Unfortunately, most of the empirical (经验主义的) research on whether immigrants make natives worse off in practice is also inconclusive except the effect, one way or the other, seems small. Most of this research has been done in America: if there were any marked influence on wages, that is where you would expect to find it, given the scale of immigration and the tendency of the newcomers to concentrate in certain areas. But most studies have compared wages and employment in areas with many immigrants to wages and employment in areas with few. For instance, one examined the impact of sudden and notorious inflow of refugees to Miami from the Cuban port of Mariel in 1980. Within the space of a few months, 125 000 people had arrived, increasing Miami’s labor force by 7%. Yet the study concluded that wages and employment among the city’s natives, including the unskilled, were virtually unaffected. Another study examined the effect of immigration on wages and employment of those at the bottom of the jobs ladder—unskilled blacks and Hispanics. It found that a doubling of the rate of immigration had no detectable effect on natives.
The most recent work, admittedly, has tended to question these findings. Using more detailed statistics and more sophisticated methods than the earlier studies, this work has tended to find that immigrants’ wages take longer to rise to the level of the natives’ wages than has been supposed. This implies a more persistent downward pressure on the host economy’s labor market.
Typically these studies find that immigration does depress unskilled natives’ wages to a small extent. But nearly all economists would agree that the effects of immigration are insignificant in relation to other influences.

The passage is intended to convey that the effect of immigration on natives’ wages is ______.

A．uncertain

B．persistent

C．inconclusive

D．insignificant

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在部长会议闭会期间，世界贸易组织( )负责解决贸易争端与审议贸易政策。

A．管理委员会
B．秘书处
C．服务贸易理事会
D． * * 事会

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在下的“订货管理”数据库中完成下列基本操作：

(1)将order_detail、order_list和customer表添加到数据库。

(2)为order_list表创建一个主索引，索引名和索引表达式均是“订单号”。

(3)建立表order_list和表order_detail间的永久联系(通过“订单号”字段)。

(4)为以上建立的联系设置参照完整性约束：更新规则为“限制”，删除规则为“级联”，插入规则为“限制”。

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在保险实务中，通常将损失分为两种形态，分别为（）。

A.实质损失和精神损失

B.直接损失和间接损失

C.财产损失和人身损失

D.责任损失和收入损失

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沥青绝缘层是由（）所组成。

A.沥青和玻璃布

B.沥青和塑料布

C.沥青、玻璃布和塑料布

D.沥青和绝缘紙

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