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问题 判断题

挖土机械按工作装置不同可分为机械式和液压式。

答案

参考答案：错

解析：挖土机械的分类：①按行走装置不同分为：履带式、轮胎式；②按工作装置不同分为：正铲、反铲、拉铲、抓铲；③按操纵机械的不同分为：机械式、液压式。见教材第七章第一节P135。

计算题

如图所示，小车停放在光滑的水平面上，小车的质量为M= 8kg,在小车水平面A处放有质量为m=2kg的物块，AB段是粗糙的水平面，BC是一段光滑的圆弧，在B点处与AB相切，现给物块一个V0=5m/s的初速度，物块便沿AB滑行，并沿BC上升，然后又能返回，最后恰好回到A点处与小车保持相对静止，求：

①从物块开始滑动至返回A点整个过程中,小车与物块组成的系统损失的机械能为多少?

②物块沿BC弧上升相对AB平面的最大高度为多少?
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单项选择题

Mayor: Four years ago when we reorganized the city police department in order to save money, critics claimed that the reorganization would make the police less responsive to citizens and would thus lead to more crime. The police have compiled theft statistics from the years following the reorganization that show that the critics were wrong. There was an overall decrease in reports of thefts of all kinds, including small thefts.
Which of the following, if true, most seriously challenges the mayor’s argument

A．When city police are perceived as unresponsive, victims of theft are less likely to report thefts to the police.

B．The mayor’s critics generally agree that police statistics concerning crime reports provide the most reliable available data on crime rates.

C．In other cities where police departments have been similarly reorganized, the numbers of reported thefts have generally risen following reorganization.

D．The mayor’s reorganization of the police department failed to save as much money as it was intended to save.

E．(E) During the four years immediately preceding the reorganization, reports of all types of theft had been rising steadily in comparison to reports of other crimes.
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