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问题 计算题

如图所示，质量为m=0.5kg的光滑小球被细线系住，放在倾角为°的斜面上。已知线与竖直方向夹角=30°，斜面质量为M=3kg，整个装置静置于粗糙水平面上。求：

（1）悬线对小球拉力的大小；

（2）地面对斜面的摩擦力的大小和方向。

答案

解：（1）对小球进行受力分析，有

 

得

（2）对斜面进行受力分析，由平衡条件得地面对斜面的摩擦力

，方向水平向左

单项选择题

Middletown’s police currently remove an apparently abandoned car from the streets about two months after it is reported. The police have been unfairly criticized for allowing such cars to be vandalized during this period. Because it is illegal for car owners to abandon cars on the street, police need not be concerned about protecting such cars.
Which of the following, if true, most seriously weakens the argument above

A．In the past, vehicles abandoned in Middletown were removed from the streets an average of four months after initial reports to police.

B．In one recent year, more than 150 abandoned vehicles were vandalized in Middletown, but police made no arrests for those actions.

C．Some cars initially classified by Middletown’s police as abandoned were later reclassified as stolen vehicles.

D．Middletown’s understaffed police force must give the disposition of abandoned cars low priority because of the number of violent crimes now committed there.

E．(E) In the most recent year for which data are available, there were 28 percent fewer reports of abandoned vehicles than the yearly average for the previous ten years.
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单项选择题

将原账“经营成本”科目的余额中未形成科技产品的余额转入新账中（）科目。

A.“经营结余”

B.“科技产品”

C.“拨出经费”

D.“专款支出”
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