题干

如图,一半径为R的半圆形轨道竖直固定放置,轨道两端等高;质量为m的质点自轨道端点P由静止开始滑下,滑到最低点Q时,对轨道的正压力为2mg,重力加速度大小为g。质点自P滑到Q的过程中,克服摩擦力所做的功为(  )

A:14 mgR

B:13 mgR

C:12 mgR

D:π4 mgR

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答案(点此获取答案解析)

C

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    26-year-old Anna Cherdantseva, from the city of Ufa in Russia, is a full-time employee of a furniture company, spending up t0 10 hours every day trying out new sofas for comfort and safety.

    Last month, Russian furniture giant M25 Group announced that it was taking application for a new position. Although all new products are tested in a specialized laboratory, management decided that in order to better meet the needs of consumers it needed some feedback(反馈) on real-life use of its sofas.  In just a few days, M25 received over 5,000 job applications.

    "We selected candidates in several stages," M25 Group spokesperson Anastasia Russkih said.  "Initially we checked their CVs, and then we let them tell us why they wanted the job. " Eventually, the furniture company shortlisted just 7 candidates, out of which Anna Cherdantseva, a 26-year-old marketing expert, proved to be the most suitable for the job.  "It was a real fight for my dream job," Anna said after getting selected.

    Despite describing herself as an active person, Cherdantseva says that spending about l0 hours a day sitting on sofas doesn't sound bad at all.  "Of course I will spend a lot of time on new sofas.  The more time I spend on them, the better it will be for my work," she told reporters.

    Anna Cherdantseva is at present on a three-month probation(试用期) at M25, but the company claims that as long as she proves to be up to the task, they plan on offering her a permanent position The young woman will receive a monthly salary of 56,000 rubles( $l,000) for the first three months.  It's not so bad for sitting on the job.

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                                                                                            宁静是一种生产力
        ①清人张潮在《幽梦影》中写道:“能闲世人之所忙者,方能忙世人之所闲。”这句话颇有意味,意思是指大家都在忙碌的事情,你能够悠闲地对待它,才会有时间、有心思去满足自己的闲情逸趣。
         ②别人忙碌追逐的,你不追赶,别人置之不理的,你用心去体会。这是一个发现的过程。发现什么呢?发现一种生活方式,发现生命的质量。《菜根谭》里说:“性躁心粗者一事无成,心平气和者百福自集。”讲的是同样的道理。
         ③宁静有时也是一种生产力。一个人能够平心静气,就能够获得一种智慧的能量,提高生命的质量和效率。
         ④我曾经看过一个小故事。一位木匠把手表掉在半人高的刨花里,让众多徒弟去找,大家乱哄哄地找,怎么也找不着。后来他的儿子独自在夜里静静地听手表走动的响声,终于找到那块手表。事情很简单,道理却很深刻。想想看,宁静难道不是一种生产力吗?
        ⑤《庄子》中说:“水静犹明,而况精神!圣人之心静乎!天地之鉴也;万物之镜也。”天地万物是可以映照在我们心中的,前提是心要安静。喧嚣的小溪把碎沫拍打在山崖上的时候,澎湃的大海把浪花摔打在沙滩上的时候,它能照见世界吗?它什么也看不见。
        ⑥一个人的心里有太多欲望,过分在意他人的赞誉和诽谤之语,这颗心就会像喧嚣的小溪碎沫和澎湃的大海浪花,鼓动着,躁动着,这样的人能静下来吗?如果不能拥有一份宁静,不能拥有一种闲适,我们能看见生命的本真吗?
         ⑦安静下来,在中国人本来的生命规律中去发现悠然的欢喜,对今天这个时代而言,也许不仅是一件锦上添花的事,说它是雪中送炭也不为过。                     (选自《趣品人生》,有删改)