题干

如图所示,质量分别为m1和m2的木块A和B之间用一轻质弹簧相连,然后将它们静置于一底端带有挡板的光滑斜面上,其中B置于斜面底端的挡板上,设斜面的倾角为θ,弹簧的劲度系数为k.现用一平行于斜面的恒力F拉木块A沿斜面由静止开始向上运动,当木块B恰好对挡板的压力为零时,木块A在斜面上运动的速度为v,则下列说法正确的是(  )

A:此时弹簧的弹力大小为m1gsinθ

B:拉力F在该过程中对木块A所做的功为F(m1+m2gsinθk

C:弹簧在该过程中弹性势能增加了F(m1+m2gsinθk12 mv2

D:木块A在该过程中重力势能增加了 m2(m1+m2)g2(sinθ)2k

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答案(点此获取答案解析)

B

同类题1

阅读下面文言文段,完成下列各题。

    予犹记周公之被逮,在丙寅三月之望。吾社之行为士先者,为之声义,敛赀财以送其行,哭声震动天地。缇骑按剑而前,问“谁为哀者?”众不能堪,抶而仆之。是时以大中丞抚吴者为魏之私人毛一鹭,公之逮所由使也。吴之民方痛心焉,于是乘其厉声以呵,则噪而相逐。中丞匿于溷藩以免。既而以吴民之乱请于朝,按诛五人,曰颜佩韦、杨念如、马杰、沈扬、周文元,即今之傫然在墓者也。

    然五人之当刑也,意气扬扬,呼中丞之名而詈之,谈笑以死。断头置城上,颜色不少变。有贤士大夫发五十金,买五人之头而函之,卒与尸合。故今之墓中全乎为五人也。

(张溥《五人墓碑记》)

    士民素德顺昌,闻其逮,不胜冤愤。吴令陈文瑞,顺昌所拔士也。夜半叩户求见,抚床为恸。公曰:“吾固知诏使必至,此特意中事耳。毋效楚囚对泣!”颜色不变。语良久,令请顺昌入治装,举家号恸。顺昌笑曰:“无事乱人怀也!”顾案上有素榜,徐曰:“此龙树庵僧属我书者,我向许之,今日不了,亦一负心事。”乃题“小云栖”三字,后识年月,投笔而起,改囚服出门。士民拥送者不下数千人。顺昌出赴使署开读巡抚毛一鹭至署诸生五六百人王节杨廷枢刘羽仪文震亭等遮中丞恳其疏救一鹭流汗不能出一语。缇骑见议久不决,手掷锒铛于地,厉声曰:“东厂逮人,鼠辈何敢置喙!”于是市人颜佩韦等前问曰:“旨出朝廷,乃东厂耶?”缇骑曰:“旨不出东厂,将谁出?”众怒,哄然而登,丛殴缇骑,立毙一人,诸司不复相顾。

(《明史纪事本末•魏忠贤乱政》)

同类题5

阅读理解

        Around four years ago, I received a call from the principal of our school as to the “Parents View” talk the next morning. He asked me to speak to the group. After the call, my whole body became feverish and panicky. The time from his call to the next morning seemed like years. The whole night, I could not sleep with many ominous_apprehensions in mind. One of them was to call the principal with regret and tell him that I could not come. Finally, I gathered some courage. I thought, “If I miss this opportunity, surely the school will never invite me again to any of their programs.”

        I reached the school in time. Before my turn came, my whole body was trembling. When my turn came and I started speaking, my heartbeat increased and my mouth went dry. I wasn't even able to read the written speech properly. I was not aware of where I was standing and what I was reading. That was the day when I realized my biggest weakness, Public Speaking.

         After my speech, I met with the principal and explained what happened to me. He told me that this happens to everyone. Even great speaker, faced the same things when they started. He suggested that I come again next time.

         Around one month later, I was invited to refer to a topic on Motivation. This time I was feeling comfortable. My speech was not only appreciated by the principal as well as the teachers, because I was able to get my idea across to them. They encouraged and praised my efforts.

         After delivering this presentation successfully, I became more confident. I said to myself, “If I can speak in front of such a learned audience, like the principal who educates others, I can now speak in front of others too.”

         I started delivering lectures in my plant, on various topics like Self Motivation, Personality Development, Personal Excellence, Spoken English and Presentation Skills. This has become a passion for me. I learned that everything is possible if we have the courage to take the first step.