题干

如图,一平行板电容器的两极板与一电压恒定的电源相连,极板水平放置,极板间距为d,在下极板上叠放一厚度为l的金属板,其上部空间有一带电粒子P静止在电容器中,当把金属板从电容器中快速抽出后,粒子P开始运动,重力加速度为g.粒子运动加速度为(   )

A:ldg

B:dldg

C:ldlg

D:ddlg

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答案(点此获取答案解析)

A

同类题3

阅读《清秋书简》,回答下列问题。

清秋书简

潘纤云

       倚灯夜读,有虫声从院角黄菊丛内传来,时停时续,忽高忽低,带点诗词里的平仄音律,不紧不慢地在秋夜里弹唱。“灯下草虫鸣”,想着这几个字,指尖慢慢滑过书页,伴着秋虫的浅吟读书,心里更觉温暖得很。

       古人说,春听鸟声,夏听蝉声,秋听虫声,冬听雪声。我觉得,这四种声音里,春夏的鸟声蝉声过于激烈,浮躁张扬了些,冬天的雪声又过于清寂,单调孤寒了些。而端坐在秋虫声里,听虫们一唱三叹的唧唧声,不紧不慢,清越激昂,犹如诗人们的雅集聚会,内心不由自主地抒情起来,或捧卷展读,或邀友煎茶,才不负这诗意秋声。

       “八月在宇,九月在户,十月蟋蟀入我床下”,《诗经》里描述了季节的转变。西风起,月冷霜寒,虫鸣断续,伴着捣衣的砧声,夜风中的箫声,思妇远望征人,虫语的低吟亦如她的心声:唧﹣﹣唧唧﹣﹣,分明是“盼﹣﹣盼啊﹣﹣”。她仰头观天,雁阵穿过暗夜的云朵长鸣而去;俯首看地,草丛里秋虫声声低语,天地间响彻它们的叫声。大雁知道南飞,蟋蟀知道归家,良人又在哪里?她的身影在秋灯里愈发孤凄。

       这种意象定格在诗书里,便是一幅幽凄伤情的画卷,也是文学里的抒情美,让人回味有加,如叶圣陶所言,虫声会引起劳人的感叹,秋士的伤怀,独客的微喟,思妇的低泣,是无上的美的境界,而常人世界里的秋虫声,又是另一番自然诗篇。

       幼时在乡下亲戚家,跟着婶婶到田野里守秋。我们爬上木柱搭成的高高的草棚,田野里的作物尽收眼底。花生、山芋、玉米等,黑黝黝的藤蔓伏在野地里,有新稻的清香从远处飘来。婶婶略有睡意,让我注意有无田鼠野獾的出没。我静静聆听,耳中灌满的却都是虫声,分不清哪是蟋蟀、蚂蚱、蝈蝈……有的若大提琴,有的若竖琴,有的若风笛,仿佛一场绝妙的交响曲,此起彼伏响个不停的交响曲,此起彼伏响个不停。当时头顶上有金黄的秋月,田野里有成熟的庄稼,草棚里有醒着的我,我们都是虫们肃穆的听众。那晚我为这秋之天籁所着迷,到黎明方才睡去。

       我钟情秋天的虫声,它们谱在自然的音符,更能令我内心充满宁静。我品味着刘墉的这句话:“秋虫声就是要这样聆听,在那细小的音韵中感触,即使到了极晚秋,只要以心灵触动,仍然可以感受到那微微的音响。”通常在这样的细小音韵中,一杯滚烫的热茶,一本心仪的书,窗前的灯影里,我在阅读,窗外的草丛里,秋虫们在吟唱。它们的话语和我心灵的声音汇合,那是无比美妙的体验。

(选自《散文》2011年第2期,有删改)

同类题4

阅读理解

    There are many differences between country life and city life in many ways. One person even does different things in the two places.

    Julia lived in the countryside, but one year she decided to visit the capital city to do some shopping and to see the sights. She stayed at a hotel near the central market. She had seldom been to the city before, and was very excited about what she would find.

    On the first morning of her visit, as she walked from the hotel to the market, she passed a beggar(乞丐). He was holding up a notice, which said, “Blind from birth. Please give generously(慷慨地).”

    Julia felt sorry for the blind beggar and she bent down and put a dollar coin into his bowl. “Thank you.” he said.

    The same thing happened again the following day.

    On the third day, however, Julia did not have a dollar coin. She had only fifty cents, so she dropped this into the beggar's bowl.

    “What have I done wrong?” the beggar said, “Why are you so stingy today? ”

    Julia was very surprised at what the beggar said. “How do you know I haven't given you a dollar?” she said, “If you're blind you can't know what coin I put into your bowl.”

    “Ah, ” explained the beggar, “The truth is that I'm not blind. I'm just looking after this place for the regular beggar while he's on holiday.”

    “On holiday?” Julia said, “And what exactly does your blind friend do on holiday?”

    “He goes into the countryside,” the man said, “and takes photographs. He's a very good photographer.”