题干

如图所示,质量为M的木块A套在粗糙水平杆上,并用轻绳将木块A与质量为m的小球B相连.现用水平力F将小球B缓慢拉起,在此过程中木块A始终静止不动.假设杆对A的支持力为N,杆对A的摩擦力为f,绳中张力为T,则此过程中(  )

A:F增大

B:f不变

C:T减小

D:N减小

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答案(点此获取答案解析)

A

同类题1

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     I was very disappointed not to be able to go to the jazz concert last Friday. The advertisement in the paper said that you could buy tickets at the theatre box in Richland Hills any day between 10:00 and 4:00. Since I work from 9:00 to 5:30, the only time I could go to the theatre was during my 45-minute lunch break.

    Unfortunately, the theatre is on theother side of the town, and the bus service between my office and Richland Hills is not very good. Butif you are lucky, you can make the round trip in 45 minutes. Last Monday, Istood at the bus stop for 15 minutes waiting for the bus, By the time I saw onecome around the corner, there was not enough time left to make the trip.

     So I gave up and went back to the office. The same thing happened on Tuesday, and again next day. On Thursday, my luck changed. I got on a bus right away and arrived at the theatre in exactly 20 minutes. When I got there, however, I found along line of people at the box office, I heard one man say he had been waitingin line for over an hour. Realizing I would not have enough timeto wait in line. I caught the next bus and headed back across the town.

    By Friday I realized my only hope was tomake the trip by taxi, it was expensive, but I felt it would be worth it tohear the concert. The trip by taxi only took 10 minutes, but felt like an hour to me. When I got to the theatre, I was relieved to see that nobody was waiting in line. The reason, however, I quickly discovered, was that they had already sold all the tickets.

同类题3

阅读下面的文言文,回答问题。

    邵民瞻①为东坡②买一宅,为钱五百缗③,东坡倾囊④仅能偿⑤之。卜⑥吉日入新第⑦,既得日⑧矣。夜与邵步月,偶至一村落,闻妇人哭声极哀,东坡徙倚⑨听之,曰:“异哉,何其⑩悲也!岂有大难割之爱,触于其心欤?吾将问之。”遂与邵推扉而入,则一老妪⑪,见东坡,泣自若⑫。东坡问妪何为哀伤至是。妪曰:“吾家有一居,相传百年,保守不敢动,以至于我。而吾子不肖⑬,遂举以售诸⑭人。吾今日迁徙来此百年旧居一旦诀别⑮宁⑯不痛心此吾之所以泣也。”东坡亦为怆然。问其故居所在,则东坡以五百缗所得者也。东坡因再三慰抚,徐⑰谓之曰:“妪之旧居,乃吾所售⑱也。不必深悲,今当以是屋还妪。”即命取屋券⑲,对妪焚之。呼其子,命翌日⑳迎母还旧第,竟不索㉑其直 。

【注释】①邵民瞻:人名。②东坡:苏轼的号。③五百缗:五百贯钱。④倾囊:尽出其所有。⑤偿:偿还。⑥卜:选择。⑦第:按一定等级建造的大宅院。⑧得日:过了几天。⑨徙倚:走过去斜靠着。⑩何其:多么。⑪老妪:老妇。⑫自若:依然如故。⑬不肖:不成材。⑭诸:相当于“之于”。⑮诀别:分别。⑯宁:难道。⑫徐:慢慢地。⑬售:这里是“买”的意思。⑲券:契据。⑩翌日:次日。㉑索:求取。 直:同“值”,价钱。