题干

在一次校运动会上,小明骑一质量为m的独轮车,以速度v匀速通过一重为G、长为L的水平独木桥,独木桥的两端由两根竖直支柱A、B支撑着,如图所示。设独轮车骑上A端支柱处为初始时刻(t=0),下面哪一个图正确地表示了B端支柱所受压力FB与时间t的函数关系?(不考虑独木桥的形变)(  )

A:

B:

C:

D:

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B

同类题1

   Except for a few times, Americans are not big present-givers. There’s no exchange of presents among business people, and if one American tries to give another a present, it may look like that he wants to bribe (贿赂) him. Americans have learned that in some other countries people like to give presents to others, but among ourselves we don’t see the need for presents.
   Even friends may never exchange presents. When I go to foreign countries, I try to bring back little things for close friends, but nobody would feel unhappy if I didn’t. I don’t often re­member a friend’s birthday, and few people outside of my family remember mine. If someone gave me presents too often, I’d get unpleasant. But a present from a foreigner — that kind from his or her homeland — won’t go wrong, except to government employees (政府职员) who can’t be given presents.
   You usually open a present at once and in front of the person who gives it except Christmas and birthday presents. You should only say, ‘It’s so nice. Thanks…’when you get Christ­mas or birthday presents.
   You may want to bring a bottle of wine (葡萄酒) or flowers to a dinner party, but you’re never asked to. Nobody will mind if you bring wine, but your friend may not use it that evening.
   At Christmas we often give presents to our family and sometimes our friends. We also give presents to people who have been helpful during the year like doormen, babysitters, housecleaners , newspaper senders — anyone who has often helped us.