题干

在水平地面上运动的小车车厢底部有一质量为m1的木块,木块和车厢通过一根轻质弹簧相连接,弹簧的劲度系数为k.在车厢的顶部用一根细线悬挂一质量为m2的小球.某段时间内发现细线与竖直方向的夹角为θ,在这段时间内木块与车厢保持相对静止,如上图所示.不计木块与车厢底部的摩擦力,则在这段时间内弹簧的形变为 (  )

A:压缩量为m1gtanθk

B:伸长量为m1gtanθk

C:压缩量为m1gktanθ

D:伸长量为m1gktanθ

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答案(点此获取答案解析)

B

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    One of the most important things in the world is friendship. In order to have friends, you have to be a friend. But how can you be a good friend at school?

    Listen — Listen when they are talking. Don't say anything unless they ask you a question. Sometimes it's not necessary for you to have anything to say; they just need someone to talk to about their feelings.

    Help them — If your friend is ever in need of something, be there to help them. You should try to put them first, but make sure you don't do everything they want you to do. Try to take an extra (额外的) pencil or pen with you to classes in case (以防) they forget one. Have a little extra money in your pocket in case they forget something they need.

    Be there for them — Try to make something for your friend to help make them feel better in hard times. Making cards and encouraging them are among the nicest things you can do for a friend. Marilyn Monroe, a famous U.S. actor, once said, “I often make mistakes. Sometimes I am out of control. But if you can't stay with me at my worst, you are sure not to deserve (值得) to be with me at my best.” Always remember this! If you don't want to stay with your friends when they're in hard times, then you don't deserve to be with them when they're having a good time!

    ______ — Try to make plans with your friends. Go shopping, go for ice cream, have a party, go to a movie and so on. Take time to know each other even better by doing something you both enjoy. By planning things together, you both can have a good time. And you'll remember these things when you're all old!