题干

如图所示,光滑杆O′A的O′端固定一根劲度系数为k=10N/m,原长为l0=1m的轻弹簧,质量为m=1kg的小球套在光滑杆上并与弹簧的上端连接,OO′为过O点的竖直轴,杆与水平面间的夹角始终为θ=30°,开始杆是静止的,当杆以OO′为轴转动时,角速度从零开始缓慢增加,直至弹簧伸长量为0.5m,下列说法正确的是(   )

A:杆保持静止状态,弹簧的长度为0.5m

B:当弹簧恢复原长时,杆转动的角速度为 102 rad/s

C:当弹簧伸长量为0.5m时,杆转动的角速度为 4103 rad/s

D:在此过程中,杆对小球做功为15J

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答案(点此获取答案解析)

A,C

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完形填空

    I believe I had the perfect life. Because I was about to 1 my high school, and on my way to college, I had great friends and a loving family. There was nothing 2. But I spoke too soon.

    It was around 9:25 a. m. that I heard the news that would 3 my life forever. My brother Zach had been in a car accident. He fought for five days before he 4. That day, I became an only child. I felt extremely 5.

    After Zach's death, I found6 in food. I ate, then I slept, then I ate again. I couldn't cry. I could hardly feel anything, and I was 7. I stopped building relationships for fear that they would end just as 8 as Zach's life. Also, I became nervous about any potentially 9 situations-driving late at night-but I couldn't express this fear of life 10 I wanted to be strong for my parents. I saw my parents' 11 worse than mine because of the losing of their son. I didn't want them to 12 me. I also experienced a lot of 13, because I was angry about why the sadness had happened to me, and I never  14from this emotion.

    Now, it has been nearly five years since Zach's death. I don't 15 life anymore: I fact it bravely. I 16 my friendships and began socializing more. I even 17 Zach's story with people around me, although my new friends met him they know about Zach.

    One lesson I learned from losing my brother was never to be 18 to say, “I love you.” I loved my brother, but it was too late to 19 it loudly. The last time I remember telling my brother I loved him was when he was when he was dying. Don't make this 20 like me.