题干

在倾角为θ的光滑斜面上有两个用轻弹簧相连接的物块A、B,它们的质量均为m,弹簧劲度系数为k,C为一固定挡板,系统处于静止状态.现用一恒力F沿斜面方向拉物块A使之向上运动,当物块B刚要离开C时,A的速度为v,则此过程(弹簧的弹性势能与弹簧的伸长量或压缩量的平方成正比,重力加速度为g)(   )

A:物块A运动的距离为 2mgsinθk

B:物块A的加速度为 F2m

C:拉力F做的功为 12 mv2

D:拉力F对A做的功等于A的机械能的增加量

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A

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    Last winter vacation, my brother got a set of bedroom furniture for his thirteenth birthday. When it arrived, I helped my parents moved the 1  furniture to the garage(车库). It was still in good condition. I thought it might be a chance for us to2 someone in our community. I started to search neighbors, anyone who 3 a bedroom set. Finally, my mom helped me call the local primary school. They told me about Jaila, the daughter of a poor single mother. I was given a telephone number and 4 and I soon found myself at Jaila's.

    The moment I went into Jaila's room, I knew I had taken on too big a task. When I pushed the old door open, it made a long high noise. But the door was nothing compared with the rest of the room. Some old paint was 5 the wall. One of the windows was 6. The bed was just a mattress(床垫)on the old carpet. It seemed that the room needed7 furniture — it cried — for a complete makeover(翻新).

    That day I spent at least an hour looking around and 8how I could possibly fixed the room. I had never painted a wall, let alone fixed a floor. The second day, I returned with a large group of helpers — my close friends. We went to collect everything needed door to door in our community. We replaced and repainted Jaila's room. We sweated in Jaila's house for more than ten hours every day that week.

    The moment we finished, we called Jaila into the room. She jumped, laughed and hugged each of us. I turned to Jaila's mother, and saw she had tears running down her face.

    "Anna," Jaila called my name 9Yes?""Welcome to my new room!"

    When we saw how happy Jaila and her mother were, our tiredness disappeared into thin air. We are really10 to make a special experience of changing someone's life.