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在光滑水平桌面中央固定一边长为0.3m的小正三棱柱abc俯视如图所示。长度为L=1m的细线,一端固定在a点,另一端拴住一个质量为m=1kg、不计大小的小球。初始时刻,把细线拉直在ca的延长线上,并给小球以v0=1m/s且垂直于细线方向的水平速度,由于棱柱的存在,细线逐渐缠绕在棱柱上。已知细线所能承受的最大张力为8N,求:

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    I grew up in New Hampshire, a small town in South Canada, where in my father's words for the seasons were “Spring, Summer, Fairtime and Winter!”At that time, a week-long fair was held in the town every autumn. Thousands of people from other towns came to sell and buy things. It was the busiest time of the year.

    When “Fairtime” came, my grandma became the busiest person of the family. Grandma was a kind old lady. She was good at cooking. All her relatives liked the food she cooked. During “Fair time”, they would come to live in her house and have meals there. Grandma was always happy to look after them.

    Year after year, many people moved to big cities. There was no longer “Fairtime”. Grandma became very old and was gradually going blind. My parents and I moved to live with Grandma. We did our best to make her life comfortable. I was at high school then. What I often did at home was to help Grandma with the daily newspaper's crossword puzzle. However, she didn't look happy. She often sat in her room for hours, without saying a word.

    To attract(吸引) people to move back, the Town Hall decided to reopen the fair. One day, when I came back from school, I saw Grandma wearing her glasses, washing the dishes in the kitchen. With a big smile on her face, she looked much younger. She told me that her two nieces(侄女) would come. “They said the food I cooked was very delicious and they wanted to stay in my house again,” Grandma said happily. “They will stay here for one week and we can have a big party. That must be the busiest week I've had in years!”

    I suddenly realized that Grandma didn't want to be looked after. She wanted to be “useful” and helpful.

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