题干

如图(甲)所示,质量不计的弹簧竖直固定在水平面上,t=0时刻,将一金属小球从弹簧正上方某一高度处由静止释放,小球落到弹簧上压缩弹簧到最低点,然后又被弹起离开弹簧,上升到一定高度后再下落,如此反复。通过安装在弹簧下端的压力传感器,测出这一过程弹簧弹力F随时间t变化的图像如图(乙)所示(忽略一切阻力)则  (       )

A:t1时刻小球速度最大

B:t2时刻小球速度最大

C:t2~t3这段时间内,小球的速度先增加后减少

D:t2~t3这段时间内,小球的加速度先增加后减少

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答案(点此获取答案解析)

C

同类题3

    When I was growing up in America, I was ashamed of my mother's Chinese English. Because of her English, she was often treated unfairly. People in department stores, at banks, and at restaurants did not take her seriously, pretended(假装) not to understand her, or even acted as if they did not hear her.
    My mother has realized the disadvantages of her English as well. When I was fifteen, she used to have me call people on phone to pretend I was she. I was forced to ask for information or even to yell at people who had been rude to her. One time I had to call her stockbroker(股票经纪人). I said in a voice that was not very convincing, “This is Mrs. Tan.” And my mother was standing beside me, whispering loudly, “Why he doesn't send me the check. It's already two weeks late.”
    And then, in perfect English I said: “I'm getting rather anxious .You agreed to send the check two weeks ago, but it hasn’t arrived.” Then she talked more loudly. “What does he want? I’ll come to New York and say it in front of his boss.” And so I turned to the stockbroker again, “I can't accept any more excuses. If I don’t receive the check immediately, I have to speak to your manager when I am in New York next week.”
    The next week we ended up in New York. While I was sitting there red-faced, my mother, the real Mrs. Tan, was shouting to his boss in her broken English. When I was a teenager, my mother’s broken English embarrassed (使困窘) me. But now, I see it differently. To me, my mother’s English is perfectly clear, perfectly natural. It is my mother tongue. Her language, as I hear it, is clear and direct. It was the language that helped me form the way I saw things, expressed ideas, and made sense of the world.