2．A、B两个物体在足够高处分别以v₀和3v₀的初速度同时做平抛运动，在初始2s内，A、B两个物体通过的水平距离之比是__________；两个物体的速度增加量之比是__________。

1．在核反应方程U＋n®X＋Sr＋2n中，原子核X含有______个核子；U是天然放射性元素，它的半衰期约为7亿年，则在距今大约21亿年前，一块铀矿石中U的原子数大约是现在的_____倍。

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六．论述题 （20分）

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五．分析说明题 （共30分 ， 每小题15分。）

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28  A     B     C     D    29   A     B     C     D   30   A     B     C     D   31.  A     B     C     D