2．A、B兩個物體在足夠高處分別以v₀和3v₀的初速度同時做平拋運動，在初始2s內(nèi)，A、B兩個物體通過的水平距離之比是__________；兩個物體的速度增加量之比是__________。

1．在核反應方程U＋n®X＋Sr＋2n中，原子核X含有______個核子；U是天然放射性元素，它的半衰期約為7億年，則在距今大約21億年前，一塊鈾礦石中U的原子數(shù)大約是現(xiàn)在的_____倍。

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六．論述題 （20分）

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五．分析說明題 （共30分 ， 每小題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