题目

如图所示，在一口地质勘探竖井的底部测出的重力加速度比地面的重力加速度小0.1%，如果将地球视为密度均匀的球体，地球半径R=6.4×103km，则可估算出的井深为                            km．                                                                                                                                                                                                                                                                              答案：6.4

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