这是我选择的，因此我说，你们也许会想，重力加速度，在三万英尺的高空，也许实际上，要比10小。
This is the one I pick and so I say, You may think that the gravitational acceleration at an altitude of 30,000 feet could be substantially less than the canonical number of 10.
作为重力为0的点，我可以选择这里。
I could have chosen my zero point of potential energy anywhere I please.
还要考虑到可能的备选方案，比如是选择混凝土重力坝还是选择填土筑堤大坝。
It also includes a consideration of possible alternatives, such as a concrete gravity dam or an earth-fill embankment dam.
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