来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  Let me now put you out of your suspense.
  让我来消除你们的疑虑吧
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  I think I've heard bits and pieces of the answer everywhere,
  我想我可以听到各种各样的答案
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  so I don't want to wait until we get it word-perfect.
  所以我不想等这大家想出完美的答案
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  The point is, the one kilogram is a matter of convention.
  关键在于 1 千克已经定下来了
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  We want to know what is the mass of the elephant.
  我们想要知道大象的质量
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  We can do the seesaw experiment, you suggested,
  我们可以做你所说的跷跷板实验
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  but the seesaw experiment requires gravity,
  但跷跷板实验需要重力
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  so we don't want to do that.
  所以我们不能那么做
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  A spring will, on the other hand, exert a force.
  但是一个弹簧可以提供一个力
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  We don't know what the force of the spring is.
  我们不知道弹簧的力多大
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  If you assume that, you are not playing by the rule,
  如果你假定了 那你就犯规了
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  we don't know what force it exerts.
  我们不知道它施加了多大的力