来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  That's a possibility.
  这是可能的
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  It's a useful concept on the Earth,
  这种观点在地球上很有用
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  because Earth is approximately inertial.
  因为地球很接近惯性状态
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  Now, we have come to the second law,
  现在 我们开始进入牛顿第二定律
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  which is "the law."
  也就是最重要的定律
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  This is the law that we all memorize and learn.
  这是一个我们都得学习牢记的定律
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  It says that, "If a body has an acceleration,
  它是这么说的 "如果一个物体有加速度
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  then you need a force
  那么你需要有一个力
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  and the relation of the force to acceleration is this thing."
  力和加速度的关系是这样的"
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  Now, I have to say a few words about units.
  我现在要讲一下单位的问题
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  Acceleration is measured in meters per second squared.
  加速度的单位是米每秒平方
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  Mass is measured in kilograms.
  质量的单位是千克