来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  because on the ground, you leave something,
  因为什么东西放在地上
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  it stays there.
  都会保持静止
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  It depends on what you leave.
  当然还要看你放地上的是什么
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  If you leave your iPod,
  如果你把 iPod 丢在地上
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  it's not going to stay there for very long.
  估计静止不了多久
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  But then you can trace it to some external forces,
  过一会儿你就能发现一些外力的作用
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  which are carrying your iPod.
  也就是有人把你的 iPod 拿走了
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  But if you don't do anything,
  如果不施加外力
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  things stay.
  物体保持静止
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  Here is the main point.
  所以关键就在这里
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  The point of Newton is,
  牛顿的观点是
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  two things in the Law of Inertia,
  惯性定律分两部分