来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  either I will know the force or I will know the acceleration.
  要么是知道力 要么是知道加速度
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  So, if this is 0, I come to the conclusion N = m · g.
  所以是 0 的话 我就得出结论 N = m·g
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  This is a very simple example,
  这是一个非常简单的例子
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  but this is how you figure out
  但也告诉我们
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  how the Newton's Laws work in this particular problem.
  牛顿定律如何应用在这种例子里
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  Okay, now I'm ready to introduce another force,
  现在我要介绍另外一个力
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  and that's the force of friction.
  也就是摩擦力
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  As you all seem to be aware,
  正如你们所知道的
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  there is generally a force of friction
  一般来说在物体 m 和桌面之间
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  between this mass m, and this table.
  有一个摩擦阻力
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  So, how do we learn there is a force of friction?
  那么我们怎么知道有摩擦力呢
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  Suppose someone said, "How do you know there is friction?"
  假设某人问 "你怎么知道有摩擦力呢"