来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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  Again, your common sense tells you,
  你的常识会又一次告诉你
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  "Look, you are pulling something whose effective mass seems to be 5,
  "你在拉一个相当于 5 千克的物体
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  the answer is 2."
  所以加速度是 2 "
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  Let's get that systematically by using free-body diagrams.
  让我们系统地用隔离法来分析一下
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  Now, there are really three bodies here.
  现在我们有三个物体
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  Block one and block two and the rope connecting them.
  物体一和物体二 然后是连着它们的绳子
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  In all these examples, this rope is assumed to be massless.
  在所有这样的题目里都假设绳子没有质量
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  We know there is no thing called a massless rope,
  我们都知道无质量的绳子是不存在的
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  but most ropes have a mass,
  是绳子就有质量
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  but maybe negligible compared to the two blocks you are pulling,
  但是相比于这两个物体就可以忽略不计了
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  so we'll take the idealized limit
  所以我们有了这个理想的条件
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  where the mass of the rope is 0.
  即绳子的质量都是 0