来自《耶鲁大学开放课程：基础物理 第3集 第3集》

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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
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  Here is the deal.
  情况是这样的
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  3 kg is being pulled by the rope on the right
  3 千克的物体受到绳子向右的一个拉力
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  with a force that I'm going to call T,
  我称它为 T
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  which stands for tension.
  也就是拉力的意思