来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  because we haven't identified all the forces.
  因为我们并没有找出所有的作用力
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  So, if you never knew about friction,
  如果你不知道摩擦力的话
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  you do this and you find,
  在做这个问题的时候你就会发现
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  "Hey, it doesn't work."
  "啊 出问题了"
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  The mass comes to rest,
  物体最终会停下来
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  whereas the theory says it should oscillate forever.
  而理论上却说是永远地振动
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  So, you are at the fork.
  所以你需要作出判断
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  Either you can say Newton's Laws are right;
  你可以说牛顿定律是正确的
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  I'm missing some forces,
  是自己忽略了一些作用力
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  I'm going to look around and find out what they are.
  然后去把它们全部找出来
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  Then, you will derive friction from that.
  然后就能从中找出摩擦力
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  Or maybe it's time to give up Newton's Laws.
  或者你可以说是时候放弃牛顿定律了