来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  Okay, that's a simple problem.
  就是这么简单
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  If you tilt it to more than that, it'll topple over.
  如果倾角超过临界值 物体就往下滑
• 
  
  So, now let's take another problem where the block has started moving;
  现在让我们再看一个物体开始运动的问题
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  it's going down the hill.
  它从斜面上滑下来
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  Now, what formula do I write?
  那现在我要写什么式子呢
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  You notice that I'm not writing every step, every time.
  你注意到我不是每次都写出每一步的
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  Each time I deal with it I'm going to do some things faster.
  每处理一个问题时 我都会加快一点速度
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  For example, I already resolved gravity to two parts,
  比如说 我刚刚跳过了分解的过程
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  without going through the whole song and dance,
  就画出了重力的两个分力
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  because you know how to do that.
  因为你们已经知道怎么做了
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  Now, I put in the friction.
  现在我引入了摩擦力
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  So now, I've got μk not equal to 0,
  此时 μk 不再是 0