来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  this is going to be the same restriction,
  所受到的限制条件是相同的
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  which is that tan(θ) has to be less than or equal to μs;
  即 tan(θ) 必须小于等于 μs
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  equal to μs is the critical value,
  其中等于 μs 已是临界状态
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  less than that is acceptable.
  小于 μs 都是可以适用的
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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
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  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