来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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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
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  mass is moving.
  物体在运动
• 
  
  You can ask, "What's the acceleration now?"
  你可能会问 "那现在的加速度是多少呢"
• 
  
  So, the horizontal equation along x is going to be
  这时横向沿着 x 轴方向的方程是