来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  as the incline becomes less and less inclined,
  当倾角越来越小的时候
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  you got to get less and less acceleration,
  你得到的加速度也应该越来越小
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  because it's going downhill less and less,
  因为它下滑趋势越来越小
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  and when θ goes to small values, ax also diminishes.
  当 θ 变成一个极小的值时 ax 就变为零
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  Or, if you take an incline and you really make it almost vertical,
  或者 如果你把斜面摆到趋近于竖直
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  and in the end you make it completely vertical,
  最后摆放到完全竖直的位置
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  the block is just falling under gravity.
  物体就直接受重力作用下落
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  So, when θ is 90°, ax becomes g.
  所以 当 θ 是 90° 时 ax 就等于 g
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  So, you can test your results.
  这样你就能验证你的结果了
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  So, be prepared for that part,
  你既然在学习这门课程
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  if you are in this course.
  就需要掌握斜面问题
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  I will give you some homework problems
  我会给你们布置一些作业题目