来自《耶鲁大学开放课程：基础物理 第4集 第4集》

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  That's called resolving the force into some other direction.
  这个过程叫做力的分解
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  Now, the key to all of this is to know that this angle,
  一切的关键就是要知道这个角
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  here, is the same as this angle here.
  在这里 和这里的这个角相等
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  In other words,
  换句话说
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  if mg is acting down,
  如果 m·g 是竖直向下的话
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  that angle θ is the same as this angle θ here.
  这个角和这个角都等于 θ
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  This is going to be used all the time,
  这个性质以后一直都用得上
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  so I'll tell you a little trick
  告诉你们一个小技巧
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  that tells you how I figured that out.
  你们来看看我是如何处理这类问题的
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  θ Is the angle between the horizontal and the incline.
  θ 是水平面和斜面之间的夹角
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  Now, you've got to agree that
  你们肯定同意这一点
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  if I draw perpendicular to the horizontal
  如果我画水平线的垂线