来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  because the center of mass is this weighted average
  因为质心是加权平均值
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  divided by the total mass,
  要除以总质量
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  so I've got to divide by M.
  所以我要给等式除以 M
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  Well, I claim, if you do this and cancel everything
  我来告诉你们 如果你这么算 把这些都约掉
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  you will get the answer of (2 / 3)·h.
  结果就是 (2 / 3)·h
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  Okay, so not surprisingly the center of mass of this
  不出意料 三角形的质心
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  is not halfway to the other end,
  没有落在中点上
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  but two-thirds of the way
  而是三分之二处
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  because it's top heavy;
  因为物体的顶部较重
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  this side of it is heavier.
  物体的这一侧更重一些
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  This is the level of calculus
  在我们的课程上
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  you should be able to do in this course,
  你们应该会做这种程度的微积分计算