来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  that the y coordinate of the center of mass must lie on this line,
  物体质心的 y 坐标一定落在这条线上
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  because if I take y to - y,
  因为如果把 y 都换成 -y
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  it maps onto itself so it looks the same.
  镜像与其本身是重合的
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  But it's supposed to reverse Y;
  但是 Y 也应该变成负的
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  therefore, Y is -Y and therefore it's 0;
  因此 Y 等于 -Y 所以 Y 等于 0
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  so, it's evidently lying somewhere on this line.
  因此 质心肯定位于这条线上的某处
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  I cannot do further calculations of this type
  我无法继续通过计算得出
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  by saying where it is on the line,
  这种物体的质心到底在哪里
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  because it has no longer a symmetry in the x direction;
  因为它在 x 方向上并不对称
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  it's symmetric on the y after flipping y,
  沿 y 轴翻转物体 它保持对称
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  but I cannot take x to - x.
  但我不能把 x 换成 -x
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  In fact, if I take x to - x
  实际上 如果我这么做