来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  we're not going to do any more work now.
  我们现在用不着那么费事儿了
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  By symmetry, I can argue that this has to be the center of mass,
  通过对称性 我可以证明这就是质心
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  because I can take every point here
  因为我可以取这里的所有点
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  and turn it into the point there by changing y to -y;
  通过把 y 换成 -y 就可以把这个点变到这里来
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  therefore, Y will become -Y,
  因此 Y 就变成了 -Y
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  but the body looks the same after this mapping.
  但物体在变换之后没有变化
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  So, Y is 0, and similarly X is 0,
  因此 Y 等于 0 同样地 X 也等于 0
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  and that's the center of mass.
  这就是质心
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  Okay, now, what if I give you this object,
  那么如果我给你们这样的物体 那会怎么样呢
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  anybody want to try that?
  有没有人想试试呢
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  Would you guys like to try this? Yes.
  你们想试试吗 请讲
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  Can you tell me where the center of mass should be?
  你能告诉我它的质心在什么位置吗