来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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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
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  be able to take some body, slice it up in some fashion,
  你们要学会将某个物体分割为小片
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  and find the location of the center of mass.
  然后找到物体质心的位置
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  You combine symmetry arguments with actual calculation.
  将对称性与实际计算相结合
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  For this sliver, by symmetry,
  对这一小片物体 利用对称性
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  you know the center of mass is at the center,
  你们就会知道物体的质心就在中部
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  you don't waste your time,
  你无须浪费时间来寻找
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  but then when you add these guys
  但将所有小片相加之后
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  there is no further symmetry you can use;
  对称性原理就不再适用
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  you have to do the actual work.
  你们必须得老老实实地计算
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  So, what have I done so far?
  到目前为止 我都做了什么
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  What I've done is point out to you
  我所做的就是给你们指出