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 when you work with extended bodies, or more than one body,
  当你们研究多个物体的体系时
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  we can now treat the entire body,
  可以将所有物体看成一个整体
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  replace the body by a single point for certain purposes.
  在某些情况下可以将物体用一个点代替
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  The single point is called a center of mass,
  这个点叫做质心
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  where it imagines all the mass concentrated at the center of mass.
  可以假设所有的质量都集中在这个点上
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  So, you have created a brand new entity which is fictitious.
  这样你就造出了一个全新的虚拟物体
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  It has a mass equal to total mass.
  它的质量是体系的总质量
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  It has a location equal to the center of mass,
  它位于体系质心所在的位置
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  and it moves in response to the total force.
  在合力的作用下能够运动
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  And it's not aware of internal forces,
  质心无法反映内力的作用
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  and that's what we want to exploit.
  这是我们想要探讨的内容
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  I already gave you a clue as to what the implications are,
  我已经给你们一条线索作为提示