来自《耶鲁大学开放课程：基础物理 第9集 第9集》

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  this is what we know to be kinetic energy,
  这是我们所知道的那个动能
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  just happened to be (1/2)·m·v^2
  只不过对于组成刚体的每一部分来说
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  for every piece that makes up the rigid body.
  恰好等于 (1/2)·m·v^2
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  For convenience,
  为方便起见
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  I've taken the rigid body
  我构造的这个刚体
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  to be made up of a discreet set of masses.
  是由一组离散的物体组成的
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  But now, if you notice,
  现在 你们会注意到
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  as you go from 1 to 3,
  当你从一到三求和时
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  m1 and m2 and m3 are three different numbers.
  m1 m2 m3 有不同的值
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  We don't know what the masses are.
  我们不知道质量是多少
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  R1 and R2 and R3 are three different numbers.
  R1 R2 R3 也是三个不同的值
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  We don't know how each one could be
  我们不知道它们各自