来自《耶鲁大学开放课程：基础物理 第8集 第8集》

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  Or if it is moving to begin with,
  否则如果它在初始时刻有速度
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  it'll maintain a constant velocity.
  它将保持这一恒定的速度
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  So, here's another example.
  这里还有另外一个例子
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  Now you can obviously generalize this to more than one dimension.
  显然你可以把这个结论推广到多于一维的空间
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  If you're living in two dimensions,
  如果你生活在二维空间中
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  you will introduce an x coordinate and introduce a y coordinate
  你要引入一个 x 坐标和一个 y 坐标
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  and then you will have the center of mass
  然后你会得到
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  as M·R double dot equals F,
  M 乘以 R 两点等于 F
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  and R would be m1·r1 + m2·r2
  R 等于 m1·r1 + m2·r2
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  divided by the total mass;
  除以总质量
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  r1 and r2 are just the location
  r1 和 r2 正好是两个物体
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  now in two dimensions of these two masses.
  在二维空间中的位置