来自《耶鲁大学开放课程：基础物理 第7集 第7集》

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  But I want it to be pointing towards the center
  但我所想要方向是指向中心
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  and that is then the force of gravity on the Moon
  这样就得到月球或是苹果
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  or on the apple or any object.
  或者任何物体受到的引力了
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  If I'm thinking of one fixed object, like the Earth,
  如果认为一个物体是固定的 比如地球
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  this is the force on the apple.
  那这就是苹果的受力
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  If you're thinking of two objects of comparable size,
  但如果考虑两个大小相当的物体
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  you'll have to say the force on one is due to the other.
  就会说是一个物体受到另一个物体作用力
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  When you write the formula,
  当你写下公式的时候
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  then you've got to make sure
  你得确定
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  that the force on one is pointing to the other one,
  其中一个所受力是指向另一个的
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  and the force on that guy is pointing towards this guy.
  那个受到的力是指向这个的
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  So, there is no universal symbol for all those things.
  所以对所有这些 没有普适的符号