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 argue that the answer is X = 0.
  就可以证明结果就是 X = 0
• 
  
  To do this, of course,
  当然想要做到这一点
• 
  
  you must cleverly pick your coordinates
  你们必须合理地选择坐标
• 
  
  so that the symmetry of the body is evident.
  好让物体的对称性显而易见
• 
  
  If you took the body like this
  如果你考虑一个像这样的物体
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  and you took a reflection around this point,
  你绕着这个点做一个镜像
• 
  
  it goes into body flipped over.
  就变成了一个翻过来的物体
• 
  
  There is not much you can say about it.
  这下子你就无能为力了
• 
  
  So, what you really want to do is to pick a point of reflection
  那么你应该做的就是选好一个镜像点
• 
  
  so that upon reflection the body looks the same;
  让这个物体的镜像看起来和自身是一样的
• 
  
  the body looks the same, the answer must be the same.
  物体看起来相同 结果也一定相同
• 
  
  But if you argue the answer must be minus of itself,
  如果你证明结果等于它自身的负值