来自《耶鲁大学开放课程：基础物理 第9集 第9集》

• 
  
  is to rotate around an axis, penetrating that point.
  就是绕着一个穿过这个点的轴转动
• 
  
  So, let's say at t = 0,
  比如 当 t = 0 的时候
• 
  
  the body looked like this,
  这个物体是这样的
• 
  
  a little later it's going to look rotated.
  过一会儿它可能就会转到那边了
• 
  
  I have to tell you what it is doing at a given instant.
  我需要描述它在给定时刻的运动
• 
  
  I have to tell you how it's oriented.
  我需要描述它的朝向
• 
  
  So, what we do for that is,
  为此 我们的做法是
• 
  
  we draw through the point of rotation some line.
  画条穿过旋转点的线
• 
  
  You can pick any line you like.
  随便什么线都可以
• 
  
  Pick a line like that
  像这条
• 
  
  and we say what angle
  然后我们记下
• 
  
  that line makes with a standard chosen direction,
  这条线和一个标准方向的夹角