来自《耶鲁大学开放课程：基础物理 第12集 第12集》

• 
  
  you are called an inertial observer.
  你就是一个惯性观察者
• 
  
  One of the laws you want is,
  你所期待的定律之一
• 
  
  if you leave something, it should stay where it is.
  就是如果你松开某物 它就会呆在原地
• 
  
  When the train is accelerating,
  而当火车加速运动时
• 
  
  that won't be true.
  则不会这样
• 
  
  You leave things on the floor when it's accelerating,
  当火车加速时你将某物放在地上
• 
  
  things will slide backwards.
  它就会向后滚
• 
  
  So, with no apparent force acting on it,
  也就是即使没有力对它作用
• 
  
  things will begin to accelerate;
  物体也会加速
• 
  
  that's a non-inertial frame.
  这就是非惯性系
• 
  
  We are not interested in that.
  我们对这个不感兴趣
• 
  
  You started out as an observer for
  你要作为一个