来自《耶鲁大学开放课程：基础物理 第14集 第14集》

• 
  
  See here, when I put v = 0 on the bottom,
  再看这里 当我令分母的 v = 0
• 
  
  because the top had a v, not v/c,
  由于分子上已经有一个 v 而不是 v/c
• 
  
  something was left over
  这项就被留了下来
• 
  
  that looked like something familiar, namely momentum.
  它似乎是我们熟悉的动量公式
• 
  
  Here, if I take v = 0 too quickly,
  这里要是我让 v = 0
• 
  
  what's left over is nothing reminiscent of anything in Newtonian mechanics.
  被留下来的项与牛顿力学毫无关系
• 
  
  So, you want to go in
  所以你想搞清楚
• 
  
  what's called the next order in v/c.
  v/c 的下一个阶是什么
• 
  
  You don't want to totally ignore it.
  你不想完全忽略它
• 
  
  You want to keep the first non-zero term,
  你想保留第一个非零项
• 
  
  so we write it
  因此我们将它写成
• 
  
  I've just written it with
  刚刚已经把它