来自《耶鲁大学开放课程：基础物理 第6集 第6集》

• 
  
  So, we know this couldn't have come from a U.
  那么我们知道这不会从 U 中产生
• 
  
  But there's a better test.
  但是有一个更好的检验方法
• 
  
  Instead of doing all that,
  不用做刚才讲到的方法
• 
  
  instead of saying I'm satisfied
  不用再说对这个
• 
  
  that this doesn't come from taking derivatives of the U
  不能通过寻找可能的 U 函数
• 
  
  by looking at possible Us are not being satisfied
  而求解 U 的导数的方法不满意
• 
  
  because maybe I'm not clever enough.
  就因为我可能不够聪明
• 
  
  There is a mechanical way to tell.
  下面要讲个机械的方法
• 
  
  The mechanical way to tell is the following.
  这个机械的方法就是下面要讲的
• 
  
  Maybe I want you guys to think for a second
  也许你们应该稍微想一下
• 
  
  about what the recipe may be.
  这个方法是什么
• 
  
  If a force came from a function U by taking derivatives,
  如果一个力来等于一个函数 U 的微商