来自《耶鲁大学开放课程：基础物理 第7集 第7集》

• 
  
  If it's possible, you are done,
  如果可能 问题就解决了
• 
  
  because you're back to writing K1 + U1 = K2 + U2.
  这就回到了 K1 + U1 = K2 + U2
• 
  
  But I tried to convince you that
  但我想让你们知道
• 
  
  it's not always going to happen.
  这不是总能办到的
• 
  
  So, let's understand why it's not always going to happen.
  那么让我们看看为什么这不是总能办到
• 
  
  It is not enough in this integral to say,
  在这个积分中
• 
  
  start at r1 and go to r2.
  仅仅知道从 r1 到 r2 是不够的
• 
  
  In fact, if I just said,
  实际上 如果我只说
• 
  
  find the integral of the work done by the force
  求出力从 r1 到 r2
• 
  
  between r1 and r2,
  所做功的积分
• 
  
  you will have to tell me--
  你们会说
• 
  
  you'll have to ask me something. You agree?
  你们会问我些什么 同意吗