来自《耶鲁大学开放课程：基础物理 第6集 第6集》

• 
  
  Now you can say, you know what, I didn't have to do all this,
  你可能认为其实我不需要推导的
• 
  
  I could have always guessed that in two dimensions,
  二维的情况完全可以猜测出来
• 
  
  when you had an x and a y
  因为这里涉及到 x 和 y 两个变量
• 
  
  you obviously have to add them.
  我们当然要把它们都加起来了
• 
  
  There's nothing very obvious about it
  但是这并不是那么简单
• 
  
  because this combination is now guaranteed to have the property
  因为这个组合确保了一个性质
• 
  
  that if I call this the work done,
  就是如果我把这定义为力所做功
• 
  
  then it has the advantage
  我们就可以认为
• 
  
  that the work done is in fact the change in kinetic energy.
  力所做的全部功就是动能的改变量
• 
  
  I want to define work so that
  我之所以想这样定义功是因为
• 
  
  its effect on kinetic energy is the same as in 1D,
  这样做功对动能的改变与一维中相同
• 
  
  namely, work done should be equal to change in kinetic energy.
  也就是说力做所功等于动能的改变量