来自《耶鲁大学开放课程：基础物理 第7集 第7集》

• 
  
  So, we have to now ask,
  这样我们就要问了
• 
  
  "What is the nature of that force?"
  "这个力的本质是什么"
• 
  
  We're going to write a formula for the force.
  我们来写一下这个力的公式
• 
  
  First, let me look at the magnitude.
  首先 我们来看下矢量的模
• 
  
  The direction is very clear,
  方向很清楚
• 
  
  it is directed towards the center of the Earth.
  方向是朝着地心
• 
  
  So, I won't write that down.
  我不会写下来
• 
  
  Later on I will write you a formula with vector symbols in it;
  之后我会给你们写一个带矢量符号的公式
• 
  
  so the vector will point towards the center of the Earth.
  矢量会指明朝向地心
• 
  
  But let's look at the magnitude.
  但是我们来看矢量的模
• 
  
  If you take an apple here, of mass m,
  假设苹果质量为 m
• 
  
  and you call M the mass of the Earth,
  地球质量为 M