来自《耶鲁大学开放课程：基础物理 第8集 第8集》

• 
  
  and I walk into this room and I say this,
  然后我走进这间教室我就这么说
• 
  
  you will say, "Well, this guy's talking about a body of mass, M,
  那你们就会说 "我们正在讨论的这个质量 M
• 
  
  undergoing some acceleration due to the force."
  会因受到外力作用而具有加速度"
• 
  
  So, the center of mass is the body
  那么质心是这样的一个物体
• 
  
  whose mass seems to be the total mass of these two particles,
  它的质量等于这两个物体的总质量
• 
  
  whose acceleration is controlled by the same as Newton's law,
  而它的加速度却同样由牛顿定律决定
• 
  
  but the right-hand side contains only the external forces;
  但等式的右边仅仅包含外力
• 
  
  this is the key.
  这就是关键
• 
  
  All the internal forces have canceled out,
  所有的内力都抵消了
• 
  
  and what remains is the external force.
  剩下的就只有外力
• 
  
  Now it turns, if you've got three bodies,
  举一反三 如果有三个物体
• 
  
  you can do a similar manipulation.
  你可以有相似的处理方式