来自《耶鲁大学开放课程：基础物理 第4集 第4集》

• 
  
  Here is a mass.
  给你一个物体
• 
  
  What experiment will you do that tells you,
  什么样的实验能告诉你
• 
  
  "Hey, there is another force called friction"?
  "喂 摩擦力确实存在" 呢
• 
  
  Okay, anybody tell me something you would do? Yes?
  有谁知道该怎么做吗 请说
• 
  
  Okay, but suppose it's on this table.
  但是假设是在这个桌面上
• 
  
  But you are around, you can do what you want.
  但你没说错 你可以按照你的想法来
• 
  
  Yes? Go ahead.
  请讲 继续
• 
  
  That's possible.
  这有可能
• 
  
  That's a very good answer.
  那是个很好的答案
• 
  
  One answer is, you find that to keep it moving at constant velocity,
  一种答案是说 你发现为了维持匀速运动
• 
  
  you have to apply a force.
  你必须施加一个力
• 
  
  That means the force you're applying is cancelled by somebody else,
  这就意味着你施加的力被其它的力抵消了