来自《耶鲁大学开放课程：基础物理 第8集 第8集》

• 
  
  and here's another context in which it's important.
  这里有另外一个重要的条件
• 
  
  Suppose there is a mass m1,
  假设这里有一个质点 m1
• 
  
  going with some velocity v1,
  以 v1 的速度运动
• 
  
  and here's the second mass m2,
  有第二个质点 m2
• 
  
  going with some velocity v2; they collide.
  以 v2 的速度运行 它俩会碰撞
• 
  
  When they collide, all kinds of things can happen.
  碰撞的时候 会有很多事情发生
• 
  
  I mean, m1 may bump its head on that and come backwards,
  m1 会一头撞上去再回来
• 
  
  or it could be a heavy object
  或者它可能是一个重的物体
• 
  
  that pushes everything in the forward direction,
  能推开朝向它运动的所有东西
• 
  
  or at the end of the day
  最后
• 
  
  you will have some m1 going with a new velocity v1',
  你会有某个以新速度 v'1 运动的 m1
• 
  
  and m2 going with a new velocity v2'.
  和新速度 v'2 运动的物体 m2