来自《耶鲁大学开放课程：基础物理第13集第13集》

I should simply reverse the sign of the relative velocity.

我只要改变相对速度的符号就行了
This is the backwards result.

倒过来就得到了这样的结果
In other words, you're in a train

也就是说如果你坐在一辆火车上
and you fired a bullet towards the front of the train.

朝火车车头开了一枪
What speed do I attribute to the bullet from the ground?

以地面为参照子弹的速度应该是多少
Well, I add the bullet speed to the train speed.

是子弹的速度与火车的速度之和
That's what the numerator is.

也就是这里的分子
The numerator says the answer is

这说明最后的答案
somewhat less than that by this number. Yes?

要比这个分子小请说
Thank you very much.

谢谢
Yeah. That's absolutely correct.

完全正确
What I really need to do--quite correct--

我真正该做的是更准确点的说