特别广播英语第 77 课：You Have a Choice 你可以选定一种解释-3

For a minute or so, our train seems to travel at a uniform speed. 约有一分钟左右，我们所乘的火车似乎是在以均匀的速度向前运行。

Our special speedometer shows that the relative speed between the two trains is 20 mph. 我们特制的测速仪显示这两列火车的相对速度是每小时20英里。

But as we look out our window, we suddenly see the last coach of the other train disappear from sight and notice the motionless station behind it. 但当我们往窗外一看，这才突然看见另一列火车的最后一节车厢从我们的视野中正在消失，于是我们看到那一列火车后面留下的是不动的火车站。

So we are not moving after all. The other train has been moving! 所以，我们所乘的火车根本就没有开动。原来是另外一辆火车在开动！

This peculiar and often frustrating experience is an effect of relative motion. 这种奇特的而又经常使人沮丧的经历就是相对运动所产生的效应。

At the train station we cannot tell whether it was our train that changed its speed from 0 mph to 20 mph or whether it was the other train that changed its speed from 0 mph to 20 mph. 在火车站上我们辨别不出窨是我们所乘坐的那一列火车把车速从每小时0英里改变成每小时20英里。

Only after the other train pulled out of the station could we see that it, and not our train, was moving. 只有当另一列火车驶出火车站之后，这我们才能明白原来是它在开动，而不是我们所乘坐的这列火车在开动。

Now let us again raise the question that was raised at the beginning of this article:现在让我们再次提出本文开头所提出过的那个问题：

can we be absolutely certain that the other train did indeed increase its speed, and in this case pull out of the station? 我们能否绝对肯定确实是另一列火车加快了车速，在此情此景驶出了火车站呢？

If your answer is yes, then you are wrong again. All we can be certain of is that the relative speed between the two trains changed. 如果你回答说是的，那么你就又错了。我们所能够完全肯定的一点只是这两列火车之间的相对速度变了。