两列火车以相同的车速并排地沿着两条平行的轨道往前行驰。我们坐在其中的一列火车上带着特制的速度测量仪,可以测出两列火车之间的相对速度。因为这两列火车是以相同的车速前进的,所以二者的相对速度是零;因此,测速仪上的读数为“0”。
The two trains run side by side at the same speed along two parallel tracks. We were sitting on one of the trains with a special speed meter, which could measure the relative speed between the two trains. Because the two trains advance at the same speed, the relative speed of the two trains is zero; therefore, the reading on the speedometer is "0".
突然间另一列火车使我们觉得好像是开始赶到我们所乘坐的火车前头去了。这时测速仪上显示出来的读数是每小时10英里。很显然,另一列火车已经加快了速度。但是,我们能不能肯定是另外那辆火车加快了速度呢?
Suddenly another train made us feel as if we were beginning to catch up with our train. The speedometer reads 10 miles an hour. Obviously, another train has speeded up. But can we be sure that the other train accelerated?