| 释义 |
1 ?四维速度 一个事件的 世界线 (world line) 的 四维速度 (four-velocity) 定义为： ；
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the position of the highest point P we first ask ourselves the question from equation number four: 0 when is the velocity in the y direction zero? 为了找到最高点P的所在,我们首先得,解答我们刚才提出的问题：,究竟何时y方向上的速度减为？ - 2
Now, I would think that it is reasonable to ask the following question: What is the average velocity, for instance, between time zero and time four? 现在，我认为,提出如下问题,是十分合理的：,比如说，平均速度是多少，在0到4秒之间？ - 3
By the time he passed four thousand feet he had reached terminal velocity, the wind was a solid beating wall of sound against which he could move no faster. 他穿越四千英尺的高度时，已经达到极速，呼啸着的海风就像一堵坚实的墙，拦在前面，使他无法以更快的速度前进。
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