If I am an inertial observer, another person moving relative to me at constant velocity will also be an inertial observer.

如果我是惯性观察者,那另一个相对于我做匀速运动的人,也是一个惯性观察者

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There's infinite number of them, but they're all moving relative to each other at constant velocity.

有无数个惯性观察者,但是他们都在相对于彼此,做匀速运动

耶鲁公开课 - 基础物理课程节选