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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But things which were at rest will move at a constant velocity, opposite of the velocity that you have relative to me.

这些静止的东西都在匀速运动,方向正好跟你相对我运动的速度方向相反

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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

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So the Earth is an inertial frame of reference, if you go in a train relative to the Earth at constant velocity, you're also inertial.

所以,地球是一个惯性参考系,如果你坐在相对于地球匀速运动的火车里,你也处于惯性状态

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That means, if you are going in a plane and you throw something up in the air, it seems to be going up and down to you.

也就是说,如果你在飞机中,你向空中扔出一物体,它也许会相对于你上下运动

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When you have a velocity in the moving plane frame and you want to find the velocity on the ground, you should add to every object in the plane the velocity of the plane with respect to the ground.

当你在运动的飞机上运动,然后你想找到相对于地面的速度,你就应该给每个物体加上,飞机相对于地面的速度

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