Wondering about SR postulate

[PeterDonis]

I know that i had to startup the train from 0 km/h to start leaving the station.

No, you don't know this. All you know is that you had to start up the train from 0 km/h *relative to the station*. You don't know whether you started "moving" in any absolute sense when you started leaving the station, or if you were "moving" when you were in the station, and gradually stopped "moving" as you accelerated, until you reached a state of "rest" when the train stopped accelerating. You can't just assume that moving at 0 km/h relative to the station means you are "really at rest". (If you're inclined to say that you can, consider that the station is on a rotating Earth, which is moving relative to the Sun, which is moving relative to the galaxy, which is moving relative to other galaxies. The fact that the station *seems* to be at rest to you is not a good reason for claiming it is "really at rest" in any absolute sense.)

In other words, just knowing that you accelerated in the past doesn't show that you are "moving", because you don't know which state of motion is "really" at rest to begin with. All acceleration does is change your state of relative motion; it doesn't, and can't, let you determine which state of relative motion is "really moving" or "really at rest".

 

[Noa Drake]

Ok, thank you for these various comments, i am learning.

 

[phinds]

Excellent. That's what all the Science Advisors and Homework Helpers and Mentors here love to hear. They spend a lot of time here trying to do just that and it is sometime an underappreciate task. I, on the other hand, am just here to kibitz