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That is what “acceleration is absolute” means. The observer can notice different phenomena, compared with the no-acceleration case, even without seeing outside the car window.

The additional meaning of “acceleration is absolute” is that we can deduce the acceleration value by measuring , , and . And, unlike velocity values, this acceleration value is identical for any two observers related by a Galilean transformation. (They are called inertial observers.)

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To see a more direct consequence of accelerating the car, we can consider an even simpler case where . Then we split the case into two, one with acceleration and one without.

In the no acceleration case, the particle 2 keeps co-moving with the car at the same velocity. In this case, the distance between particle 2 and a point of the car is always the same, no matter what the car’s velocity is. So it is impossible to deduce that velocity by observing only the distance between any two points within the car.

The car velocity value observed is different for different observers, depending on each observer’s velocity. That is what “velocity is relative” means.

However, if the car accelerates, although for an observer co-moving with the car, what he sees is not the car accelerating, but the particle 2 accelerates in the opposite direction; he can measure the acceleration based on the separations (between points within the car), the separation changes, and rate of separation changes; and he will get an acceleration value of the same magnitude.

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The key point is that the observer within the car can see the separation changes among some objects, including the car itself, within the car.

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— Me@2023-12-06 11:06:23 AM

— Me@2023-12-24 05:58:54 PM

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2023.12.26 Tuesday (c) All rights reserved by ACHK