# Questions about accelerated frames of reference

1. Mar 19, 2013

### velixo

Suppose we have two frames of reference, with one being accelerated (not inertial). This could be you standing on the platform of a train station as a train in front of you is starting. From the train's point of view, you are accelerating, but one obviously knows that it is actually the train that is accelerating, because you feel no force (acceleration) being applied to you.

However, if we have two reference frames where we are not a part of either of them, but instead we are merely observing/measuring them, how do we know which one is the non-inertial one?

Thanks for any replies in advance.

2. Mar 19, 2013

### A.T.

You can measure the proper acceleration of your frame with an accelerometer. Then you can determine the proper acceleration of other frames based on their coordinate acceleration relative to you frame.

3. Mar 19, 2013

### velixo

I'm probably not being fully clear :P I'm kind of thinking along the lines of "How do we sense acceleration at all?"

4. Mar 19, 2013

### ModusPwnd

I think he answered your question. We sense acceleration by the force that goes along with it. So we can know our frame is not accelerating. We then measure each other frame, with respect to ours, and see which one has non-zero acceleration.

5. Mar 19, 2013

### A.T.

No idea what you mean by "sense". Physics is about measuring things. Proper acceleration is measured by accelerometers. Coordinate acceleration is measured with rulers and clocks.

6. Mar 19, 2013

### velixo

I mean, if you are standing still in the center of the accelerating frame, the inertial frame appears to accelerating (like standing on the train, looking at the person on the station platform). But from the point of view of the inertial frame, the accelerating frame appears to be, well, accelerating (like the person on the platform watching the train).

But given that both frames can appear to be accelerating because of the reasoning above, what would determine which one is actually accelerating? My point of the example above is that you could say that the train station (or more generally, the earth) is accelerating relative to the train.

7. Mar 19, 2013

### ModusPwnd

You can check whether your frame in accelerating without looking at another frame. That is, you can measure your acceleration without appealing to any other external frame.

The same cannot be said about velocity. You cannot measure your velocity without looking at another frame.

Velocity - frame dependent, depends on what frame you are comparing too.
Acceleration - not frame dependent. You can measure it within your frame without looking at another one. (measure it personally by sensing the force or measure it scientifically with an accelerometer)

8. Mar 20, 2013

### velixo

I'm sensing that I have to go and understand how an accelerometer works more deeply... :P Thanks for the help, I will check out the accelerometer before I ask any more questions!