Rest Frames in Special Relativity?

Question

I have a physics test coming up and one of the topics is special relativity. For the most part I understand the concept but I seem to be getting stuck when it comes to determining which frame is the rest frame. I don't understand how to find the rest frame and consequently keep getting the proper length wrong. Any help?

Answer 1

The rest frame is the frame you are using to measure the object moving relative to you.



If you are moving and are measuring a stick sitting on a desk, YOU are in the rest frame.



If you are trying to measure a box on a *fast* moving truck, you are once again in the rest frame.

Answer 2

The rest frame of an object is the frame in which the object appears to be still, i.e. at rest. For example, if you have a person on a moving train, and a person standing on a platform, then if you were to see things from the rest frame of the person on the platform, then you would see the platform as still, and the train as moving. If you were to see things from the rest frame of the person in the train, the train would be still, but the platform would be moving past instead. And if you were looking from the rest frame of a bird in the sky, the bird wouldn't appear to be moving, but the train and the platform and everything else would!



Hope this helps :)

Answer 3

One of the first principles of Special Relativity is that there are no privileged frames of reference, no frames that you can establish as being at "rest" relative to all other frames. Instead, all frames must be considered relative to the frame of the observer. I can consider myself at rest as I sit and watch the train go by. Even though my frame of reference (the Earth) is itself in motion, it is only the motion of the train relative to my frame of reference that matters. To me, an object on the train appears contracted because it is in motion relative to my (apparently) rest frame. So if you know which frame is that of the observer, you know which frame is considered to be at rest.

Answer 4

There is no rest frame, everything in the universe is in motion, you can only gauge your movement relative to another moving body.