# A very basic question about relativity

1. Dec 11, 2005

### nazgjunk

A very basic (or maybe not?) question about relativity

This is something I have been doubting about for a rather long time, but now I want to try and ask other people to explain it to me.
I know it might sound rather weird, but please try and think about my problem, and about where my thinking is going wrong.
We all know, movement is relative. I.e., from my viewpoint, the train is moving, but from the train's viewpoint, I move.
It seems to me that this does not completely apply for acceleration. After all, I don't feel any forces when the train next to me accelerates. I know this sounds rather confused, but why is this the case?

Last edited: Dec 11, 2005
2. Dec 11, 2005

### Danger

I was confused about that too. Someone here explained to me a few months back that the object which expends energy (has a force act upon it) in order to move is the one that is considered to be moving. That's not exactly what he said, but seems to be the gist of it.

3. Dec 11, 2005

### Staff: Mentor

Special relativity informs us that all inertial frames (uniform unaccelerated motion) are equivalent--there's no way to say who's "really" moving and who's "really" stationary.
But you can distinguish accelerated motion! As you seem to realize, the person in the accelerated train can certainly tell that they are accelerating. (You, in your uniformly moving train, will not feel the inertial forces that the accelerating train observers feel. You can tell who's accelerating and who's not.)

4. Dec 11, 2005

### Reshma

Correct. Motion is relative but you must define your frame of reference.
If you are watching a moving train from a place where you are at rest, you are in a Rest frame and the train is the Moving frame.
If you are the observer from the train observing an object on the ground, the train is the Rest frame and the object is in the Moving frame.
In this case there is relative motion between the two frames.
In this case, there is no relative motion between the two trains, so both the trains belong to the same inertial frame(either rest or moving).

Last edited: Dec 11, 2005
5. Dec 11, 2005

### Staff: Mentor

At least one of those trains is accelerating, according to the original post.

6. Dec 11, 2005

### Reshma

Oops, sorry I missed the word "accelerating"!!

7. Dec 11, 2005

### Staff: Mentor

Huh? You might want to explain what you mean by that. You can distinguish uniform motion from accelerated motion, but both are moving.

8. Dec 11, 2005

### nazgjunk

Uhmm... Well, when the train next to me accelerates, i guess there is some relative movement. The train next to me after all moves faster than i am.

9. Dec 11, 2005

### Reshma

Sorry for my oversight. Yes,there is relative motion in this case.

10. Dec 11, 2005

### X-43D

The point is whether it be can measured or not. Only the speed of light can be measured.

If you were moving in a straight line in intergalactic space, with constant velocity, and there were no windows to see the outside, there is no way you can tell what speed you are moving at (or, for that matter, whether you are moving at all) with any measurement. Thus, speed has only meaning relative to something else.

Last edited: Dec 11, 2005
11. Dec 11, 2005

### nazgjunk

Duh.... this is not the problem. The problem was more something like "what is acceleration". Read the previous posts to know what you are talking about.

12. Dec 11, 2005

### sigh hens

if an object is stationary and suddenly everything else in the universe is vanished then you couldn't tell if the object was stationary or moving.

if an object is moving at a constant velocity and suddently everything else in the universe is vanished then you couldn't tell if the object was stationary or moving.

if an object is accelerating and suddently everything else in the universe is vanished then you still wouldn't be able to tell if the object was stationary or moving with a constant velocity or moving with acceleration. However the object would logically stop accelerating because there would be no outside force to keep it accelerating.

I don't really know what my point was lol.

13. Dec 11, 2005

### Danger

Sorry Doc; I really didn't express that very well. We were partying pretty heavily at the time, and N/W's nutball nephew kept unplugging the cable modem to plug in his karaoke machine. By the time I got back to it, I guess I'd forgotten the original question.
My reference was to the first part of the post, about relative movement rather than acceleration. When it was first explained to me by one of you biggies (Integral, I think), it was in response to my question as to which twin in the famous paradox is actually moving, since each one appears to be from the other's perspective. He said that whichever one had to expend energy in order to leave the other was the one considered to be moving. The exact wording eludes me right now.
Sorry for the confusion there.

14. Dec 11, 2005

### nazgjunk

Well, at least I finally sort of understand. Thanks all. Hopefully I'll understand this even better when I'll start my tech physics in two years ;)

15. Dec 11, 2005

### X-43D

Acceleration requires force. It can be measured using a pendulum. Mass reacts to acceleration but it doesn't react to constant velocity.

16. Dec 11, 2005

### kawikdx225

Acceleration is absolute. You can always tell if an object is accelerating or not.

17. Dec 12, 2005

### bernhard.rothenstein

E.A Desloge and J.Philpott , "Uniformly accelerated reference frames in special relativity", Am.J.Phys. 55 (3) March 1987 pp. 252-261

18. Dec 12, 2005

### nazgjunk

Well, I think I finally understand it. Funny thing is I haven't even known my question till now:

If a body, moving at constant speed, is at rest in it's own frame of reference, then why is an accelerating body moving through it's own frame of reference?

As said, I do understand it now. Might not yet be able to explain it, but I do understand.

Thanks all.

19. Dec 12, 2005

### Staff: Mentor

It's not moving with respect to its own frame of reference, but it can tell that it is accelerating.

20. Dec 12, 2005

### nazgjunk

OK, but why?

I know this sounds stupid, but i feel i am losing it.