A very basic question about relativity

[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?

 

[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.

 

[Doc Al]

We all know, movement is relative. I.e., from my viewpoint, the train is moving, but from the train's viewpoint, I move.

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.

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?

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.)

 

[Reshma]

We all know, movement is relative. I.e., from my viewpoint, the train is moving, but from the train's viewpoint, I move.

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.

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?

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).

 

[Doc Al]

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).

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

 

[Reshma]

Oops, sorry I missed the word "accelerating"!

 

[Doc Al]

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.

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

 

[nazgjunk]

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).

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.

 

[Reshma]

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.

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

 

[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.

 

[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.

 

[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.

 

[Danger]

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

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.

 

[nazgjunk]

He said that whichever one had to expend energy in order to leave the other was the one considered to be moving.

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 ;)

 

[X-43D]

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

 

[kawikdx225]

snip

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.

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

 

[bernhard.rothenstein]

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

 

[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.

 

[Doc Al]

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?

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

 

[nazgjunk]

OK, but why?

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

 

[Doc Al]

I'm not sure I can answer "why?", but I can explain what I mean. Picture this: You are in your spaceship with the windows closed. If you are moving at a constant speed in a straight line, then there is no possible way of telling (by any experiment or measurement you can do totally within the ship) that you are moving at some speed or are at rest. But measurements inside the ship can easily determine that you are accelerating.

 

[nazgjunk]

Well, that I knew. I know after all how it feels to be in a a rollercoaster. But my question now still is "Why". Thankis though, because I do understand it a bit more now.

 

[robphy]

Well, that I knew. I know after all how it feels to be in a a rollercoaster. But my question now still is "Why". Thankis though, because I do understand it a bit more now.

“Say, Pop, I noticed something: When I pull
the wagon the ball rolls to the back of the wagon, and when
I’m pulling it along and I suddenly stop, the ball rolls to the
front of the wagon,” and I says, “why is that?” And he said,
“That nobody knows,” he said. “The general principle is that
things that are moving try to keep on moving and things that
are standing still tend to stand still unless you push on them
hard.” And he says, “This tendency is called inertia but nobody
knows why it’s true.”
-Feynman, The Pleasure of Finding Things Out