Help with special relativity (frames of reference)

[thisisdom]

Well I've just been learning about special relativity, and I think I understand everything I need to know, except frames of reference (for A level). I need to know where you are allowed to take frames of reference from, and where you are not.

I understand that you can't take a frame of reference from something which is accelerating or rotating. I don't understand how you can define if something is accelerating though.

So for example, say I have 2 spaceships (spaceship 1 and spaceship 2), which are a certain distance apart from each other, and are at rest (relative to each other).

Lets say the person inside spaceship 1 turns his boosters on, and accelerates towards spaceship 2, and when he gets near to spaceship 2, he turns around accelerates backwards, eventually returning to his original position.

Am I right in thinking you would have to take the frame of reference from spaceship 2? since it's not "really" accelerating?

So my questions are:
- Why couldn't I say that spaceship 2 was actually the one that was accelerating, and use the frame of reference from spaceship 1?

- If everything is relative, why would only the person in spaceship 1 feel a force, when technically they are both accelerating away from each other?

 

[JesseM]

I understand that you can't take a frame of reference from something which is accelerating or rotating. I don't understand how you can define if something is accelerating though.

The one that accelerates feels G-forces, which can be measured with an accelerometer. So, acceleration isn't relative the way velocity is.

 

[thisisdom]

The one that accelerates feels G-forces, which can be measured with an accelerometer. So, acceleration isn't relative the way velocity is.

So why does only one feel a force, when technically they are both accelerating towards each other.

What defines who will feel the force when you have two things accelerating towards each other?

Is it to do with "how" they are accelerating?

 

[DrGreg]

So why does only one feel a force, when technically they are both accelerating towards each other.

What defines who will feel the force when you have two things accelerating towards each other?

Is it to do with "how" they are accelerating?

The force doesn't just magically appear from nowhere, it has to be supplied by something. In your example in post #1, it is the boosters of spaceship 1 that supply the force.

Terminological note: acceleration of one thing relative to something else is called "relative acceleration" or "coordinate acceleration". The sort of acceleration that is measured by an accelerometer, and that you can feel as a "g-force", is called "proper acceleration". Technically it is acceleration relative to the inertial frame in which you are momentarily at rest.

In your example, each spaceship has acceleration relative to the other, but only one undergoes proper acceleration, caused by boosters.

 

[thisisdom]

The force doesn't just magically appear from nowhere, it has to be supplied by something. In your example in post #1, it is the boosters of spaceship 1 that supply the force.

Terminological note: acceleration of one thing relative to something else is called "relative acceleration" or "coordinate acceleration". The sort of acceleration that is measured by an accelerometer, and that you can feel as a "g-force", is called "proper acceleration". Technically it is acceleration relative to the inertial frame in which you are momentarily at rest.

In your example, each spaceship has acceleration relative to the other, but only one undergoes proper acceleration, caused by boosters.

Ok thanks a lot for the explanations :) I understand now.

So "proper" accelleration is always at the point where momentum would be bieng conserved. Thinking about it, spaceship 2 would be magically gaining momentum.

 

[JesseM]

So why does only one feel a force, when technically they are both accelerating towards each other.

No, technically only one is accelerating, the one that feels the g-force (he's also the only one whose velocity is changing relative to any inertial frame, the other one has constant velocity in all inertial frames)