Frames?

[Asimov Pupil]

I was reading in the paper on special relativity from David W. Hogg and i was hopping it would explain frames, but it hasn't so far, though it has mentioned them plenty of times. Please explain to those of us who go to high school in some hick town out in the middle of nowhere.

 

Thank you

[Klaynos]

This is really hard to explain, I shall try my best.

 

A frame is a view posision, so take a car, if you are stood by the side of the road it is moving at 30m/s in your reference frame. So it appears to be moving at that speed compared to you. If you are sat in the car it is moving at 0m/s so that is the cars rest frame, the observer is at rest with it. if you are in a car heading in the oter direction and the first car has appears to you to be moveing at 60m/s, then in the original frame the new car has a celocity of -30m/s in the direction of motion of the first car.

 

I hope this has helped...

[Asimov Pupil]

yes, thank you......................what about light if it always travels at c does it have a rest frame or is c the rest frame

[Klaynos]

Light travels at c in all frames. As far as I know it has no true rest frame, but you can consider a rest frame for a system, but c will always be the speed of light in that frame.

 

"The speed of light c is a universal constant, the same in any inertial frame" one of the postulates of special relativity.

[Bob182]

Special relativity deals with inertial frames. A frame of reference, as described by Klaynos, is what we choose to measure things relative to. Someone standing at the side of a railtrack sees a train fly past at 70mph relative to the track, someone sat in that train sees the person at the trackside move past them at 70mph (in the opposite direction) relative to the train.

 

An inertial frame is one in which Newton's laws hold, that is, a frame which is not accelerating. Try this link for an explination of inertial/noninertial frames with a pretty diagram.

 

Special relativity states two things:

 

For any inertial frame the laws of physics are the same - perform an experiment in one inertial frame and you get the same result as in any other inertial frame.

 

The speed of light, as measured in any inertial frame, will be constant.

 

Using these, and some geometry, results appear that differ from our common sense notions for adding velocities etc.

[Saint]

I've read, and heard, that there is no inertial (rest frame) for light. Where does this idea come from? Did it originate simply from the statement that the speed of light is constant in all frames - therefore there cannot exist an inertial frame in which the speed of light is zero? Or is there something more to back it up?

[Johnny5]

I've read, and heard, that there is no inertial (rest frame) for light. Where does this idea come from? Did it originate simply from the statement that the speed of light is constant in all frames - therefore there cannot exist an inertial frame in which the speed of light is zero? Or is there something more to back it up?

 

I don't understand your questions. Do you know what an inertial frame is? If you do, please explain it to me.

[Bob182]

An inertial frame is one that is not accelerating. If you travel in your space ship at constant velocity and then closed the curtains you would have no way of telling how fast you were moving, or indeed if you were moving at all.

 

If you performed an experiment - say hitting a monkey with a cricket bat - you would observe the same physical laws applying as someone travelling in another space ship travelling twice as fast as your one.

 

You would be aware if the ship underwent an acceleration. Say it undergoes a uniform acceleration, you would experience a force towards the back end of the ship. The ship is no longer an inertial frame, but by taking the force into account (the same way we take gravity into account in earth bound labs) you can continue to hit your monkey with a bat and deduce that it obeys the same physical laws as someone in a non accelerating frame.

[Johnny5]

An inertial frame is one that is not accelerating.

 

Let me ask you something then Bob.

 

Suppose you are in a rocket, passing by a moon, which is at rest in deep space.

 

And your ship's engines are on.

 

You look out your spaceship's window.

 

Is the moon accelerating?

[5614]

Is the moon accelerating?

Yes!

 

Speed and position is relative, so if there's only your ship and the moon and one starts to move you cannot tell which is moving without comparing it to a 3rd thing.

 

I mean accelerating means it's speed is increasing... if there is only your ship and the moon and the ship turns on it's engines then the moon is moving away from the ship or the ship is moving away from the moon... it's the same thing until you introduce a 3rd reference frame. But even then its you being the reference frame and you are comparing yourself to the moon, it's only two, they're both moving away from each other, or only one from the other, you cannot tell, although you do know one (or both) is/are moving.

[J.C.MacSwell]

Let me ask you something then Bob.

 

Suppose you are in a rocket' date=' passing by a moon, which is at rest in deep space.

 

And your ship's engines are on.

 

You look out your spaceship's window.

 

Is the moon accelerating?[/quote']

 

Not in any inertial frame.

[Johnny5]

Not in any inertial frame.

 

Exactly.

 

 

But in the rest frame of the ship, the moon is accelerating.

And

In the rest frame of the moon, the ship is accelerating.

 

The rest frame of the moon was stipulated to be an inertial reference frame.

 

So since, in the rocket frame, the moon is accelerating without an external applied force (no action reaction pair) it necessarily follows that the rest frame of the rocket isn't an inertial reference frame.

 

But still, the center of inertia of the moon is accelerating in the rest frame of the ship.

 

Yes.

 

 

Just switch back and forth between the frames, and you know what I'm taking about.

[Johnny5]

Yes!

 

Speed and position is relative' date=' so if there's only your ship and the moon and one starts to move you cannot tell which is moving without comparing it to a 3rd thing.

 

[/quote']

 

Aye, a third frame.

[J.C.MacSwell]

Exactly.

 

 

But in the rest frame of the ship' date=' the moon is accelerating.

And

In the rest frame of the moon, the ship is accelerating.

 

The rest frame of the moon was stipulated to be an inertial reference frame.

 

So since, in the rocket frame, the moon is accelerating without an external applied force (no action reaction pair) it necessarily follows that the rest frame of the rocket isn't an inertial reference frame.

 

But still, the center of inertia of the moon is accelerating in the rest frame of the ship.

 

Yes.

 

 

Just switch back and forth between the frames, and you know what I'm taking about.[/quote']

 

 

Not quite. I could be wrong but I would not call these (the ones I bolded) "rest frames". I would say that the ship is constantly changing rest frames as it accelerates.

[Johnny5]

Not quite. I could be wrong but I would not call these (the ones I bolded) "rest frames". I would say that the ship is constantly changing rest frames as it accelerates.

 

To any object in the universe, there is a center of mass of that object. You can attach a frame to that object, so that the center of inertia of that object is always located at the origin of the frame. This is what I mean by "rest frame" of something.

 

So for example, the rest frame of the moon I was talking about, is a three dimensional coordinate system, whose origin is always the center of inertia of the moon. wherever the moon goes, so does the coordinate system.

 

Likewise for the rest frame of the rocket.

 

Somewhere inside the rocket, is the center of mass of the rocket.

Let that point be the origin of a three dimensional rectangular coordinate system.

 

Wherever the rocket goes, the coordinate system goes, the coordinate system is the rest frame of the rocket, in other words, the rocket is at rest in the coordinate system, even if the rocket is accelerating in someone else's frame.

 

PS: You didnt bold anything, and I don't understand what you mean by "constantly changing rest frames as it accelerates."

[J.C.MacSwell]

To any object in the universe' date=' there is a center of mass of that object. You can attach a frame to that object, so that the center of inertia of that object is always located at the origin of the frame. This is what I mean by "rest frame" of something.

 

So for example, the rest frame of the moon I was talking about, is a three dimensional coordinate system, whose origin is always the center of inertia of the moon. wherever the moon goes, so does the coordinate system.

 

Likewise for the rest frame of the rocket.

 

Somewhere inside the rocket, is the center of mass of the rocket.

Let that point be the origin of a three dimensional rectangular coordinate system.

 

Wherever the rocket goes, the coordinate system goes, the coordinate system is the rest frame of the rocket, in other words, the rocket is at rest in the coordinate system, [i']even if [/i] the rocket is accelerating in someone else's frame.

 

PS: You didnt bold anything, and I don't understand what you mean by "constantly changing rest frames as it accelerates."

 

I understand what you are saying. I will try to find out the correct definition, or if there is more than one accepted use of the term in physics.

[Johnny5]

I understand what you are saying. I will try to find out the correct definition, or if there is more than one accepted use of the term in physics.

 

Good thank you.

[J.C.MacSwell]

I understand what you are saying. I will try to find out the correct definition, or if there is more than one accepted use of the term in physics.

 

OK, I have:(from "the international dictionary of physics and electronics, second edition 1961, D. Van Nostrand Co.)

 

Rest Frame: Lorentz frame in which the total momentum of a system vanishes.

[Bob182]

In your moon/rocket example an observer inside the ship would feel the acceleration due to the engines. It is not an inertial frame, although I realise you are talking about the instantaneous rest frame of the rocket.

 

The moon would be seen to accelerate away from the bloke in the rocket, but if you were to close the curtains on the ship he would still be aware of an acceleration. The same cannot be said of someone on the moon.

 

The rest frame of an entire system is also referred to as the "zero momentum" frame just like J.C. said, where the individual momenta involved sum to zero. It is useful when dealing with collisions such as in particle accelerators.

[Johnny5]

OK' date=' I have:(from "the international dictionary of physics and electronics, second edition 1961, D. Van Nostrand Co.)

 

Rest Frame: Lorentz frame in which the total momentum of a system vanishes.[/quote']

 

What does it tell you that a Lorentz frame is?

[Johnny5]

In your moon/rocket example an observer inside the ship would feel the acceleration due to the engines.

 

 

You cannot feel an acceleration, you feel a force.

[Bob182]

indeed

[J.C.MacSwell]

You cannot feel an acceleration, you feel a force.

 

You feel a force and force gradients within yourself. I would say if you feel force gradients in this way you "feel" the acceleration, but I think it is open to interpretation.

[Johnny5]

You feel a force and force gradients within yourself. I[/b'] would say if you feel force gradients in this way you "feel" the acceleration, but I think it is open to interpretation.

 

Let me ask you a direct question.

 

Is it possible to accelerate without feeling a force gradient within yourself?

[J.C.MacSwell]

What does it tell you that a Lorentz frame is?

 

I checked last night, but will re-check to get the right wording.