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Author Topic: Are frames of reference even more misunderstood than centripetal force?  (Read 30626 times)

Offline Geezer

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Every time I get into a debate about the convenient, yet non-existent, centrifugal force, somebody always tries the old, "ah, but in a rotating frame of reference" argument.
 
I'm probably just too old-fashioned, but I was under the impression that you can't flip-flop between different frames whenever the going gets slightly tough. If you apply the Earth frame to a model, you have to describe EVERYTHING with respect to that frame. You can't suddenly claim that the frame is rotating because, by definition, if there is any rotation, it is because everything is rotating around that frame. (I think that has been tried already, but it didn't get too far.)
 
Seems to me that "frames" are being used as a sort of scientific "Three Card Monte" by people who really have no idea what they are on about, or am I just being too old-fashioned?
 
 


 

Offline Soul Surfer

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I think that the problem lies deep in most people's understanding of "reality"  we are all very used to the experience that everyone else sees things happening much as we see it.  We can easily see and understand that if we were standing somewhere else the view would be slightly different because it is not to difficult to visualise this and we may have already seen the different view ourselves.  However when it comes to time we all think things happen at the same time.

Now when we get to the relativistic case things that we seem ay happen at different times or happen at different rates.  A lot of people tend to assume that a person somewhere else would experience these distortions of time and not that they would have a completely different view of when things happened or how fast.  The same is of course true for gravitational distortions of reality.

The other big problem of course is the true scale of things related to space time and gravity  Peole jus do not grasp differences once they get bigger than two or three orders of magnitude and have no concept whatever of ten or one hundred orders of magnitude (both of which appear in scales of this nature)  far less the  ten to one hundred and much more orders of magnitude that commonly occur in mathematics.

Also illustrations often require extreme compressions of scale or totally unnatural viewpoints and the populsr literature just fails to get the relative scale of things over

One of the things that I have found useful in this is a local scale model of the solar system where a model of the sun (about the size of a large box van can be seen from several hundred yards away form a model of the earth and moon as a medium and small ball bearing a couple of feet apart with the outer planets stretched out along a canal several miles away and the nearest star about 47,000 miles away!  see http://en.wikipedia.org/wiki/Somerset_Space_Walk .
 

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Offline Ęthelwulf

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Every time I get into a debate about the convenient, yet non-existent, centrifugal force, somebody always tries the old, "ah, but in a rotating frame of reference" argument.
 
I'm probably just too old-fashioned, but I was under the impression that you can't flip-flop between different frames whenever the going gets slightly tough. If you apply the Earth frame to a model, you have to describe EVERYTHING with respect to that frame. You can't suddenly claim that the frame is rotating because, by definition, if there is any rotation, it is because everything is rotating around that frame. (I think that has been tried already, but it didn't get too far.)
 
Seems to me that "frames" are being used as a sort of scientific "Three Card Monte" by people who really have no idea what they are on about, or am I just being too old-fashioned?

For a moderator who likes to play the ''don't be condescending card'' you are pretty hypocritical right?

Anyway, frames of reference are very important in physics. Even in rotating frames of reference. The Coriolis Effect is a perfect example of a frame-dependant phenomenon which rotates.
 

Offline JP

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There really is no ambiguity here.  Can you make your "force" vanish by changing to an inertial (non-accelerating) reference frame? 

Yes:  It's not a force.
No: It's a force.
 

Offline Pmb

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Every time I get into a debate about the convenient, yet non-existent, centrifugal force, somebody always tries the old, "ah, but in a rotating frame of reference" argument.
The reason people automatically start talking about rotating frames is because the only place centrifugal forces exist are in rotating frames. It is impossible to construct a non-inertial frame of reference in an inertial frame.
 

Offline @/antic

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Hi

I'm not certain if this is the correct place to ask this question, but here it is anyway:

I'm confused about frames of reference when we speak of time dilation at the speed of light. If one object is moving at speeds close to that of light relative to another, then for whom would time dilate? Because both could be regarded as moving at the speed of light depending on the frame of reference. For whom does time slow down then?

What am I not getting here?

Cheers
Atlantic
 

Offline Pmb

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I'm confused about frames of reference when we speak of time dilation at the speed of light. If one object is moving at speeds close to that of light relative to another, then for whom would time dilate?
Let us first note that no clock can move "at the speed of light".

Define the following frames:

Let S represent an inertial frame of reference in which the observer is at rest and for which there is a clock C.

Let S' represent an inertial frame of reference which is moving parallel to the X'-axis in which there is a clock C' at rest.

Let S' represent an inertial frame of reference which is moving parallel to the X'-axis in which there is a clock C’’at rest.

Let S' be moving with speed U = 0.999999999999c in the X’ direction.
Let S’’ be moving with speed V = 0.999c in the X’’ direction.

We now have 3 objects which are moving relative to each other. Using relativity it can then be shown that each clock runs at a different rate than from the other clock.

I'm confused about frames of reference when we speak of time dilation at the speed of light. If one object is moving at speeds close to that of light relative to another, then for whom would time dilate? Because both could be regarded as moving at the speed of light depending on the frame of reference. For whom does time slow down then?
This question can’t be answered since it is predicated on the assumption that one of the clocks is moving at the speed of light, which is physically possible.
 

Offline Soul Surfer

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You are just not reading my reply above! Time does not dilate for anyone!  Dilation only applies when you look at someone else's clock that is moving in a different way to yours. Your clock is always working quite normally.  The same is of course true for the other person looking at their clock and yours.   Theirs looks perfectly normal to them yours is slow.
« Last Edit: 16/04/2012 08:48:07 by Soul Surfer »
 

Offline Soul Surfer

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Geezer the centrifugal force problem is similar.  We are all used to the behaviour of rigid bodies and centrifugal force only appears when you tie yourself to a rotating rigid body which by the nature of its rigidity is supplying the opposition to the centripetal force and you then have to supply this force yourself to go with the rigid body.  Now rigid bodies are on the whole quite rare through the universe but we happen to be on one (well reasonably rigid that is)
 

Offline Pmb

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You are just not reading my reply above!
Please specify who your comments are directed to, otherrwise it can be confusing.  For example; I can't tell if you're comments here are directed to the OP or to me.

Thanks.
 

Offline simplified

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Hi

I'm not certain if this is the correct place to ask this question, but here it is anyway:

I'm confused about frames of reference when we speak of time dilation at the speed of light. If one object is moving at speeds close to that of light relative to another, then for whom would time dilate? Because both could be regarded as moving at the speed of light depending on the frame of reference. For whom does time slow down then?

What am I not getting here?

Cheers
Atlantic

The theoretical disagreement exists, however  experimental disagreement does not exist.You see slowed  clock of International Space Station.Observer of ISS sees your clock is faster.
 

Offline Ęthelwulf

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Geezer the centrifugal force problem is similar.  We are all used to the behaviour of rigid bodies and centrifugal force only appears when you tie yourself to a rotating rigid body which by the nature of its rigidity is supplying the opposition to the centripetal force and you then have to supply this force yourself to go with the rigid body.  Now rigid bodies are on the whole quite rare through the universe but we happen to be on one (well reasonably rigid that is)

Agreed.

 

Offline Ęthelwulf

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Hi

I'm not certain if this is the correct place to ask this question, but here it is anyway:

I'm confused about frames of reference when we speak of time dilation at the speed of light. If one object is moving at speeds close to that of light relative to another, then for whom would time dilate? Because both could be regarded as moving at the speed of light depending on the frame of reference. For whom does time slow down then?

What am I not getting here?

Cheers
Atlantic

The theoretical disagreement exists, however  experimental disagreement does not exist.You see slowed  clock of International Space Station.Observer of ISS sees your clock is faster.

Of course, it is all relative. Frame-dependant if you wish.
 

Offline Soul Surfer

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PMB I was referring all of you including yourself.

  you say 

"We now have 3 objects which are moving relative to each other. Using relativity it can then be shown that each clock runs at a different rate than from the other clock."

This is totally untrue and misleading to others.  All the clocks at their different locations and speeds are running at exactly the same rate.  It is only that the "observers" by the clocks looking away from their clock (running normally) and towards one of the other clocks "sees" a clock running at a different rate and in every case it is slower.
« Last Edit: 16/04/2012 19:17:06 by Soul Surfer »
 

Offline simplified

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Hi

I'm not certain if this is the correct place to ask this question, but here it is anyway:

I'm confused about frames of reference when we speak of time dilation at the speed of light. If one object is moving at speeds close to that of light relative to another, then for whom would time dilate? Because both could be regarded as moving at the speed of light depending on the frame of reference. For whom does time slow down then?

What am I not getting here?

Cheers
Atlantic

The theoretical disagreement exists, however  experimental disagreement does not exist.You see slowed  clock of International Space Station.Observer of ISS sees your clock is faster.

Of course, it is all relative. Frame-dependant if you wish.
I don't see any physics in your words.Is electron an frame-dependant in accelerator? :-\
 

Offline Ęthelwulf

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Hi

I'm not certain if this is the correct place to ask this question, but here it is anyway:

I'm confused about frames of reference when we speak of time dilation at the speed of light. If one object is moving at speeds close to that of light relative to another, then for whom would time dilate? Because both could be regarded as moving at the speed of light depending on the frame of reference. For whom does time slow down then?

What am I not getting here?

Cheers
Atlantic

The theoretical disagreement exists, however  experimental disagreement does not exist.You see slowed  clock of International Space Station.Observer of ISS sees your clock is faster.

Of course, it is all relative. Frame-dependant if you wish.
I don't see any physics in your words.Is electron an frame-dependant in accelerator? :-\

What's an accelerator got to do with this?

This discussion in this thread is explicitely about frame-dependance and the effects of a centrifugal force.
 

Offline Ęthelwulf

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Strangely however, I'm looking back at my statement and I think I have qouted the wrong person.
 

Offline CliffordK

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It is my belief that there is one universal frame.  Call it the fabric of space (or the fabric of space-time, but I prefer simply space). 

It is often convenient to define a local frame, such as being inside of a moving train, but one should always consider it with respect to the universal frame, the fabric of space.

As far as a "rotating frame", some phenomena such as describing the trajectory of a football thrown by the quarterback work well considering the earth in a fixed frame, flat, under constant gravity, with the football, quarterback, and receiver all in the same frame (yes, I consider football the game in which the ball is carried with people's hands).

Other phenomena such as describing the forces on a geosynchronous or geostationary satellite require considering the frame of at least the solar system, if not the galaxy or universe (fabric of space).

One could, of course, consider the football's motion with respect to the fabric of space, but then there are so many additional variables, most of which cancel themselves out, or make a very small contribution, that it is unnecessary.

What a speeding spaceship frame. 
Again, understanding the forces experienced by the occupants of the spaceship with respect to the vessel, it can be convenient.  However, it runs into problems.  For example, say the spaceship is travelling at 90% of the speed of light (c) with respect to Earth.  In it's own frame, it is not moving at all.  So, can it accelerate again to 90% of the speed of light?  And, thus be at 180% of the speed of light with respect to Earth?  Is acceleration different depending on the direction?

Referring back the the universal fabric of space frame, it becomes obvious that the spaceship can't continue to accelerate without doing something very funky with the clocks.

Where are we with respect to the fabric of space?  A good estimate is the cosmic microwave background radiation, which we are travelling through at about 370 km/s, slower than the Milky Way at 552 km/s, due to the current orbital position and motion around the galaxy.  What is this a measurement of?  Actually, it measures the redshift/blueshift of the 21cm hydrogen line, I think, which then is corrected to a neutral frame.  Now, it is certainly possible that this is not in a rest frame, but it is the best estimate that we have.

Anyway, by considering a universe frame, or a universal fabric of space frame, the problem of a rotating frame becomes irrelevant, and one can understand the orbital motion of satellites, as well as why a spaceship can't keep accelerating within it's local frame past the speed of light.
 

Offline Ęthelwulf

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It is my belief that there is one universal frame.  Call it the fabric of space (or the fabric of space-time, but I prefer simply space). 

I agree with this as well. This is what I believe.

The Dirac equation and negative holes in the vacuum actually gave rise to the Dirac Sea. Today, not many agree with this interpretation because it involved the aether theory. But his theory did predict an aether. Also John Bell said the aether was rejected on wrong grounds and that an aether could help resolve the spooky action at a distance, I will recite a part from wikipedia now:

''John Bell, interviewed by Paul Davies in "The Ghost in the Atom" has suggested that an aether theory might help resolve the EPR paradox by allowing a reference frame in which signals go faster than light.[2] He suggests Lorentz contraction is perfectly coherent, not inconsistent with relativity, and could produce an aether theory perfectly consistent with the Michelson-Morley experiment. Bell suggests the aether was wrongly rejected on purely philosophical grounds: "what is unobservable does not exist" [p.49]. Einstein wrote that the Special Theory of Relativity "does not compel us to deny the Aether. We may assume the existence of an Aether".''
 

Offline Ęthelwulf

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If space is some kind of absolute reference frame, then we may perhaps call it a type of quantum aether.
 

Offline graham.d

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Don't you think this has all gone a bit beyond Geezer's question. I don't think we really need to get into General Relativity and the fabric of space time to discuss the question of whether we should describe the feeling of being thrown out when rotating as a "force" (centrifugal) or not. I know Geezer supports the view expressed by physics teachers around the mid 20th century (and maybe still) that the words "centrifugal force" should not be used. The reasons for this (I think) is that it can cause confusion and a misunderstanding of the mechanism: the only force in the rest frame is the tension in the string (centipetal) and that Newton's laws then adequately describe the motion. The feeling experienced by every child on a roundabout is then not explained as a force but as the tendency of objects to continue in a straight line unless acted upon by an external force. Personally I never had a problem with looking at this in either way although I can see why this method of teaching was encouraged. To me the idea of "real" forces that have names and "other" forces that, it is decreed, shall not have names is not a sensible one. Giving the force, that we all feel and know as a force whilst whizzing round on a roundabout, a name and being able to explain how it comes about only enhances people's understanding and I do not see a good reason to prevent this use of language. It is open to debate whether it confuses children learning the physics for the first time (I think not, but that's just my opinion), but it certainly would not confuse most people versed in physics to some extent. It just restricts language use to describe what we feel as a force on a roundabout (or utilise in a centrifuge).
 

Offline Pmb

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Quote from: Geezer
... I was under the impression that you can't flip-flop between different frames whenever the going gets slightly tough. If you apply the Earth frame to a model, you have to describe EVERYTHING with respect to that frame. You can't suddenly claim that the frame is rotating because, by definition, if there is any rotation, it is because everything is rotating around that frame. (I think that has been tried already, but it didn't get too far.)
I don't follow your assertion. Can you please give an illustrative example?

Thanks
 

Offline Ęthelwulf

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It is open to debate whether it confuses children learning the physics for the first time (I think not, but that's just my opinion), but it certainly would not confuse most people versed in physics to some extent. It just restricts language use to describe what we feel as a force on a roundabout (or utilise in a centrifuge).

Maybe I have had bad lecturers then, because I don't ever recall them ever telling me that the centrifugal force was a myth.
 

Offline Ęthelwulf

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It is open to debate whether it confuses children learning the physics for the first time (I think not, but that's just my opinion), but it certainly would not confuse most people versed in physics to some extent. It just restricts language use to describe what we feel as a force on a roundabout (or utilise in a centrifuge).

Maybe I have had bad lecturers then, because I don't ever recall them ever telling me that the centrifugal force was a myth.

Or maybe I am a bad student. Or even better, like Geezer said, maybe I don't know what I am talking about.
 

Offline Geezer

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Sorry everyone! Geezer's view is very simplistic.
 
It goes along the lines that a "frame of reference" is absolutely contained within that frame. Peeking outside the frame is not allowed.
 
If I use the Earth as my frame of reference, I have to describe everything relative to that frame, so it's likely I will assume the Universe revolves around the Earth (which was not an uncommon viewpoint in the past.)
 
You must pick your frame of reference. You cannot swap a different frame half-way through an argument. That's my point.
 
If you want to argue about this stuff, please define your frame of reference, and try to stick to it.
 

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