Are frames of reference even more misunderstood than centripetal force?

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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?
 
 
There ain'ta no sanity clause, and there ain'ta no centrifugal force ęther.

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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.

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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.

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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.

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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

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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.

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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 »
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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)
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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.

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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.

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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.


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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.

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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 »
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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? :-\

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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.

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Offline Ęthelwulf

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

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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.

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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".''

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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.

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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).

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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

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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.

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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.

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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.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ęther.

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Offline JP

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The fact is that it's all about trying to use F=ma and all the laws of Newtonian mechanics that can be derived from that in an accelerating reference frame.  The problem is that F=ma and those other laws are first taught in non-accelerating reference frames, since they are exactly the same in all such frames, and they have to be modified if you're in an accelerating reference frame!

But if you're in a reference frame undergoing uniform circular motion, the modifications to these equations are added to them just like a force usually is in a non-accelerating reference frame so you can cheat and pretend its a force.  This is fine if all you want to do is use the equations in a reference frame undergoing uniform circular motion.  But if you make the mistake of thinking centrifugal force is a real force then you'll get the equations wrong as soon as you have to compute motion in any other reference frame.

If you understand this, there's not really a problem with treating it like a force in a reference frame undergoing uniform circular motion.  But if you confuse centrifugal force with a real force, you're setting yourself up to make some pretty big mistakes.

Textbooks these days are for the most part very careful about making this distinction for precisely this reason.  For the most part it is carefully stressed that only real forces get added to Newton's laws as F=ma in non-accelerating reference frames.  Only later, when students have the mathematical sophistication to reformulate Newton's laws in accelerating reference frames, can they fundamentally grasp why the centrifugal "force" appears and why it isn't a real force.

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Offline Pmb

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Geezer - Nothing in that post makes any sense to me. You made may assertions in that frame and never explained what you meant nor did you think to back it up with reasonsing. Why is that?

For example: you said It goes along the lines that a "frame of reference" is absolutely contained within that frame.

Please state what you mean by one frame being absolutely contained within another frame. Also please explain what frame you're talking abiut when you said "that" frame? What frame are you talking about?

etc.

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Offline Pmb

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The problem with this thread is that posters in this thread keep referring to inertial forces as non-real/fictitious/pseudo etc. or claiming that using inertial forces is cheating. All that is contrary to classical mechanics as it stands today, especially to general relativity.

Since physics in rotating frames is one of those subjects that keeps popping up in physics discussion forums I decided a long time ago to create a web page on the subject. The page is very thorough. I now post a link to the web page whenever the subject comes up.

It now appears to me that it's not being read. That's all fine and dandy but I don’t know whether it’s being read or not and whether their viewpoints on the layman viewpoint of inertial forces, like the Coriolis force, is “not real” or their own personal viewpoint that inertial forces are not real. Don’t get me wrong. There are still physicists who think of inertial forces as being non-real.

So, what do I do? Do I assume it’s not being read? If it’s being read and you don’t agree with its content then please tell me where you think its wrong.

Thanks.

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Offline imatfaal

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Peter - I can't see where you have posted the link to your webpage on this thread; so Yes, I think you can assume it isn't being read.
There’s no sense in being precise when you don’t even know what you’re talking about.  John Von Neumann

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Offline Pmb

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Peter - I can't see where you have posted the link to your webpage on this thread; so Yes, I think you can assume it isn't being read.
Ha!! Son of a gun! You're right. I didn't post that URL in this thread.

Here it is now: http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm

All comments welcome.

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Offline graham.d

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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.

I don't know when you were at school, but for me in the late 60's  (and at university doing Physics) it seemed to be the norm to decry the use of the words "centrifugal force". You may have had enlightened teachers. To me the centrifugal force is simply an inertial force to be considered as such.

Geezer, it is not jumping from one frame to another; just viewing a system from alternative frames. Of course you have to be consistant with a particular frame with any calculations, but I think it more enlightening to explain an event (as you suggest, if you are on a roundabout) in that frame where, if you are an external observer, from that external frame. Both are valid. Another thing I note is that although not liking the phrase "centrifugal force" everyone seemed happy with the "Coriolis Force" to which the same arguments apply.

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Offline imatfaal

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There’s no sense in being precise when you don’t even know what you’re talking about.  John Von Neumann

At the surface, we may appear as intellects, helpful people, friendly staff or protectors of the interwebs. Deep down inside, we're all trolls. CaptainPanic @ sf.n

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Offline JP

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The problem with this thread is that posters in this thread keep referring to inertial forces as non-real/fictitious/pseudo etc. or claiming that using inertial forces is cheating. All that is contrary to classical mechanics as it stands today, especially to general relativity.

Since physics in rotating frames is one of those subjects that keeps popping up in physics discussion forums I decided a long time ago to create a web page on the subject. The page is very thorough. I now post a link to the web page whenever the subject comes up.

It now appears to me that it's not being read. That's all fine and dandy but I don’t know whether it’s being read or not and whether their viewpoints on the layman viewpoint of inertial forces, like the Coriolis force, is “not real” or their own personal viewpoint that inertial forces are not real. Don’t get me wrong. There are still physicists who think of inertial forces as being non-real.

So, what do I do? Do I assume it’s not being read? If it’s being read and you don’t agree with its content then please tell me where you think its wrong.

Thanks.

Your page proves exactly why you can't group inertial forces together with "real" forces (or whatever other term you want to use for them.)  They're caused by different things and behave differently under transformations of reference frames (in particular, inertial forces vanish in inertial reference frames).  Grouping them all together is misleading. 

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Offline Pmb

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Quote from: graham.d
Geezer, it is not jumping from one frame to another; just viewing a system from alternative frames.
I have no idea what that means. To view a system at all one chooses a frame of reference.

With physics one has to be very precise about what one means and how that is expressed in writing. Geezer's response was probably very clear to Geezer but it might be read different ways by different people, hence my confusion.

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Offline imatfaal

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Quote from: graham.d
Geezer, it is not jumping from one frame to another; just viewing a system from alternative frames.
I have no idea what that means. To view a system at all one chooses a frame of reference.

With physics one has to be very precise about what one means and how that is expressed in writing. Geezer's response was probably very clear to Geezer but it might be read different ways by different people, hence my confusion.

modnote - Peter, a fair percentage of your posts are critiquing other members language and comprehensibility (in my opinion unfairly) - could you tone it down a bit please.  Many thanks.

on a substantive note - whilst one chooses a frame of reference and works within it, there is no reason not to then choose to model the exact same physical process from an alternative reference frame.  the ball can be described as rising vertically from the hand and falling back into it from the perspective of a fellow train passenger, or describing a (near) parabola travelling 20 metres horizontally according to the man watching the train rush past.  mixing measurements from the two frames is dangerous, but making sure that the results from the two frames are consistent is crucial.
There’s no sense in being precise when you don’t even know what you’re talking about.  John Von Neumann

At the surface, we may appear as intellects, helpful people, friendly staff or protectors of the interwebs. Deep down inside, we're all trolls. CaptainPanic @ sf.n

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Offline Pmb

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Quote from: imatfaal
modnote - Peter, a fair percentage of your posts are critiquing other members language and comprehensibility (in my opinion unfairly) - could you tone it down a bit please.  Many thanks.
I never meant my posts to seem irritating to people. Since you read things in a way that they aren't meant then it seems that I'd be better of leaving this forum. Good luck in seeking to understand physics! BEst wishes to you all.

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Offline simplified

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Quote from: imatfaal
modnote - Peter, a fair percentage of your posts are critiquing other members language and comprehensibility (in my opinion unfairly) - could you tone it down a bit please.  Many thanks.
I never meant my posts to seem irritating to people. Since you read things in a way that they aren't meant then it seems that I'd be better of leaving this forum. Good luck in seeking to understand physics! BEst wishes to you all.
If you wish to state some valuable thing then you should not worry about irritation of people.

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Offline Geezer

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@Graham
 
(I'd insert your post, but the stu****  system won't let me.)
 
I think my point has been more than made. I'm sure you are not confused, but half the punters on this forum seem to be convinced that "centrifugal force" actually exists by virtue of rearranging "frames of reference".
 
Personally, I believe this is a case of science pandering to common misconceptions. For me, it's the thin end of the wedge.
 
If you allow this, pretty soon you're allowing alternatives to the theory of evolution - after all, it was just a "theory", wasn't it?
 
(Your bud G.W. Bush fell for that one.)
 
There ain'ta no sanity clause, and there ain'ta no centrifugal force ęther.

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Offline Geezer

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So, here's the challenge:
 
Does a moving body, or does a moving body not, travel in a straight* line in the absence of other forces?
 
*"Straight" allows for curvature of spacetime.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ęther.

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Offline graham.d

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So, here's the challenge:
 
Does a moving body, or does a moving body not, travel in a straight* line in the absence of other forces?
 
*"Straight" allows for curvature of spacetime.

Yep! Within the spirit of this discussion.

So from the external frame this is a perfectly good explanation of the motion of a rotating (or any other system). For most systems this is the best way to analyse and understand the behaviour as the maths is usually easier. My only objection is to the apparent denial that an inertial force (like the centrifugal force), that is patently experienced by someone in an accelerated frame, should not have a name. It is fine to explain how it arises from Newtons laws but it is wrong, in my opinion, to deny that, for the person in an accelerated frame, that what they experience as an obvious force is not a force and should not have a name which would make discussions simpler. It would be like saying to a
trainee pilot in a centrifuge "you are feeling an effect because you trying to go in a straight line and outer wall of the centrifuge is accelerating radially inwards". This is true but is is perfectly valid to say that in his frame he is experiences centrifugal force. As I said before we don't have the same attitude to the Coriolis force - mainly because it is not such a common experience so the subject does not come up so much.

Is denial of centrifugal force (as a force) like saying that inertial forces are not forces at all? I am not arguing the causation issues are unimportent but that I don't see the need to restrict the use of language in these cases. I don't think kids are confused by this and physicists are certainly not - it merely serves to make descriptions harder because you have to *****-foot around with words. When the bloke in the centrifuge drops his coffee cup he then has to say "damn, I've let go of my cup and it is flying off in a diverging path from my rotating one and the floor has now converged with it so that has splashed coffee on my new trainers".

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Offline graham.d

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I hate computer systems that censor perfectly good words. The asterisks are what we affectionionately call a cat but which can have other connotations. Maybe I should not have hyphenated the word. I tried to correct this but I can't get the "modify" feature to work any more. Does this need fixing or did I do something wrong?

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Offline JP

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...but inertial forces aren't the same as forces.  Hence the word inertial.  :)

Actually, I used to have the same view as you.  Then I taught introductory physics and realized how completely confusing it was to the students group inertial forces and "real" forces together under one term.  Most of them have a lot of trouble grasping how describing circular motion works in an inertial reference frame, let alone how you can use the same equations in the rotating frame by introducing an inertial force. 

The standard curriculum now teaches that forces are the F=ma in inertial reference frames, and that inertial forces are things that appear in non-inertial reference frames.  Since most students don't have the mathematical sophistication to deal with non-inertial reference frames until their second year, a thorough discussion of inertial forces needs to wait until then.  If centrifugal force is mentioned, it's usually done in the way I did above: you can use F=ma in a rotating frame, but only by hand-waving that it works.  A thorough derivation of why it works has to wait for future courses, when students have the necessary calculus to really understand how to change reference frames.

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Offline graham.d

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...but inertial forces aren't the same as forces.  Hence the word inertial.  :)


True, but they are forces nonetheless and I see no reason for them not to be named. I value your view as someone who has had to teach Physics, but I still do not see why it cannot be explained in just the way you have done. Speaking for myself, I had no problem with understanding this and I felt the non-scientists' confusion was just in remembering the word to use (centripetal or centrifugal) with centrifugal getting used more (and incorrectly) just because it was a word they knew and had heard more often.

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Offline JP

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I don't quite understand your complaint that they aren't named.  They have a variety of names: inertial force, fictitious force, pseudoforce, etc.  All these names include the term "force" but also distinguish them from "real" forces which have different properties.  (I do dislike the terms real and fictitious force, hence the quotes--I would prefer force vs. inertial force.) 

In terms of teaching, most students taking physics 101 these days are concurrently taking basic calculus for the first time.  They won't learn to solve general differential equations until their second year of college.   This means they don't have the mathematical sophistication to solve the differential equations of motion in inertial reference frames, let alone non-inertial ones. 

The only practical solution is to teach Newton's laws and their solutions in inertial reference frames, since the solutions then hold in all inertial reference frames.  Then you teach them that the equations you showed are only valid in inertial reference frames and that trying to apply them elsewhere will lead to wrong answers.  I suppose you could pick a few rotating coordinate systems and teach them the solutions to Newton's laws in those, but it's far more efficient to put that on hold until they have the mathematics to formulate and solve Newton's laws in general coordinate systems, since physics 101 also usually covers waves and thermodynamics as well as Newtonian mechanics.

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Offline graham.d

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I don't quite understand your complaint that they aren't named.  They have a variety of names: inertial force, fictitious force, pseudoforce, etc.  All these names include the term "force" but also distinguish them from "real" forces which have different properties.  (I do dislike the terms real and fictitious force, hence the quotes--I would prefer force vs. inertial force.) 

What different properties? Isn't it a point that in a closed system you could not differentiate between an inertial force and any other force? You could only do so by obsevations outside your frame but the properties of the force itself are not different - or so it is contended. Of course you could test for being in a rotating system because of the change in force as you move radially, but the nature of the force is otherwise indistinguishable from that from a gravity field over a small distance.

Anyway my point is not to avoid teaching the behaviour of a system using Newtonian mechanics from the simplest perspective - in this case the external frame - but to not be so rigid about giving the inertial force experienced in a rotating frame the name of "centrifugal force". Every child experiences this as a force and it is absolutely "real" to them and most even know the name before doing the physics. By all means explain the reason for it and say it results as a consequence of rotation and how it arises, but don't extinguish the words from the English language. To me this is a negative approach even if it makes life easier for the teacher. In any case there is no reason to not use such words when one does understand the nature of the forces.

I guess this is really a matter of opinion of whether you wish to name specific inertial forces or not. Historically "centrifugal force" was a name given and I see no good reason to forbid its use provided it is properly understood. I understand that you have to guide students and build their understanding. To use an analogy: teaching someone to walk before they can run; but it would be a shame if when teaching a child to walk he was never able to see that running was a possibility.

OK, not a great analogy :-)

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Offline Pmb

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Quote
modnote - Peter, a fair percentage of your posts are critiquing other members language and comprehensibility (in my opinion unfairly) - could you tone it down a bit please.  Many thanks.
Please note that when I wrote my last response I was going through a difficult time. I retract my statement that I'm leaving.

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Offline Pmb

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Quote from: imatfaal

     
Quote from: Pmb
           
Quote from: graham.d
              Geezer, it is not jumping from one frame
              to another; just viewing a system from
              alternative frames.
           
       
     
     I have no idea what that means. To view a system at all one
     chooses a frame of reference.
     
With physics one has to be very precise about what one means and how that is expressed in writing. Geezer's response was probably very clear to Geezer but it might be read different ways by different people, hence my confusion.
[/quote]

Quote
modnote - Peter, a fair percentage of your posts are critiquing other members language and comprehensibility (in my opinion unfairly) - could you tone it down a bit please.  Many thanks.
You're welcome. You're missing what I was attempting to do. I was trying to be as polite as possible given no idea what the other person's knoweledge base is.

that . The problem is that I have no idea what the other person's
on a substantive note - whilst one chooses a frame of reference and works within it, there is no reason not to then choose to model the exact same physical process from an alternative reference frame.  the ball can be described as rising vertically from the hand and falling back into it from the perspective of a fellow train passenger, or describing a (near) parabola travelling 20 metres horizontally according to the man watching the train rush past.  mixing measurements from the two frames is dangerous, but making sure that the results from the two frames are consistent is crucial.
[/quote]
Quote from: imatfaal
modnote - Peter, a fair percentage of your posts are critiquing other members language and comprehensibility (in my opinion unfairly) - could you tone it down a bit please.  Many thanks.
I never meant my posts to seem irritating to people. Since you read things in a way that they aren't meant then it seems that I'd be better of leaving this forum. Good luck in seeking to understand physics! BEst wishes to you all.
If you wish to state some valuable thing then you should not worry about irritation of people.

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Offline Geezer

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Graham,
 
If you are in a rotating thingy and you know it is rotating, you are in a rotating thingy, not in a rotating frame, so it should not be a big surprise that you are aware of a reaction to centripetal force.
 
If you are in a place where objects tend to move mysteriously in a radial fashion from some point, you either think there is some unknown force acting on them, or you infer you are in a thing that is rotating about that point, in which case, when an object hits the wall, it is obviously constrained by a centripetal force and there will be an equal reaction to that force. You certainly don't think "Wow, I'm in a rotating frame! I can finally use the elusive centrifugal force."
 
You can call the reaction to a centripetal force "centrifugal force" if you want, as long as you understand that it will cease as soon as the centripetal force is removed. Children on roundabouts are very aware of this. They know that as soon as they let go, they will have to start running very quickly along a tangential, not radial, path.
 
Perpetuating the "centrifugal force" myth to lay persons is not, IMHO, a good idea. It's making science more complicated than it needs to be by creating the illusion of some "force" that really does not exist.
 
When my grandchildren ask me why they seem to be forced against the side of the car when it goes round a corner, my explanation is that it's because they would prefer to go straight on, but the car has another plan. I'm not about to start telling them it's because of a force that's not really a force unless, of course, they happen to be in a rotating frame.
There ain'ta no sanity clause, and there ain'ta no centrifugal force ęther.

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Offline graham.d

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Then try to explain how a centrifuge separates materials of differing density by saying that some have a greater urge to go in a straight line than others :-) I am not trying to restrict how things are explained but, on the contrary, trying to prevent unnecessarily complicated explanations that result from a refusal to apply a name to an inertial force. Even if I were to wholly accept that kids have trouble understanding the physics, I certainly do not see that the word "centrifugal" should not be used by those who do understand it.

I actually think we have done this to death now as it is a matter of opinion and not a dispute about the facts.

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Offline Ęthelwulf

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Here's a new one. If the centrifugal force is a myth, then why have scientists taken it seriously for so long? And, let me demonstrate a specific case.

Penrose and Hawking specifically worked out in their singularity theorems whether the centrifugal force partly counteracts gravity and keeps a singularity from forming. They figured it could not happen, but not by the reasoning that the centrigufal was a real effect (even though technically, it is a psuedoforce.)