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Author Topic: Are frames of reference even more misunderstood than centripetal force?  (Read 30593 times)

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.
 

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.
 

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.
 

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.
 

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.
 

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.
 

Offline imatfaal

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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. 
 

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.
 

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.
 

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.
 

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.
 

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.)
 
 

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.
 

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".
 

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?
 

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.
 

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.
 

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.
 

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 :-)
 

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.
 

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.
 

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.
 

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.
 

Offline Æthelwulf

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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.)
 

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