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Author Topic: Are frames of reference even more misunderstood than centripetal force?  (Read 30552 times)

Offline yor_on

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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.

I liked that one :)
Elegant.

 

Offline yor_on

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But reading Graham, Yeah, your point is valid too :)
'Forces' that act on you will be perceived as 'forces' locally. although having been outside your closed black centrifuge you might define it as a centripetal force, but if not knowing of any 'outside'? But then again, to get this effect, doesn't it presume another 'frame of reference' from where it can exist?

 

Offline Geezer

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Actually, my question was less about the name given to centripetal reaction and more about frames of reference.
 
I was under the impression that, when a frame is selected, all phenomena within that frame (not just within a subset of space) have to be described relative to that frame, even it that means you have to invent new math. You cannot explain something by selecting data from two different frames simultaneously.
 
This may seem like a major inconvenience, but science is under no obligation to be convenient.
 

Offline graham.d

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Actually, my question was less about the name given to centripetal reaction and more about frames of reference.
 
I was under the impression that, when a frame is selected, all phenomena within that frame (not just within a subset of space) have to be described relative to that frame, even it that means you have to invent new math. You cannot explain something by selecting data from two different frames simultaneously.
 
This may seem like a major inconvenience, but science is under no obligation to be convenient.

I don't think anyone would disagree with that, Geezer. Your "motto" of ... "there ain'ta no centrifugal force either" implies more than that though, hence the lengthy discussion.
 

Offline yor_on

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A 'frame of reference' is a tricky subject to me. But yeah, choose one positionally in time and space, and feel free to define it as your 'center', then define everything else from it. But it is also so as with Mach's Principle  How do you define a 'frame of reference' when not having another to prove it against?
 

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.) 

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.


Sorry for dragging this up now.  I was busy this weekend.

Anyway, I disagree with you on this point.  The only forces which will be indistinguishable from inertial forces are those which are proportional to mass.  If I was in a spaceship with no windows, I wouldn't be able to tell if it was accelerating or sitting still in some external gravitational field because in both cases, the objects would experience motion in my reference frame that was proportional to mass.

If I was in a spaceship under an external electromagnetic field, I would be able to tell if I was accelerating or experiencing a force because acceleration would effect all objects proportionally to their mass, while the electromagnetic field would effect objects proportional to their charge.  There are relatively simple experiments I could conduct in my closed spaceship to determine this.
 

Offline Geezer

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If I was in a spaceship with no windows, I wouldn't be able to tell if it was accelerating or sitting still in some external gravitational field because in both cases, the objects would experience motion in my reference frame that was proportional to mass.
 

Ahem! I beg to differ.
 
The motion of objects in a gravitational field would be independent of their mass. Didn't some Italian geezer establish that a wee while ago?
 

Offline JP

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D'oh.  You're absolutely right.  I should say apparent force is proportional to mass. 

(I'm going to blame the benadryl I took for my cold this morning for that one! :p  )

My point still stands, though.  You can only generally eliminate forces that are proportional to mass by choosing an appropriately accelerating reference frame.  For others, you can devise experiments within your closed spaceship that will detect the difference from reference frame effects.
 

Offline Geezer

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I'm going to blame the benadryl I took for my cold this morning for that one
 

But it specifically says on the bottle,
 
"Do not operate heavy machinery or alter your frame of reference after taking."
 

Offline graham.d

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JP, I was loose with language in saying "any other force". I should have said gravitational force. Of course one can distinguish EM, Strong, Weak and gravitational forces. I rather took that for granted.
 

Offline Geezer

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Graham,
I'm sorry if you find my signature disturbing. I suppose I better keep schtum on the question of the tooth fairy.
 

Offline yor_on

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Yeah, that was how I read you Graham, discussing 'gravity'. And gravity is in my eyes a geometry. But 'frames of reference' is tricky. I still don't know how to define a frame without having another to define it from?

a little like that Chinese thing

Jim and Jam?
 

Offline JP

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JP, I was loose with language in saying "any other force". I should have said gravitational force. Of course one can distinguish EM, Strong, Weak and gravitational forces. I rather took that for granted.

So I guess you agree with my point, then?  There is a distinction between inertial forces (which includes gravity if you bring in general relativity) and "real" (insert a better term there) forces. 
 

Offline graham.d

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If you class all gravitational forces as inertial forces and all other forces as "real" forces then, by definition, you have made a distinction between these forces. I still don't see a reason not to give some of the inertial forces names though - e.g. Centrifugal, Coriolis - as it is useful to do so.  I guess the point you are making is that you can't have an inertial force without the action of a "real" force. But, as I think you said, "real" is not a good name as the inertial forces are also real (in the more normal use of the word) to anyone experiencing them.
 

Offline Airthumbs

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Hi all, I would just like to jump in here at this point..... I have read all the above posts and I really am finding this difficult to grasp so I have a few questions that I would really appreciate some answers to.

When you talk about a frame of reference are you talking about a fixed moment in time? 

Ok then, what is a frame of reference?

Is this just a question of English or is the actual definition of centrifugal force incorrect?

If centrifugal force describes the forces exerted on an object in a spin then why is it incorrect. 

How does a centrifuge work, or is this improperly named?

It was Geezer who introduced me to the concept of centripetal force as opposed to centrifugal and I can categorically state that in my case it is certain that frames of reference are more misunderstood then the other.

I liked the analogy of a roundabout as I can relate to this but it would be very useful if someone could add in a frame of reference to this analogy and explain it a little further if possible.

Thanks all.
 

Offline graham.d

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A frame of reference is position from which you observe and calculate the motion of any objects. As long as you are totally consistant in doing this, the outcome of your calculation will agree to that of any other observer.

So, in the case of a roundabout or a centrifuge, an external observer sees the mechanical structure of the rotating body and observes that it is rotating. He sees the "centrepetal" force created by the rigidity of the rotating body as a measurable tension in a radial arm and he sees that a person being rotated is being constrained by this force so as to prevent him continuing on in a straight line. This is the simplest way to observe this particular motion and is sometimes called the "rest" frame. From the perspective of the man being swung around, he feels a force pushing him outwards. This is an inertial force and is called a centrifugal force. If he moves in a radial direction he will also experience a side force, which is another inertial force called the Coriolis force. It is possible for him to calculate all these forces and motions from his (accelerated) frame though this is mathematically complex. Such concepts are not encouraged (as JP says) in teaching Newton's laws of motion as these are taught before the student has the capability to work through the maths in a rigorous way. There can be confusion because, in my opinion, children aer aware of "centrifugal" force and usually know the name, before they understand the physics; they then get confused between centripetal and centrifugal force. I understand this issue but feel it could be handled better rather than trying to say the centrifugal force does not exist. Explaining inertial forces is not so difficult, I believe, and, as I think JP agrees, using the word "real" (and implying that inertial forces are not real) is not ideal.

A simpler inertial force is that which you would experience in the proverbial constant acceleration rocket ship. You would not be able to distinguish such a force from gravity (provided you could not look out of the window). In General Relativity it is a tenet that the two are indistinguishable.
 

Offline imatfaal

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wish we had a "like" button or rep system for that post Graham - nicely done.
 

Offline Geezer

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Yes, but centrifugal force is still not a "real" force. It is the real reaction to the real centripetal force.
 
The reason it's not "a real force" (call it what you will) is because there is no physical phenomenon to explain it, whereas there is a very simple explanation for centripetal force.
 

Offline Geezer

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If we use the argument that children should be taught that centrifugal force is a real force because they can feel it, should we also teach them that the Earth is at the center of the Universe because they can see everything rotating around it?
 
Wouldn't it be much better to take advantage of the situation to teach them that forces have reactions, and what they are feeling is simply the reaction to a force that results from one of the most fundamental scientific principles?
 

Offline graham.d

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You have to then say gravity is not a "real" force too but a consequence of the geometry of space-time. It is all well to define things in these ways but sometimes it is better to use words in the way people already understand them.
 

Offline Geezer

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You have to then say gravity is not a "real" force too but a consequence of the geometry of space-time.

I have no objection to that at all, but it doesn't have much bearing on the question I posed:
 
If centrifugal force is "a real force", what physical phenomenon produces it?
 
(BTW - I don't have any objections to "the centrifugal effect" or something similar.)
 

Offline graham.d

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People doing Newtonian mechanics are told gravity is a force. They are not told it is an effect and a consequence of spacetime geometry. There is not any distinction between gravity and an inertial force except for one's frame of reference; they are regarded as equivalent in their nature and behaviour. Centrifugal force is an inertial force, as is the Coriolis force. Note the use of the word "force" here!

I am happy to say how this force comes about as a result of being in an accelerating frame. I am just saying there is nothing wrong with using a common name for it. If you are in a racing car or a centrifuge you would not say I am experiencing a tendency to carry on in a straight line in reaction to being constrained by a centripetal force. You say I am experiencing so much g-force and in a centrifuge it would not be surprising to call that force a centrifugal force.

I feel that this has become an argument over semantics. I think most people in this discussion are not confused by the actual causes and effects so I am happy to bow out and leave the discussion to others. I've had my say.
 

Offline Geezer

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Graham,
 
You may think it's about semantics, but I think it's an important issue about how best to teach basic physics.
 
A student is likely to ask the question that I asked, but there is no satisfactory way to answer it without introducing abstract concepts that are only likely to confuse the student (and possibly the teacher) even more.
 
 

Offline Ćthelwulf

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Yes, but centrifugal force is still not a "real" force. It is the real reaction to the real centripetal force.
 
The reason it's not "a real force" (call it what you will) is because there is no physical phenomenon to explain it, whereas there is a very simple explanation for centripetal force.

Just because it is a psuedoforce should not mean to take it as not having a consequence in the mathematics. As I told you before, the black hole and singularity theorems takes the centrifugal force very seriously.

In fact, I am sure I have mentioned this before, but gravity is a psuedoforce of types.
 

Offline Pmb

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Quote from: Ćthelwulf link
"...gravity is a psuedoforce of types..."
Why is it so important to keep refering to gravity as a"psuedoforce of types"?
 

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