Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Geezer on 15/04/2012 06:54:36
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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?
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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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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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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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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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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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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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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.
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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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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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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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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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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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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.
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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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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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Strangely however, I'm looking back at my statement and I think I have qouted the wrong person.
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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 (http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation), which we are travelling through at about 370 km/s (http://en.wikipedia.org/wiki/Sun), slower than the Milky Way at 552 km/s (http://en.wikipedia.org/wiki/Milky_Way), 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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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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If space is some kind of absolute reference frame, then we may perhaps call it a type of quantum aether.
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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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... 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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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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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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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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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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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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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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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.
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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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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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(https://www.thenakedscientists.com/forum/proxy.php?request=http%3A%2F%2Fimgs.xkcd.com%2Fcomics%2Fcentrifugal_force.png&hash=ca180f3d668dd86803d26f0a2736f21c)
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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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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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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.
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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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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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@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.)
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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.
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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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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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...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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...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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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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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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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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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]
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]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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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.
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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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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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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.
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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?
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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.
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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.
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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 (http://www.bun.kyoto-u.ac.jp/~suchii/mach.pr.html) How do you define a 'frame of reference' when not having another to prove it against?
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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.
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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?
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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.
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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."
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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.
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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.
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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?
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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.
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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.
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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.
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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.
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wish we had a "like" button or rep system for that post Graham - nicely done.
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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.
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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?
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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.
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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.)
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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.
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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.
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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.
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"...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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"...gravity is a psuedoforce of types..."
Why is it so important to keep refering to gravity as a"psuedoforce of types"?
Because if you study the quality of Geezers arguement, you essentially keep finding the same thing. He keeps making a mention that the centrifugal force is a psuedoforce, that it is not ''real''.
Well, neither is gravity then, but we still take it as being within the framework of field theories. See, originally, Geezer said, ''there is no such force'' --- to ''there is only the centripetal force.'' Now, gravity is well-understood of as the curvature of space, but no one hangs about to say, ''well gravity isn't real because it's a psuedoforce.''
That kind of thinking is never heard of... only from cranks who can't accept that gravity is something we all come to experience in.
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Not to put words in Geezer's mouth, but I don't think he's saying that a pseudoforce can't have real effects in some reference frame. I think he's saying (quite reasonably) that you shouldn't group "centrifugal force" together with non-inertial forces by labeling it a force.
It's certainly quite easy to look back at the idea of pseudoforces once you've had advanced courses in mechanics and say that it's obvious where pseudoforces come from and what they physically mean. It's incredibly confusing to have your instructor in physics 101 tell you that there are reference frame dependent forces which vanish depending on the reference frame you choose: it's hard enough learning to apply Newton's laws in inertial reference frames. It makes a lot of pedagogical sense to break "force" into two categories: one which has the properties of "real" forces and one which has the properties of pseudoforce--simply because they behave differently in physics, and the latter category requires a lot more mathematical sophistication to tackle properly.
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"...gravity is a psuedoforce of types..."
Why is it so important to keep refering to gravity as a"psuedoforce of types"?
If we take a case that we are probably all familiar with - the case where we are spinning around on a roundabout, or a carousel, our senses tell us that a force is acting on us in an outward direction radial to the rotation of the device. This is commonly referred to as "centrifugal force".
However, if you accept that Newton's laws of motion have not yet been repealed, there is no actual force acting in that direction. There is a force acting in the opposite direction - the centripetal force, and without that force, we would not be rotating at all. The apparent "centrifugal force" is only a reaction to the real centripetal force. That's why it is referred to as a pseudoforce, a fictional force, etc., etc.
It's actually impossible to experience any "centrifugal force" in the absence of centripetal force, because, without a centripetal force, there would be no rotation at all.
I do know there are other methods, like Hamiltonian Mechanics, that can be used to describe these phenomena, but they go to great lengths to avoid the discussion of any forces, so it's not very likely they will help to resolve the issue.
The so-called "centrifugal force" is a consequence of Newton's Laws of Motion. I really don't have any objection to alternative scientific interpretations, but we may not use an aberrant interpretation of Newton's Laws of Motion to disprove Newton's Laws of Motion.
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Not to put words in Geezer's mouth, but I don't think he's saying that a pseudoforce can't have real effects in some reference frame. I think he's saying (quite reasonably) that you shouldn't group "centrifugal force" together with non-inertial forces by labeling it a force.
The discussion has became so semantic and technical, I've almost lost track of what people are really intending in their discussions.
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The discussion has became so semantic and technical, I've almost lost track of what people are really intending in their discussions.
It's very simple.
If you can't explain the physical phenomenon that produces a force, it's not a real force. It's an artifact.
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If someone asked me, ''is the centrifugal force real?''
I'd reply with the question ''is the force of gravity just as real as the centrifugal force?''
If the answer is yes, then that is all I need.
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Let me remind you why you created this thread. It was to denounce what I asked... if an elevator was going straight to the heavens and into space, what would stop it from being ripped apart by the centrifugal forces?
You said, ''that's easy. It won't because the centifugal force does not exist.''
Since that remark, we have made some headway. I have shown that black holes are pivotal to understanding centrifugal forces (taken seriously by physicists) whether it is or not a real force created by physical mediators. We've also established time and time again, that gravity is a psuedoforce, but in light of this we do not go around saying it does not exist.
And what are we stuck on here? Semantics.
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The interesting thing to me is that we presume two frames of reference to proof a force :)
And it doesn't really seem to ah, matter what I call it, or deem it to be.
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I think we all agree on the physics, but we disagree on what terms should be used to describe different parts of the physics. The problem with writing this agreement off as semantics is that you're indicating that semantics isn't important in physics. But the precision of the definitions we give to terms is absolutely critical to science!
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I think we all agree on the physics, but we disagree on what terms should be used to describe different parts of the physics. The problem with writing this agreement off as semantics is that you're indicating that semantics isn't important in physics. But the precision of the definitions we give to terms is absolutely critical to science!
I'm not saying semantics is really not that important, only that it has muddled this conversation to tenebrous levels :P
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Let me try to dispel the muck with a simple experiment:
We have a rotatable circular platform, say, 4 meters in diameter. We step onto the platform, and the platform starts to slowly rotate. We continue standing on the platform as it slowly rotates.
Question: Why do we rotate with the platform?
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Let me try to dispel the muck with a simple experiment:
We have a rotatable circular platform, say, 4 meters in diameter. We step onto the platform, and the platform starts to slowly rotate. We continue standing on the platform as it slowly rotates.
Question: Why do we rotate with the platform?
To be extremely general, because as the platform starts to rotate it forces the muck to exert a force between the platform and us. This causes a force on us which causes us to rotate.
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Let me try to dispel the muck with a simple experiment:
We have a rotatable circular platform, say, 4 meters in diameter. We step onto the platform, and the platform starts to slowly rotate. We continue standing on the platform as it slowly rotates.
Question: Why do we rotate with the platform?
To be extremely general, because as the platform starts to rotate it forces the muck to exert a force between the platform and us. This causes a force on us which causes us to rotate.
Is the force is conveyed by friction between our shoes and the platform?
EDIT: Wait a minute. I thought when you apply a force to something it is supposed to go in a straight line. Weren't we just taught that?
Why doesn't the force make us go in a straight line until we fall off the platform?
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I find this thread to be not very good at all.
Centrifugal force (and coriolis force) are pseudo forces due to a combination of rotation of the reference frame and momentum; and they are needed when you are analysing a situation in a rotating reference frame to get the correct movements; together they explain why the stars spin around the Earth (for example).
You use rotating reference frames when it's more convenient to use them; for example a space elevator would rotate with the Earth, and are thus are particularly easy to analyse in the rotating reference frame.
Another example is the Lagrange points; these are stationary, but only in a rotating reference frame, they are balanced between gravity and the centrifugal (pseudo)force.
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Why doesn't the force make us go in a straight line until we fall off the platform?
Because you're not moving at all in the rotating reference frame that rotates with the platform, at least until the centrifugal force exceeds the friction of your shoes.
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I find this thread to be not very good at all.
The quality of a thread is determined by it's usefulnes to address/answer a person's query/questions. If Geezer gets his question answered to his satisfaction then the thead was a very good thread.
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Is the force is conveyed by friction between our shoes and the platform?
Yes. So long as you have a good understanding of kinetic coefficient of friction, static coefficient of friction and how to apply them to a rotating platform, shoes and muck.
EDIT: Wait a minute. I thought when you apply a force to something it is supposed to go in a straight line. Weren't we just taught that?
It’s a bit more complicated than that. When an object is sitting on a rotating platform there is a force of friction, which is perpendicular to the direction of motion, which then makes the object move in a circular motion. Eventually, as you increase the rotational motion of the platform the kinetic coefficient of friction increases so as to make the kinetic coefficient of friction becomes greater than the static coefficient of friction and the object then moves in a straight line.
The dynamics are similar to that of a charged particle moving in a magnetic field. See http://home.comcast.net/~peter.m.brown/sr/cyclotron.htm
This is a tricky thing to describe so you may want to rethink it until you get it right since I probably got something wrong in the explanation. :-'(
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It’s a bit more complicated than that. When an object is sitting on a rotating platform there is a force of friction, which is perpendicular to the direction of motion, which then makes the object move in a circular motion. Eventually, as you increase the rotational motion of the platform the kinetic coefficient of friction increases so as to make the kinetic coefficient of friction becomes greater than the static coefficient of friction and the object then moves in a straight line.
Ah right, but there can only really be one force acting on our hypothetical person via their shoes. Presumably it must act in a direction that continuously changes as the platform rotates?
(It seems Wolfkeeper is unaware of Newton's First Law. The so called "rotating reference frame"* could not exist in the absence of centripetal force.)
*What?! Reference frames do not rotate. Everything else around them might, but they don't. If they did, they would hardly be a reference.
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Nope, no centripetal force is necessarily involved.
The whole point is to give the same results as a non rotating reference frame. As in, you can take any situation in an inertial reference frame and model the same situation in any rotating reference frame, and the same thing must happen, otherwise you're doing it wrong.
The motion won't usually look the same, but if you do the rotation mapping back to the inertial frame, everything must be the same as if you did it all in the inertial frame.
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Nope, no centripetal force is necessarily involved.
You're wrong on that point.
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Ah right, but there can only really be one force acting on our hypothetical person via their shoes.
That is not quite true. There is the force due to the centripital force which is due to choice in frame of reference, there is the force of friction and there is the force of friction. The total force is the sum of all the forces acting on the shoes and it is that total force which is responsiple for explanation of the motion of the shoes.
Presumably it must act in a direction that continuously changes as the platform rotates?
I agree with that.
*What?! Reference frames do not rotate. Everything else around them might, but they don't. If they did, they would hardly be a reference.
In Newtonian Dynamics that would be incorrect. In General Relativity that is incorrect.
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Nope, no centripetal force is necessarily involved.
You're wrong on that point.
Nope.
What you do to derive a rotating reference frame is take the normal inertial frame of reference and calculate a coordinate transformation to transfer everything, all the Newtonian mechanics, to that rotating frame.
When you do that, all the normal physics still works, but two pseudo accelerations/pseudo forces appear, the coriolis and centrifugal, if you apply them both, then (somewhat counterintuitively) Newton's laws still work perfectly, they sort of cancel out the effects of the rotating reference frame.
There is no centripetal that appears at all, although if there was one in the original situation in the inertial frame of reference, then there will be one afterwards, but if there wasn't, then there won't be one afterwards.
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Nope, no centripetal force is necessarily involved.
You're wrong on that point.
Nope.
What you do to derive a rotating reference frame is take the normal inertial frame of reference and calculate a coordinate transformation to transfer everything, all the Newtonian mechanics, to that rotating frame.
When you do that, all the normal physics still works, but two pseudo accelerations/pseudo forces appear, the coriolis and centrifugal, if you apply them both, then (somewhat counterintuitively) Newton's laws still work perfectly, they sort of cancel out the effects of the rotating reference frame.
There is no centripetal that appears at all, although if there was one in the original situation in the inertial frame of reference, then there will be one afterwards, but if there wasn't, then there won't be one afterwards.
But that's my point.
The above is all very well, and mathematically correct, but it only obfuscates this particular problem.
Unless you instantaneously jump into a new frame, there must be a centripetal force, and why would you feel the need to jump into a different frame half-way through a problem?
If we have to instantaneously "change our frame of reference" when we stand on a rotating platform, do we also have to "change our frame of reference" when we step onto a linear moving platform like an escalator, or moving sidewalk, and then describe all the dynamics in terms of pseudo forces? Likewise when we are walking, or rollerblading down the street?
I suppose we could do that, but it seems like an academic exercise.
Of course there are complex situations where these treatments are useful, but from a teaching perspective, if you introduce them too early, you're only going to confuse a lot of students, or other TNS members.
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What you do to derive a rotating reference frame is take the normal inertial frame of reference and calculate a coordinate transformation to transfer everything, all the Newtonian mechanics, to that rotating frame.
When you do that, all the normal physics still works, but two pseudo accelerations/pseudo forces appear, the coriolis and centrifugal, if you apply them both, then (somewhat counterintuitively) Newton's laws still work perfectly, they sort of cancel out the effects of the rotating reference frame.
There is no centripetal that appears at all, although if there was one in the original situation in the inertial frame of reference, then there will be one afterwards, but if there wasn't, then there won't be one afterwards.
But that's my point.
The above is all very well, and mathematically correct, but it only obfuscates this particular problem.
Unless you instantaneously jump into a new frame, there must be a centripetal force,
Nope. It turns out when you do the maths there's a steady 'force' that is proportional to the distance from the axis that points away from the axis, that is proportional to the distance (the centrifugal force), and there's a coriolis force that acts when something moves* (where "moves" means relative to the rotating reference frame), which acts at 90 degrees to the movement and the axis.
and why would you feel the need to jump into a different frame half-way through a problem?
You don't "jump" in, you just do it all as if it wasn't rotating, and then add on those two fixup force to deal with the fact that you're working in a rotating frame. In many cases either the centrifugal or coriolis cancel out anyway. On the surface of the Earth, the centrifugal mostly cancels for example.
If we have to instantaneously "change our frame of reference" when we stand on a rotating platform, do we also have to "change our frame of reference" when we step onto a linear moving platform like an escalator, or moving sidewalk, and then describe all the dynamics in terms of pseudo forces? Likewise when we are walking, or rollerblading down the street?
You can, but you wouldn't normally. You only use accelerated reference frames when it's convenient.
I suppose we could do that, but it seems like an academic exercise.
Using rotating reference frames is what the weather forecast people do. If they didn't the maths gets even more horrible. They pretty much have to use rotating reference frames.
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Nope. It turns out when you do the maths there's a steady 'force' that is proportional to the distance from the axis that points away from the axis, that is proportional to the distance (the centrifugal force), and there's a coriolis force that acts when something moves* (where "moves" means relative to the rotating reference frame), which acts at 90 degrees to the movement and the axis.
You can do the math all you like (and I agree with your math) but it does not serve to provide a good explanation for what actually happened. It's only complicating things unnecessarily.
In this example, our subject experienced a force that continuously changed direction, otherwise he would have traveled in a straight line. "Centrifugal force" does not help to explain the phenomenon. In fact, it's downright misleading.
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Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.
But I'd also like to point out that you can't directly feel gravity either; if you fall off a roof, you can not feel anything, yo cannot feel gravity, but the deceleration hurts a lot at the bottom. So you indirectly feel gravity that way. Likewise you can feel centrifugal or coriolis force when something prevents it from acting.
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Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.
But I'd also like to point out that you can't directly feel gravity either; if you fall off a roof, you can not feel anything, yo cannot feel gravity, but the deceleration hurts a lot at the bottom. So you indirectly feel gravity that way. Likewise you can feel centrifugal or coriolis force when something prevents it from acting.
Agreed, but gravity has little direct bearing on the particular example.
My only purpose in making such a big fuss about this is, where possible, to encourage the use of the most straightforward explanations of physical phenomena. Sometimes things do get very complicated and it's necessary to introduce things like "coriolis force" because they can greatly simplify the problem at hand. That's fine. All I'm trying to do is make sure we don't forget that the apparent coriolis force is really just a consequence of Newton's first law.
More importantly, I don't want a lot of casual scientists who visit TNS to get the idea that there are any more mysterious forces known to science than the existing set of mysterious forces.
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Well, in that case, magnetism doesn't really exist either; it's just moving charged particles and relativity.
(At least it is unless or until somebody finds a magnetic monopole, if that ever actually happens, in the meantime all known magnetic fields are just electric fields.)
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Well, in that case, magnetism doesn't really exist either; it's just moving charged particles and relativity.
(At least it is unless or until somebody finds a magnetic monopole, if that ever actually happens, in the meantime all known magnetic fields are just electric fields.)
I'm sorry. Why does this help to explain why our subject rotates? Are you saying he is a particle with no mass, or are you just being obtuse?
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It's often convenient to choose a rotating reference frame in which the subject doesn't rotate; that's the main point.
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Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.
The coriolis field cannot be felt. Regarding the gravitation field, that's only true under special cases, i.e. when the gravitation tidal forces are too week to feel.
As I recall it's the gravitational tidal force which is responsible for reorienting artificial satelites.
This is also wrong.
Likewise you can feel centrifugal or coriolis force when something prevents it from acting.
In those cases its the electric force between you and the rotating is what you feel.
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It's often convenient to choose a rotating reference frame in which the subject doesn't rotate; that's the main point.
Yes, it can be convenient, but we should not forget that it does not overturn the fundamental physics. I'm pretty sure a significant number of TNS subscribers are now convinced that, because they can "feel" a centrifugal force, it's a real force. I think that's unfortunate.
Essentially, Newton said that, in the absence of a force, things stay put, or travel in straight lines. This is just as valid today as it was when Newton figured it out. The only refinement in the meantime is that, in certain cases, "straight lines" are not quite what he might have thought they were.
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So... we all know forces (except gravity) are mediated by fields which are made up of virtual particles. We have photons for electromagnetism, gluons for the strong force, W and Z bosons for the weak force...
So are there any centrifugons?
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I'm pretty sure a significant number of TNS subscribers are now convinced that, because they can "feel" a centrifugal force, it's a real force. I think that's unfortunate.
What I think is unfortunate is that people think that inertial forces are not real forces and that real forces are different from non-inertial force in some significant way.
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Your page proves exactly why...
To assert that my page proves something cannot be determined unless s msjority of readers step up and vote on that point, and that never happens, especially since there
is no way to determine who is reading it but not posting in it.
you can't group inertial forces together with "real" forces (or whatever other term you want to use for them.)
I disagree. Why do you belive that they can't be so grouped?
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.
That is not true. One can't say "This kind of force transforms in this way while this other force transforms in this other way." How something transforms is determined only by the theory being used.
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Centrifugal force (and coriolis force) are pseudo forces due to a combination of rotation of the reference frame and momentum; and they are needed when you are analysing a situation in a rotating reference frame to get the correct movements; together they explain why the stars spin around the Earth (for example).
The Centrifugal force and Coriolis force are found in the same frame that you re sitting in right at this minute. During WWII the deck guns used to fire shots over 20 miles away to plument a beach has to be calibrated to take into account the Coriolis force, otherwise the shells won't hit their target.
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Wellll.... as a matter of fact, you can't directly feel coriolis or centrifugal force.
But I'd also like to point out that you can't directly feel gravity either;
coriolis forces and centrifugal forces are considered gravitational forces in general relativty.
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That is not true. One can't say "This kind of force transforms in this way while this other force transforms in this other way." How something transforms is determined only by the theory being used.
To be clearer, inertial forces are proportional to mass, and can be transformed away by an appropriate reference frame choice, since changes in reference frames introduce terms proportional to mass. Non-inertial forces need not be proportional to mass (electromagnetism, for example) and cannot be transformed away in general, since they appear in Newton's equations as proportional to charge, so subtracting a term that depends on mass will not, in general, cancel them out.
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By the way, what model are we assuming for this debate? The mantra centrifugal force is not a force is usually used when teaching Newtonian mechanics in inertial reference frames. In that case, it's absolutely correct. Not only is centrifugal force not a force, it doesn't even enter the equations!
It only becomes an issue when you move to non-inertial reference frames, in which case a proper frame-independent treatment of Newton's equations shows what I said above: some terms appear proportional to mass and others not, so there's still a distinction. You could also make a distinction based on which forces are still there in an inertial reference frame, since this makes it clear how this model ties into the previously learned inertial-frame models.
My position is that we should teach Newton's laws in inertial reference frames first, since they're hard enough to understand for beginning students. Since these students have heard the term "centrifugal force" before and it's something that intuitively makes sense, its important to stress that it is not a force in this model. At this point, they know F=ma and they're ready to change reference frames (when they have some fancy calculus to back it up.) Then it's still important to start from what they know, F=ma and show how changing reference frames introduces extra terms to that equation. Since you're building on F=ma, it makes sense to still treat the F's in inertial reference frames as one type of thing, and those terms which appear due to a non-inertial reference frame as another type of thing. It also makes sense to draw attention to the fact that transforming away from an inertial reference frame gets you new terms that are proportional to mass (because the "a" spits out extra terms, which are multiplied by m). This has the extra benefit of getting them ready to notice that gravity is also proportional to mass, which leads to general relativity...
You can, of course, argue that that's not an effective way to teach the material, but its widely used these days and having taken and taught it, it seems effective.
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To be clearer, inertial forces are proportional to mass, ...
I thought I had stated that already. Sorry if I missed that. :(
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To be clearer, inertial forces are proportional to mass, ...
I thought I had stated that already. Sorry if I missed that. :(
You probably did. :) I don't think my statements are all 100% clear, so I wanted to reiterate that I'm talking about mass-dependent terms as opposed to forces which aren't proportional to mass.
I assume we all agree on the physics and the math involved, so the whole argument here is about coming up with physical/mathematical reasons for grouping terms in the equations under different labels (or not).
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By the way, what model are we assuming for this debate? The mantra centrifugal force is not a force is usually used when teaching Newtonian mechanics in inertial reference frames.
I don't know where you got that idea. Typically it's undergrad courses which some teach that, i.e. Physics I, II, III. Not in more advanced courses. See
More modern teachings give the following
http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm
Most notably,
From Gravitation, by Misner, Thorne and Wheeler, Box 6.1, page 164
A tourist in a powered interplanetary rocket feels "gravity." Can a physicist by local effects convince him that this "gravity" is bogus? Never, says Einstein's principle of the local equivalence of gravity and accelerations. But then the physicist will make no errors if he deludes himself treating true gravity as a local illusion caused by acceleration. Under this delusion, he barges ahead and solves gravitational problems by using special relativity: if he is clever enough to divide every problem into a network of local questions, each solvable under such a delusion, then he can work out all influences of any gravitational field. Only three basic principles are invoked: special-relativity physics, the equivalence principle, and the local nature of physics. They are simple and clear. To apply them, however, imposes a double task: (1) take spacetime apart into locally flat pieces (where the principles are valid), and (2) put the pieces together into a comprehensible picture. To undertake this dissection and reconstruction, to see curved dynamic spacetime inescapably take form, and to see the consequences for physics: that is general relativity.
From Introducing Einstein's Relativity, by Ray D'Inverno, Oxord/Clarendon Press, (1992) page 122
Notice that all inertial forces have the mass as a constant of proportionality in them. The status of inertial forces is again a controversial one. One school of thought describes them as apparent or fictitious which arise in non-inertial frames of reference (and which can be eliminated mathematically by putting the terms back on the right hand side). We shall adopt the attitude that if you judge them by their effects then they are very real forces.
From Nature, Albert Einstein, February 17, 1921 issue
Can gravitation and inertia be identical? This question leads directly to the General Theory of Relativity. Is it not possible for me to regard the earth as free from rotation, if I conceive of the centrifugal force, which acts on all bodies at rest relatively to the earth, as being a "real" gravitational field of gravitation, or part of such a field? If this idea can be carried out, then we shall have proved in very truth the identity of gravitation and inertia. For the same property which is regarded as inertia from the point of view of a system not taking part of the rotation can be interpreted as gravitation when considered with respect to a system that shares this rotation. According to Newton, this interpretation is impossible, because in Newton's theory there is no "real" field of the "Coriolis-field" type. But perhaps Newton's law of field could be replaced by another that fits in with the field which holds with respect to a "rotating" system of co-ordinates? My conviction of the identity of inertial and gravitational mass aroused within me the feeling of absolute confidence in the correctness of this interpretation.
A.P. French - Inertial force is defined as the force on a body that results solely from observing the motion of the body from a non-inertial frame of reference. This in addressed in Newtonian Mechanics, A.P. French, The M.I.T. Introductory Physics Series, W.W. Norton Pub. , (1971) , page 499. After describing the inertial force as seen from an accelerating frame of reference French writes
From the standpoint of an observer in the accelerating frame, the inertial force is actually present. If one took steps to keep an object "at rest" in S', by tying it down with springs, these springs would be observed to elongate or contract in such a way as to provide a counteracting force to balance the inertial force. To describe such force as "fictitious" is therefore somewhat misleading. One would like to have some convenient label that distinguishes inertial forces from forces that arise from true physical interactions, and the term "psuedo-force" is often used. Even this, however, does not do justice to such forces experienced by someone who is actually in the accelerating frame of reference. Probably the original, strictly technical name, "inertial force," which is free of any questionable overtones, remains the best description.
Cornelius Lanczos - The subject of inertial force is also addressed in The Variational Principles of Mechanics - 4th Ed., Cornelius Lanczos, Dover Pub., page 98.
Whenever the motion of the reference system generates a force which has to be added to the relative force of inertia I’, measured in that system, we call that force an “apparent force.” The name is well chosen, inasmuch as that force does not exist in the absolute system. The name is misleading, however, if it is interpreted as a force which is not as “real” as any given physical force. In the moving reference system the apparent force is a perfectly real force, which is not distinguishable in its nature from any other impressed force. Let us suppose that the observer is not aware of the fact that his reference system is in accelerated motion. Then purely mechanical observations cannot reveal to him that fact.
From Cosmological Physics, John A. Peacock, Cambridge University Press, (1999), page 6-7 (See URL, last quote at bottom)
One last comment - It seems like a very common thing that is done on discussion forums that when someone uses the term maths (not the "s" at the end of math) it seems to come from someone who doesn't know math very little or at all. If you want to give the impression that you know math then I recommend that you don't use the term "maths" or if you need to then leave the "s" off the end of maths.
Pete
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By the way, what model are we assuming for this debate? The mantra centrifugal force is not a force is usually used when teaching Newtonian mechanics in inertial reference frames.
I don't know where you got that idea. Typically it's undergrad courses which some teach that, i.e. Physics I, II, III. Not in more advanced courses. See
More modern teachings give the following
http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm
Most notably,
From Gravitation, by Misner, Thorne and Wheeler, Box 6.1, page 164
. . .
Yes, I'm well aware of general relativity as well as non-relativistic formulations of classical mechanics in arbitrary reference frames. My point still holds that when we Newtonian mechanics in inertial reference frames (usually in the first course of an undergraduate physics education), centrifugal force is not a force in that model. It doesn't matter what models they'll learn later--it doesn't make centrifugal a force magically a force in inertial Newtonian mechanics simply because general relativity exists.
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My point still holds that when we Newtonian mechanics in inertial reference frames (usually in the first course of an undergraduate physics education), centrifugal force is not a force in that model. It doesn't matter what models they'll learn later--it doesn't make centrifugal a force magically a force in inertial Newtonian mechanics simply because general relativity exists.
In Notonian physics that is not always true. That's why I posted examples from Newtonian texts. It appears that you skipped through post you're quoting and cut out all the Newtonian texts/examples. The ones you skipped over are very important physics texts. Please look over the examples I used before you skip over them
http://home.comcast.net/~peter.m.brown/gr/inertial_force.htm
A.P. French - Inertial force is defined as the force on a body that results solely from observing the motion of the body from a non-inertial frame of reference. This in addressed in Newtonian Mechanics, A.P. French, The M.I.T. Introductory Physics Series, W.W. Norton Pub. , (1971) , page 499. After describing the inertial force as seen from an accelerating frame of reference French writes
From the standpoint of an observer in the accelerating frame, the inertial force is actually present. If one took steps to keep an object "at rest" in S', by tying it down with springs, these springs would be observed to elongate or contract in such a way as to provide a counteracting force to balance the inertial force. To describe such force as "fictitious" is therefore somewhat misleading. One would like to have some convenient label that distinguishes inertial forces from forces that arise from true physical interactions, and the term "psuedo-force" is often used. Even this, however, does not do justice to such forces experienced by someone who is actually in the accelerating frame of reference. Probably the original, strictly technical name, "inertial force," which is free of any questionable overtones, remains the best description.
The subject of inertial force is also addressed in The Variational Principles of Mechanics - 4th Ed., Cornelius Lanczos, Dover Pub., page 98.
Whenever the motion of the reference system generates a force which has to be added to the relative force of inertia I’, measured in that system, we call that force an “apparent force.” The name is well chosen, inasmuch as that force does not exist in the absolute system. The name is misleading, however, if it is interpreted as a force which is not as “real” as any given physical force. In the moving reference system the apparent force is a perfectly real force, which is not distinguishable in its nature from any other impressed force. Let us suppose that the observer is not aware of the fact that his reference system is in accelerated motion. Then purely mechanical observations cannot reveal to him that fact.
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Please read what I said above. I specifically stated (twice) that it's Newtonian mechanics in inertial reference frames where centrifugal force doesn't exist. This is 100% true.
You keep citing things on non-inertial reference frames and general relativity, which are different models. I agree with the French quote you give above, but its irrelevant to Newtonian mechanics in inertial reference frames.
By the way, "math" is the standard American English spelling...
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The correct spelling for the short term of mathematics is "math" and not "maths". In any case I never seen a mathematician of physicist use the term "maths" In the last 30 years.
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The correct spelling for the short term of mathematics is "math" and not "maths". In any case I never seen a mathematician of physicist use the term "maths" In the last 30 years.
Being a Yank, I use the American spelling for that one. However, I did spend a year in England when I was six, and learned to read there. I tend to use "grey" and "theatre" instead of the preferred American English "gray" and "theater" as a result.
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I think 'math' is an Americanism, and this is a UK website and show. But that aside...
So... we all know forces (except gravity) are mediated by fields which are made up of virtual particles. We have photons for electromagnetism, gluons for the strong force, W and Z bosons for the weak force...
So are there any centrifugons?
No. But science certainly isn't restricted to fundamental particles. Topics like chaos theory involve no fundamental particles, but is certainly part of science.
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Well that's part of my point that this entire argument hinges on which model you choose to use.
Anyway, regarding the fundamental particles, I wasn't trying to prove or disprove anything. I'm legitimately curious. I suspect there might actually be "centrifugons." After all, we suspect that gravity is governed by gravitons, and "centrifugal force" is an inertial force like gravity. Would inertial forces also be governed by gravitons? I don't know, but its interesting.
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I find the following comments confusing:
My point still holds that when we Newtonian mechanics in inertial reference frames ... centrifugal force is not a force in that model.
What you've said here, literally, is "In inertial frames of reference, inertial forces don't exist and when they don't exist they are not forces?"
See my confusion? Sorry I didn't cacth myself earlier. :(
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Well that's part of my point that this entire argument hinges on which model you choose to use.
Not from what I've seen. There are two classes of physicists who chime in on this subject:
Class 1) The Newtonian viewpoint: gravitation is merely an artifact of looking at things from the 'wrong' point of view. (See Peacock in link above)
Class 2) From the standpoint of an observer in the accelerating frame, the inertial force is actually present. If one took steps to keep an object "at rest" in S', by tying it down with springs, these springs would be observed to elongate or contract in such a way as to provide a counteracting force to balance the inertial force. To describe such force as "fictitious" is therefore somewhat misleading. (See A.P. French)
Anyway, regarding the fundamental particles, I wasn't trying to prove or disprove anything. I'm legitimately curious. I suspect there might actually be "centrifugons." After all, we suspect that gravity is governed by gravitons, and "centrifugal force" is an inertial force like gravity. Would inertial forces also be governed by gravitons? I don't know, but its interesting.
It seems to me that if gravitons existed then they must exist in inertial frames. In fact their existance should be frame dependant, just like inertial forces.
A similar thing should happen in electromagnetic field. See http://home.comcast.net/~peter.m.brown/ref/falling_charge.htm
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I find the following comments confusing:
My point still holds that when we Newtonian mechanics in inertial reference frames ... centrifugal force is not a force in that model.
What you've said here, literally, is "In inertial frames of reference, inertial forces don't exist and when they don't exist they are not forces?"
See my confusion? Sorry I didn't cacth myself earlier. :(
That's exactly my point. In introductory physics we restrict ourselves to models in which inertial forces do not exist and therefore "centrifugal force is not a force" is perfectly valid within that model. The only reason we have to stress this is that students have already heard the name and confuse it with centripetal force, which does exist in that model.
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Class 1) The Newtonian viewpoint: gravitation is merely an artifact of looking at things from the 'wrong' point of view. (See Peacock in link above)
Ah yes, but my example was careful to avoid gravity. I'm not entirely confident about this, but I suspect it's virtually impossible to explain the case of the subject on a rotating platform without a centripetal force component. The trick, if there is a trick, is that the platform was initially stationary.
Class 2) From the standpoint of an observer in the accelerating frame, the inertial force is actually present. If one took steps to keep an object "at rest" in S', by tying it down with springs, these springs would be observed to elongate or contract in such a way as to provide a counteracting force to balance the inertial force. To describe such force as "fictitious" is therefore somewhat misleading. (See A.P. French)
If you are in an accelerating frame, and you determine that it is rotating, you have just established that you are rotating within an inertial frame, in which case there is no "centrifugal" anything, and if you are in an accelerating frame without reference to any other frame, there still is no "centrifugal" anything because there is no rotation.
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Ah yes, but my example was careful to avoid gravity.
I was talking to JP, wasn't I?
Pete
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Ah yes, but my example was careful to avoid gravity.
I was talking to JP, wasn't I?
Pete
Will I have to conclude that my superior logic prevails?
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Will I have to conclude that my superior logic prevails?
Geezer - Can you restate what point you were trying to make? I'm ashamed to say that I losttrack of your argument.
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I think that after one did a survey of the literature tat one would conclude that whether inertial forces are refered to as real or not depends on the author. I've found it difficult to conclude what the census is.
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I'll make this one last comment before I go back to bed. If one uses Netonian physics but insist that inertial forces are not real then one must accept that the gavitational force is not real because in Newtonian dynamics the force is proportial to the mass of body.
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I'll make this one last comment before I go back to bed. If one uses Netonian physics but insist that inertial forces are not real then one must accept that the gavitational force is not real because in Newtonian dynamics the force is proportial to the mass of body.
That's why I was trying to leave gravity out of it :)
In the example I gave, the subject stepped on to the platform while it was stationary. The platform then started to rotate, and the subject followed a curved path.
Here's a restatement of the question:
How was it possible for the subject to follow a curved path without a component of the force acting on him pointing towards the axis of rotation?
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I'll make this one last comment before I go back to bed. If one uses Netonian physics but insist that inertial forces are not real then one must accept that the gavitational force is not real because in Newtonian dynamics the force is proportial to the mass of body.
That's why I was trying to leave gravity out of it :)
In the example I gave, the subject stepped on to the platform while it was stationary. The platform then started to rotate, and the subject followed a curved path.
Here's a restatement of the question:
How was it possible for the subject to follow a curved path without a component of the force acting on him pointing towards the axis of rotation?
Sorry. It's too late and my brain has turned to mush. lol! I'll get to this tomorrow.
Pete
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How was it possible for the subject to follow a curved path without a component of the force acting on him pointing towards the axis of rotation?
Geezer - Please accept my appologies for not being able to get around to your question in full. I'm spending all of my spare time learning about Dark Matter. After that it will be something else ad. infinitum. Sorry. But I do post here so I'll do my best to get around to what I best can. My thoughts and my allegiencies are all over the place right now.
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PMB - No problem!