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Thread: Centrifugal force is really inertia and not a force!

  1. #31
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    PAY NO ATTENTION TO ANY OF MY POSTS IN THIS THREAD, BECAUSE MY SCIENCE IS IN ERROR! ljbrs

    I was writing about SCIENTIFIC dictionaries. The American Heritage Dictionary (a favorite of mine) is not a scientific dictionary. I have a load of scientific dictionaries and encyclopedias and all of them define (when they define it at all) *centrifugal force* as being *inertia* and not a force. It really surprised me to find out, a long time ago, that *centrifugal force* was not defined in science dictionaries as a *force*, but as *inertia*, instead.

    Really, the professional astronomers and physicists of my limited acquaintance use the term *centrifigal force* and it really does not bother me. They know what they are describing and the words *centrifugal force* can be very useful. Everybody knows what they are describing. However, many ordinary people do not know that centrifugal force acts like inertia in movement at a tangent to the curve when the centripetal force causing the circular motion has ended. If centrifugal force were a real force, it would act radially to the curve rather than tangentially.

    Oh, well, you know about dogs with bones. They refuse to relinquish them...

    Arf arf! Bow wow! Woof woof! Snarl snarl!

    Granny ljbrs [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img] [img]/phpBB/images/smiles/icon_rolleyes.gif[/img]

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    <font size=-1>[ This Message was edited by: ljbrs on 2002-03-31 15:14 ]</font>

  2. #32
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    It is my opinion, that this dead horse has been beaten sufficiently.[img]/phpBB/images/smiles/icon_smile.gif[/img]

  3. #33
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    I think I understand the point ljbrs is making. The big problem isn't the use of the term "centrifugal force" itself, it's that the average person doesn't know that it isn't a true force in physics terms. Unless everyone knows what you are really talking about, you are technically making an error.

    But if you think about it, the average person hasn't a clue what physics considers a "force" anyway, and frankly, I don't think they really need to. You can function just fine without that knowledge. It'd be nice if we could get the word out to everyone, but that's a real uphill battle.

    I think the best thing to do is to make sure the average person is reached in terms he or she understands first, and worry about the details of terms like this when necessary.

    So what I'm saying is, in effect, that even though the term isn't accurate, in this case the BA's intended audience doesn't need to know it isn't real to understand the point he's trying to get across.

  4. #34
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    On 2002-03-31 11:03, Kaptain K wrote:
    It is my opinion, that this dead horse has been beaten sufficiently.[img]/phpBB/images/smiles/icon_smile.gif[/img]
    Kaptain, why couldn't you have waited just a few more minutes so I could get my post up first. Then I wouldn't look like such a putz for posting after you. [img]/phpBB/images/smiles/icon_razz.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img]

    I agree though. This is one over-whipped horse carcass.

  5. #35
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    mea culpa

  6. #36
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    PAY NO ATTENTION TO ANY OF MY POSTS IN THIS THREAD, BECAUSE MY SCIENCE IS IN ERROR! ljbrs

    David Hall:

    Thank you for your post. Of course, the horse has been long gone. It was just one of my favorite finds many years ago when I looked up centrifugal force and found that it was not a force.

    I used to be a good horsewoman (many long years ago), but this *force* horse would be exceedingly difficult to mount and would be entirely unable to respond to my direction. But you know women. Yack, yack, yack...

    Kaptain K:

    Oops. I just fell off my hobby horse!

    ljbrs



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    <font size=-1>[ This Message was edited by: ljbrs on 2002-03-31 15:18 ]</font>

  7. #37
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    PAY NO ATTENTION TO ANY OF MY POSTS IN THIS THREAD, BECAUSE MY SCIENCE IS IN ERROR! ljbrs

    EXCEPT THIS ONE...

    Of course, in my silly and dogmatic mental state, I completely forgot that there are only four known forces (in physics): The strong force, the weak force, the electromagnetic force, and gravity. So there goes nothing... Perhaps quintessence/lambda/dark energy could eventually be added to this short list.

    Now if I could only find my blankety-blank *drawing board* which got lost somewhere along the way.

    ljbrs [img]/phpBB/images/smiles/icon_redface.gif[/img] [img]/phpBB/images/smiles/icon_frown.gif[/img] [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_eek.gif[/img] [img]/phpBB/images/smiles/icon_confused.gif[/img] [img]/phpBB/images/smiles/icon_cry.gif[/img]

    <font size=-1>[ This Message was edited by: ljbrs on 2002-03-31 15:32 ]</font>

    <font size=-1>[ This Message was edited by: ljbrs on 2002-03-31 15:39 ]</font>

  8. #38
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    'Borrowing' Hat Monsters diagram...



    We have to consider here the net resultant force on the orbiting object. The balance of the gravity against the inertia results in a perfect 'free fall' orbital state. The yellow object travelling along the green tangental line would try to climb up the gravity well produced by the black object. Since it does not have the velocity to achieve this climb, it curves round in the orbit described by the red circle. The problem with describing this force as centrifugal or centripidal is that it does not explain the whole of what is going on, but I don't think there is anything wrong with describing this with a simple name, even though it is a combination of more than one factor. Imagine being in AC Clarks 2001 space station. On the outer ring would be a force holding you to the floor on the outer edge. This force would be indistinguishable from the gravity we all feel day after day. You want to go in a straight line but the moving floor keeps 'catching' you to prevent that. It is little different to the effects of gravity, whereby you want to fall to the centre of the Earth, but the ground prevents this. This force has a name. Centripital force. It can be measured. It exists. It may be a combination of other things, but it is there nonetheless. I say the BA's got it right. So there.

  9. #39
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    On 2002-03-31 10:22, ljbrs wrote:
    If centrifugal force were a real force, it would act radially to the curve rather than tangentially.
    Centrifugal force does act radially, not tangentially, though.

  10. #40
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    Whatever...

    Look up centripetal force and centrifugal force (sic) in any physics textbook for a complete explanation of both. The terms are often used casually (often interchangeably) by scientists, and that is fine with me. It is not a big deal...

    ljbrs [img]/phpBB/images/smiles/icon_smile.gif[/img] [img]/phpBB/images/smiles/icon_confused.gif[/img]

  11. #41
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    O.K. I looked it up in The Feynman Lectures on Physics, in order to get it from the horse's mouth (Richard Feynman being considered by many physicists as the greatest 20th Century American physicist). Feynman mentions (briefly) centrifugal force in both Chapters 7 (page 5) and 12 (page 11). In Chapter 12, page 11 (12-11) he states:

    **Another example of pseudo force is what is often called *centrifugal force.* An observer in a rotating coordinate system, e.g., in a rotating box will find mysterious forces, not accounted for by any known origin of force, throwing things outward toward the walls. These forces are due merely to the fact that the observer does not have Newton's coordinate system, which is the simplest coordinate system.**

    A pseudo force in this instance is not considered to be a real force, by reason of its *pseudo* appelation. Some refer to *centrifugal force* as a *fictitious force*. Physicists and astronomers use the term *centrifugal force* all of the time. I am certain that they all know it is a pseudo (fictitious) force. Whatever...

    ljbrs [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img] [img]/phpBB/images/smiles/icon_confused.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]

  12. #42
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    I like to read Feynman's lectures, because he seems to boil everything down to its essence, and then present it in an intuitive way. Thank you for prompting me to dig them out of the bookshelf.

    Still, he uses the term centrifugal force on page 7-5, to explain the tides and the motion of the Earth, and he says that the moon's attraction on the Earth is balanced by the centrifugal force, just as the BA has done. If you accept Feynman as the ultimate authority, I think you'd have to accept BA's use of the term.

    Also, further on in chapter 12, on page 12-12, he says "if we distort the geometry sufficiently, it is possible that all gravitation is related in some way to pseudo forces; that is the general idea of the Einsteinian theory of gravitation." That is the point I tried to make earlier--that gravity itself could be considered a pseudo force. That doesn't mean that we shouldn't use the term "gravity" though.

  13. #43
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    Right. Feynman also stated that gravity was kind of a pseudo force. So, there is no problem there. This is what makes science so much fun, it never stands still.

    ljbrs [img]/phpBB/images/smiles/icon_biggrin.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img] [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]

  14. #44
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    Okay, I had kind of dropped out of this discussion, but I'd like to clarify a couple of things about my stand on this issue.

    The term "centrifugal force" is commonly used to describe the effect in question. Because of the nature of language, that in itself is enough to make it "right" (nova, Panama hat, camel's hair brush, etc.), but it is still technically wrong.

    On 2002-04-02 04:31, GrapesOfWrath wrote:
    . . . gravity itself could be considered a pseudo force. That doesn't mean that we shouldn't use the term "gravity" though.
    Nothing was ever said that would imply an objection to the term "gravity." An objection to the term "gravitational force," yes, but nobody ever said we shouldn't have different words for different specific instances or effects of inertia.

    Another example, though, would be the "g-forces" experienced by astronauts during launch (and jet pilots during acceleration). Again, it's just the person's inertia resisting the engines' acceleration, but it still has come to be termed a "force."

    As for the BA's usage of the term that started this whole thread, since he says "The Earth feels . . . " it can be argued that it's perfectly valid anyway since it's dealing with the perception of a force more so than the actual existence of a force.

    I still think it might've been better to say something along the lines of "The Earth feels a gravitational pull towards the Sun while it's inertia tries to carry it away," but that's just me. [img]/phpBB/images/smiles/icon_smile.gif[/img]


    Sometimes you win, sometimes you learn

  15. #45
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    On 2002-04-03 09:16, SeanF wrote:
    Nothing was ever said that would imply an objection to the term "gravity." An objection to the term "gravitational force," yes, but nobody ever said we shouldn't have different words for different specific instances or effects of inertia.
    So, you would have the same objection to "gravitational force" that you would have to "centrifugal force"? I would have thought that there would be no objection to that.

  16. #46
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    On 2002-04-03 09:43, GrapesOfWrath wrote:
    On 2002-04-03 09:16, SeanF wrote:
    Nothing was ever said that would imply an objection to the term "gravity." An objection to the term "gravitational force," yes, but nobody ever said we shouldn't have different words for different specific instances or effects of inertia.
    So, you would have the same objection to "gravitational force" that you would have to "centrifugal force"? I would have thought that there would be no objection to that.
    I could've worded that better.

    I mentioned in a previous post about how the "acceleration" of gravity is different than the "acceleration" of centrifugal force.

    My only point here was that the objection was based on the word "force," and could thus imply an objection to "gravitational force" but not to "gravity."


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    <font size=-1>[ This Message was edited by: SeanF on 2002-04-03 10:03 ]</font>

  17. #47
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    Of course the correct Simpsons quote would be:

    Lisa: Bart, I've read about what happens to kids who's parents no longer love and cherish each other. They go through eight separate stages. Right now I'm in stage three, Fear. You're in stage two, Denial.
    Bart: [whines] No I'm not.
    Lisa: Yes you are.
    Bart: [angrily] No I'm not!
    Lisa: Yes you are!
    Bart: Am not! Am not! Am not!
    Lisa: I stand corrected

    There you go... just thought I'd mention this... [img]/phpBB/images/smiles/icon_smile.gif[/img]

  18. #48
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    I believe that I have made enough dogmatic, and therefore sometimes idiotic, statements about force and inertia which I took much too seriously. The nature of science is such that new ideas come into use forcing old ideas often to drop by the wayside. If I could stay alive forever, it would be exciting just to see where all of this is going to end. Then again, by the time everything eventually becomes positronium (if everything does become positronium), new ideas will have long ago come to an end, there having been no intelligent species around to cogitate about them).

    ljbrs [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_confused.gif[/img] [img]/phpBB/images/smiles/icon_wink.gif[/img]

  19. #49
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    Don't worry lj (may I call you lj?). The thread you started has been entertaining and informative, especially to newbees just coming onto the boat. Like you said, many things turn positive in the end. And such was this.

    Besides, you weren't really wrong at all. In fact you were right in the basic way: centrifigul force is not a 'real' force per se. Most of the disagreement comes from interpreting the phrase with looser or tighter standards of meaning.

    And BTW, you really like the smilies, don't you?

  20. #50
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    Hmmm...

    I'm not clear on why this is so controversial. Is it because of differing interpretations of "real" vs. "fictional"? Here's the way I think about it, in case this helps anyone.

    Start from Newton's Second Law, which states that F=ma, or that there is a simple proportional relationship between the force acting on an object and the acceleration that object undergoes, with the proportionality being termed its "mass" (or "inertial mass", to be more accurate). One can turn that equation around and deduce from an object's observed acceleration that a force must have been applied to the object.

    So far, so good. We exert a force, we see acceleration; we observe acceleration, we deduce a force. Now, we next note that F and a are both vectors (don't know how to do vector notation in HTML, sorry), which have to be defined relative to some coordinate system. If that coordinate system is rotating, then the vectors also have to rotate in time. But force and acceleration are both second time derivatives of the position! Which means that it's not *just* the position of the object that's changing with time, but your description of the position has to change with time as well. This adds extra terms to the acceleration that serve to account for the changing coordinate system, which in turn then demand adding extra force terms on the left side of the equation. These force terms are called "centrifugal" and "coriolis".

    So, these *are* "forces" in the sense that they are necessary deductions from the observed acceleration (in the rotating coordinate system). A ball released on a carousel will look (from the point of view of someone standing on the carousel) like it is being accelerated/forced radially outward. Newton's second law demands we call that a force, since the mass is clearly undergoing acceleration. However, that "acceleration" is just due to the rotating coordinates, not due to anything actually *acting* on the ball, and therefore the force is called "fictional". You can switch into an inertial reference frame and make fictional forces vanish, but real forces will never go away in an inertial reference frame.

    A "centripital" force is an inwardly directed, *real* radial force that keeps an object in a circular orbit. In the rotating frame, the object is stationary, the centripital force having canceled out the centrifugal force, but in an external, non-rotating frame, the centripital force would be seen to be simply pushing perpendicularly to the object's tendency to want to go straight. The force, and hence acceleration, is acting crosswise to the velocity vector, leading to an acceleration that doesn't change the speed of the object, just its direction. It could be caused by any of the four forces, although in practice it's probably only going to be caused by gravity or electromagnetism. Centrifugal force, on the other hand, is just "caused" by the rotation, in so far as it only is part of an accurate description of the equations of motion in a rotating reference frame. (It's not really *caused* by anything, since it's only a description.)

    So there's nothing wrong with talking about centrifugal force, as long as you know it's a shorthand for "a mathematical term we need to introduce into Newton's second law in order to deduce the observed trajectory of the object" and not "a magical, sourceless force that throws an object away from you while you are spinning". If you are defining "force" in a Quantum Field Theory kind of way, as a change in momentum mediated by the exchange of a boson, then the centrifugal and coriolis forces are *not* forces, but if you define force as being the mass times the observed acceleration, then they are.

    Simple, no? [img]/phpBB/images/smiles/icon_smile.gif[/img]

    Don

    <font size=-1>[ This Message was edited by: DoctorDon on 2002-04-04 12:26 ]</font>

  21. #51
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    [quote]
    On 2002-03-30 21:59, The Bad Astronomer wrote:
    "Actually, there is a theory that the weirdly shaped nebulae (butterfly shaped, or more technically bipolar) may be due to the stars consuming their planets when they expand into red giants." [img]/phpBB/images/smiles/icon_wink.gif[/img]
    I hate stating this without a source, but isn't there another theory that novas (which result in nebula) explode out from the poles of the star? This might result in the giant expanding spheres of gas too. [img]/phpBB/images/smiles/icon_smile.gif[/img]

  22. #52
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    If you are defining "force" in a Quantum Field Theory kind of way, as a change in momentum mediated by the exchange of a boson, then the centrifugal and coriolis forces are *not* forces, but if you define force as being the mass times the observed acceleration, then they are.
    Inertia has no acceleration, observed or otherwise. So-called *centrifugal* force is not a force (being inertia). I am not backing down from that. I am backing down from making any kind of issue about it. It is a useful term used casually (hopefully) by all of the astronomers I know. My fault lay in taking issue with the use of the word *force*. No big deal. But it is not really a force, but is a pseudo force (to re-quote the late Richard Feynman in his *Lectures on Physics*). No big deal...

    David Hall:

    Don't worry lj (may I call you lj?). The thread you started has been entertaining and informative, especially to newbees just coming onto the boat. Like you said, many things turn positive in the end. And such was this. *** And BTW, you really like the smilies, don't you?
    Thanks. The smilies are there to hide my glowering frown...

    ljbrs [img]/phpBB/images/smiles/icon_frown.gif[/img] [img]/phpBB/images/smiles/icon_eek.gif[/img] [img]/phpBB/images/smiles/icon_smile.gif[/img]

  23. #53
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    Inertia has no acceleration
    Well, I never said it did, so I'm not sure why you're saying this.

    So-called *centrifugal* force is not a force (being inertia). I am not backing down from that. .... But it is not really a force, but is a pseudo force
    Did you *read* what I wrote? That's just what I said, only I explained just what exactly a "pseudo-force" is: a term introduced into Newton's second law to explain the observed acceleration in a rotating reference frame. I don't understand what you're protesting.

    Don

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    [quote]
    On 2002-04-04 13:01, Chip wrote:
    isn't there another theory that novas (which result in nebula) explode out from the poles of the star? This might result in the giant expanding spheres of gas too. [img]/phpBB/images/smiles/icon_smile.gif[/img]
    That's different than a planetary nebula. I have page about this here. Expanding spheres are relatively rare in astronomy; you usually get some degree of rotational symmetry (like rings, or barrel shapes).

  25. #55
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    On 2002-04-04 21:45, The Bad Astronomer wrote: "That's different than a planetary nebula. I have page about this here. Expanding spheres are relatively rare in astronomy; you usually get some degree of rotational symmetry (like rings, or barrel shapes)."
    Thanks for the info. I learned things new to me from the bitesized page. Nick Strobel's linked website was interesting too.

    (I wonder if astronomers have ever spotted a pre-nova red super giant that is in a binary orbit with a high UV-light star that illuminates the giant's solar wind patterns. Just wondering.)

    Chip

  26. #56
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    Quote:
    --------------------------------------------------------------------------------


    Inertia has no acceleration


    --------------------------------------------------------------------------------



    Well, I never said it did, so I'm not sure why you're saying this.

    Quote:
    --------------------------------------------------------------------------------


    So-called *centrifugal* force is not a force (being inertia). I am not backing down from that. .... But it is not really a force, but is a pseudo force


    --------------------------------------------------------------------------------



    Did you *read* what I wrote? That's just what I said, only I explained just what exactly a "pseudo-force" is: a term introduced into Newton's second law to explain the observed acceleration in a rotating reference frame. I don't understand what you're protesting.

    Don
    I have gone back to re-read it now, and I have not the foggiest idea how I came to write what I did. Perhaps my personal *inertia* turned suddenly into a *force* (fictitious or otherwise) and *caused* me to write like an idiot.

    All of what you write is very interesting, and I must have been in a hurry, very tired, or both. No excuses will properly explain that ridiculous post of mine. Sorry! I somehow missed your above post and failed to respond earlier.

    ljbrs [img]/phpBB/images/smiles/icon_rolleyes.gif[/img] [img]/phpBB/images/smiles/icon_confused.gif[/img]

  27. #57
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    On 2002-04-04 23:21, Chip wrote:
    I wonder if astronomers have ever spotted a pre-nova red super giant that is in a binary orbit with a high UV-light star that illuminates the giant's solar wind patterns.
    No, but the star Sher 25 is a blue supergiant that is a bit hotter and more massive than the star that blew up to become Supernova 1987A. It gives off enough UV to flouresce its nebula (which 87A didn't do before it blew up). Sher 25 can be seen in my logo, actually, as the bright star in the "A" of "Astronomy", on this very page. The nebulosity can be seen in the "T".

    Check out the Hubble page about Sher 25. I also plan (someday, sigh) of writing a page about the image for my logo, since I am fascinated with this object.

    <font size=-1>[ This Message was edited by: The Bad Astronomer on 2002-04-06 11:30 ]</font>

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