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

On page 71 of your wonderful *Bad Astronomy*, you write:

*The Earth feels a gravitational pull toward the Sun and a centrifugal force away away from it.*

However, whenever I look up *centrifugal force* in any of my many science dictionaries, and when I am lucky to find the term *centrifugal force* defined at all, I get something like this. (I selected as my source, a FAVORITE dictionary of mine in a FAVORITE SERIES OF FACTS ON FILE DICTIONARIES in the sciences: *The Facts on File Dictionary of Physics* (THIRD EDITION) by John Daintith & John O. E. Clark*.) It clearly states the following:

*centrifugal force A force supposed to act radially outward on a body moving in a curve. In fact there is no real force acting; centrifugal force is said to be a 'fictitious' force, and the use of the term is best avoided. The idea arises from the effect of inertia on an object moving on a curve. If a car is moving around a bend, for instance, it is forced in a curved path by friction between the wheels and the road. Without this friction (directed toward the center of the curve) the car would continue in a straight line. The driver also moves in the curve, constrained by the friction with the seat, restraint from a seat belt, or a 'push' from the door. To the driver it appears that there is a force radially outward pushing his or her body out -- the centrifugal force. In fact this is not the case; if the driver fell out of the car he or she would move straight forward at a tangent to the curve. It is sometimes said that the centrifugal force is 'reaction' to the centripetal force -- this is not the case. (The 'reaction' to the centripetal force is an outward push on the road surface.) See also centripetal force.*

So, Bad Astronomer, you have made a bad bad astronomy mistake. I find astronomy professors misusing the term regularly at my favorite university (where my astronomy club meets). I guess that astronomy professors have a secret love for forbidden terms.

Anyway, when there is nothing to stop the motion of a person thrown from a car, that person flies off at a tangent to the curve. In other words, *centrifugal force* is in fact *inertia* (a lack of a force and not a force at all)!

When I first learned about *centrifugal force* not actually being a force, I was elated to discover something about which I had been previously mistaken until that very moment of enlightenment. It was one of those instances of *the pleasure of finding things out* so beautifully stated by Richard Feynman.

Anyway, I KNOW that you KNOW about this. But others might not know about *centrifugal force* actually being a *fictitious force* and being equal to *inertia* which is the LACK OF A FORCE and NOT A FORCE AT ALL!

This is so much fun!

Phil Plait: *Bad Astronomy* is a great book. I am reading it VERY, VERY SLOWLY. You give many interesting and varying answers to many astronomical puzzles.

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


_________________
*Nothing is more damaging to a new truth than an old error.* Goethe

<font size=-1>[ This Message was edited by: ljbrs on 2002-03-27 19:19 ]</font>

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

Not a force? So I should stop trying to detect the Centrifutron?

ljbrs,

I mentioned a similar misuse of this term in this thread.

If you read that thread, you'll find that the regulars on this board apparently have no problem with the term "centrifugal force."

For what it's worth, though, I'm with you! [img]/phpBB/images/smiles/icon_smile.gif[/img]

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

So do certain physicists. The term centrifugal force appears without reservation in the "bible" of general relativity, Gravitation, by Charles Misner, Kip Thorne, and John Wheeler. John Wheeler was the physicist who named "black holes" and Kip Thorne invented the concept of worm holes, I believe. The term "centrifugal force" is often used. Sometimes it is avoided because readers might be confused, but I disagree that it is a forbidden term.

It is used on this Exploratorium website, Boeing website, and even in an excerpt from Neil Comin's Heavenly Errors: Misconceptions About the Real Nature of the Universe (it is not listed as a heavenly error in Neil's book).

Anything that causes an acceleration is a force. An acceleration is, by definition, a change in an object's speed or direction of travel.

Centripetal force is a force on an object not moving in a straight line as seen by an outside observer, say, someone on the ground as the object moves past.

But in the frame of reference of the moving object, the forces are different because the coordinates have changed. In that frame, as I understand it, centripetal force is actually centrifugal.

Many people claim centrifugal force is fictional, but I prefer to think that it is just as valid as centripetal, just in a different reference frame. I suspect that's why those other sources have no problem with it.

<img src=http://sushi.dbestern.net/forumstuff/forces.png>

I just can't agree with the BA here. No matter what the frame of reference, there are no outward forces.
Imagine the above as a binary star system, the BA spinning his daughter in a circle, or me whirling a stone on a string around my head.

The only way to get a centrifugal force is through tides, such as how the moon is receding.

__________________
"We want a few mad people now. See where the sane ones have landed us!" - George Bernard Shaw

Clearly, you've never ridden in a car with my Dad.

Are you all also saying that we shouldn't talk about coriolis forces either? Is that also a forbidden term?

Sighs, hasn't this been talked over in out left right and then some...

<font size=-1>[ This Message was edited by: Mr. X on 2002-03-28 13:04 ]</font>

I thought it wasn't considered a force because there's no exchange particle.

To be honest, I've never heard of coriolis forces, just the coriolis "effect" . . . hmm.

If you're sitting on a stool and somebody pulls the stool out from under you, would you say that, from your frame of reference, a force pushed you off the stool? How would you name this force?

If it's a chair with a back, and the chair is pulled so that the back hits you, does a force push you against the back of the chair? Again, what's the name of this force?

If you're rolling across the floor on a castered chair when somebody pulls it, what's the name of the force that pushes you against the back of the chair?

It's all just inertia, isn't it?


_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-03-28 15:34 ]</font>

In a rotating frame of reference, centrifugal force is obvious. Get on a good merry-go-round, and tell me that you don't feel an "outward force."

The same "force" is what causes the earth's equatorial bulge.

Rotating frames of reference are always suspect, since they are accelerating, and we have an understandable preference for inertial frames of reference. But some problems resolve more nicely in a r.f.r, just as some problems are more tractable in polar coordinates than in cartesian coordinates.

Whirl your partner round and round,
Lift 'er in the air and set her down,
Did she orbit, or did you spin?
Cen-TRI-fug-al force, it ain't no sin!

Silas

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

Everything depends upon whether *centrifugal force* is a force or simply inertia. I still go with inertia because it travels off on a tangent to the curve in a straight line when released to move on its own.

I personally like the term *centrifugal effect* instead. I have well over 20 science dictionaries and many scientific encyclopedias. Some of them do not list *centrifugal force* at all. Some simply state *undefined* as a definition of *centrifugal force*. Those dictionaries which define *centrifugal force* usually explain that it is inertia and not a force and/or call it a *fictitious force*. I have yet to find a single one which defines it as a real force. Whatever...

I always enjoy finding things out which go contrary to ideas I have previously held dear. I think that, since the term *centrifugal force* has been used for so long by so many, it will continue to be so used in the future.

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]

_________________
*Nothing is more damaging to a new truth than an old error.* Goethe

<font size=-1>[ This Message was edited by: ljbrs on 2002-03-31 15:05 ]</font>

You may be in denial. [img]/phpBB/images/smiles/icon_smile.gif[/img]

I just looked for the term in the first three books I have at hand, the American Heritage Dictionary, Gravitation by Misner, Thorne, and Wheeler, and College Physics by Franklin Miller and "coriolis force" appears in all of them. Which texts do not have the term?

Lisa: I've moved on to the anger stage, and you're still in denial.
Bart: I am not in denial.
Lisa: Yes, you are.
Bart: No, I'm not.
Lisa: Yes, you are.
Bart: No, I'm not!
Lisa: I stand corrected.

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

Texts? Who said anything about texts? In the last year or so that I've been posting here, have I at some point given you the impression that I'm educated? [img]/phpBB/images/smiles/icon_smile.gif[/img]

Seriously, I was just thinking of my own everyday experience. I will not attempt to debate you on whether the term is used in physics textbook and the like.

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

Well then, the least you could do is recant your statements that the use of the term centrifugal force is a misuse. But, I'll accept any other quotation from the Simpsons.

Not that it can't be misused.

Oh, I don't know - just because it's used a lot doesn't mean it's not a misuse. [img]/phpBB/images/smiles/icon_smile.gif[/img] At any rate, I still think it's a misleading term.

(BTW, I used "misuse" above to refer back to my post of a while ago. That particular usage of "centrifugal force" basically said "If the driver doesn't sharply bank the car, centrifugal force will make it keep going straight" and I'm going to stand by my description of that as a "misuse").

I still say that "centrifugal force" is just a specific application of inertia. I mentioned having a stool pulled out from under you - isn't describing that as "a force pushed me off the stool" at least somewhat incorrect?

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

True. However, not true in this case.

To some people perhaps.

You said it was a similar misuse. I took that to mean that you thought this one was too.

Well, since Einstein, "gravity" is just a specific application of warped space. In that sense, gravity is just an inertial force too. You wouldn't argue that the use of the word gravity is a misuse, would you? As long as it is understood what is meant by the term, I don't see how it can be called a misuse.

Oh, I like that! An orbit is entirely inertial - both towards the orbited body and away from it. Cool! [img]/phpBB/images/smiles/icon_smile.gif[/img]

An object in an Einsteinian gravity well is behaving inertially four-dimensionally, but not three-dimensionally, while centrifugal "force" is an object simply behaving inertially three-dimensionally. I don't think it's necessarily misleading to have a word ("gravity") that specifically refers to that four-dimensional warping - it is technically just inertia, but it's not, um, "normal" inertia.

In regards to the term "gravitational force", however -- There was an earlier thread in which it was directly asked why we refer to gravity as a "force" if it's really just an inertial result of warped spacetime. Your last response regarding that question was:

which, to me, suggests that you agree that it is somehow not entirely correct to refer to (inertial) gravity as a "force." Have you changed your mind?

BTW, you still haven't answered my question about the stool - what's the "force" that pushes you off a stool when somebody else pulls it out from under you?

__________________
SeanF

"Ask to understand, but don't challenge unless you have the knowledge."--NEOWatcher

The contents of this post are ©2008 by SeanF and may not be copied or retransmitted in any form without the express written consent of SeanF

Isn't that simply gravity? Am I missing something basic here? Gravity is indeed a force, in that it causes acceleration. If there were no force on you, you would float over the ground when the stool is removed.