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ljbrs
2002-Mar-28, 12:13 AM
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 /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_rolleyes.gif


_________________
*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>

Chuck
2002-Mar-28, 01:52 AM
Not a force? So I should stop trying to detect the Centrifutron?

SeanF
2002-Mar-28, 12:52 PM
ljbrs,

I mentioned a similar misuse of this term in this thread (http://www.badastronomy.com/phpBB/viewtopic.php?topic=170&forum=4&38).

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! /phpBB/images/smiles/icon_smile.gif

GrapesOfWrath
2002-Mar-28, 01:25 PM
On 2002-03-27 19:13, ljbrs wrote:
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.

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 (http://www.exploratorium.edu/xref/phenomena/centrifugal_force.html), Boeing website (http://www.hughespace.com/sat101.html), and even in an excerpt from Neil Comin's Heavenly Errors: Misconceptions About the Real Nature of the Universe (http://www.aspsky.org/mercury/ezine/articles/zine0301.html) (it is not listed as a heavenly error in Neil's book).

The Bad Astronomer
2002-Mar-28, 02:49 PM
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.

Hat Monster
2002-Mar-28, 03:55 PM
<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.

GrapesOfWrath
2002-Mar-28, 04:44 PM
On 2002-03-28 10:55, Hat Monster wrote:
I just can't agree with the BA here. No matter what the frame of reference, there are no outward forces.

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?

Mr. X
2002-Mar-28, 05:57 PM
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>

Chuck
2002-Mar-28, 08:07 PM
I thought it wasn't considered a force because there's no exchange particle.

SeanF
2002-Mar-28, 08:31 PM
On 2002-03-28 11:44, GrapesOfWrath wrote:

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


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>

Silas
2002-Mar-28, 08:57 PM
I just can't agree with the BA here. No matter what the frame of reference, there are no outward forces.


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

ljbrs
2002-Mar-29, 05:08 AM
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 /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_rolleyes.gif

_________________
*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>

GrapesOfWrath
2002-Mar-29, 06:44 AM
On 2002-03-28 15:31, SeanF wrote:
To be honest, I've never heard of coriolis forces, just the coriolis "effect" . . . hmm.

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

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?

SeanF
2002-Mar-29, 12:13 PM
On 2002-03-29 01:44, GrapesOfWrath wrote:


On 2002-03-28 15:31, SeanF wrote:
To be honest, I've never heard of coriolis forces, just the coriolis "effect" . . . hmm.

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

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.

/phpBB/images/smiles/icon_smile.gif



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?


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? /phpBB/images/smiles/icon_smile.gif

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.

GrapesOfWrath
2002-Mar-29, 01:12 PM
On 2002-03-29 07:13, SeanF wrote:
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.

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.

SeanF
2002-Mar-29, 02:30 PM
Oh, I don't know - just because it's used a lot doesn't mean it's not a misuse. /phpBB/images/smiles/icon_smile.gif 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?

GrapesOfWrath
2002-Mar-29, 03:26 PM
On 2002-03-29 09:30, SeanF wrote:
Oh, I don't know - just because it's used a lot doesn't mean it's not a misuse. :)

True. However, not true in this case.


At any rate, I still think it's a misleading term.

To some people perhaps.


(BTW, I used "misuse" above to refer back to my post of a while ago.

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


I still say that "centrifugal force" is just a specific application of inertia.

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.

SeanF
2002-Mar-29, 06:09 PM
On 2002-03-29 10:26, GrapesOfWrath wrote:

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! /phpBB/images/smiles/icon_smile.gif

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 (http://www.badastronomy.com/phpBB/viewtopic.php?topic=797&forum=2) 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:



Savanna's question is basic, and not yet really answered, I think.


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?

The Bad Astronomer
2002-Mar-29, 06:20 PM
On 2002-03-29 13:09, SeanF wrote:
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?


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.

SeanF
2002-Mar-29, 06:22 PM
On 2002-03-29 13:20, The Bad Astronomer wrote:


On 2002-03-29 13:09, SeanF wrote:
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?


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.


Not the downward movement, the sideways movement. You were on top of the stool, now you're several inches to the side of it . . . my point is that it's just inertia.

If there were a back to the chair (and it's pulled the right direction), you would be pressed against the back. But there's no force pushing you towards the back, it's just your inertia resisting the back pushing towards you.

And that's all "centrifugal force" is - there's no force pushing you against the far "side", it's just your inertia resisting the far side pushing against you.

_________________
SeanF

<font size=-1>[ This Message was edited by: SeanF on 2002-03-29 13:25 ]</font>

Hat Monster
2002-Mar-29, 06:28 PM
If the stool was removed instantly (like whipping the cloth from a table and leaving objects on top of it untouched) then you'd go straight down.

It is not. The friction of the stool on you pulls you to a tangent.

<hr>

Riding in a car:
You are not pulled "outwards" on corners. You attempt to continue in your original direction. This is extremely basic inertia. It appears to be an outwards force, but is nothing of the sort.

ljbrs
2002-Mar-29, 07:14 PM
PAY NO ATTENTION TO ANY OF MY POSTS IN THIS THREAD, BECAUSE MY SCIENCE IS IN ERROR! ljbrs

A little over a year ago, I went off the road on black ice and went straight down a ravine to end inbetween (luckily) a tree and bushes, both of which pinned me in my car. There was a large indentation at the front of my car where I had knocked down the *turn sign* on my way. I went off at a tangent and not in a radial direction. I love to tell people that it was centrifugal force showing that it was inertia. If it had been radial it would have been the a force creating the movement.

Actually, the force is the centripetal force doing the turning. My tangential movement (since my foot was no longer on the pedal and the engine was idling) showed that there was no sidewise force involved, but only inertia involved. I would have traveled in a radial direction and missed the bush and the tree completely if the centrifugal effect had, indeed, been a centrifugal force.

It was my first accident in my entire long, long life and I had not realized that there was ice on the road. Dumb mistake!

I guess that I am really *stubborn as a mule*. Of course, it really does not matter. However, the force being applied/supplied is the centripetal force and it is about time it gets the credit for all that it accomplishes.

Nag, nag, nag, nag... Just like a woman! Cackle, cackle, cackle...

ljbrs /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_rolleyes.gif

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

<font size=-1>[ This Message was edited by: ljbrs on 2002-03-29 14:18 ]</font>

<font size=-1>[ This Message was edited by: ljbrs on 2002-03-29 14:20 ]</font>

<font size=-1>[ This Message was edited by: ljbrs on 2002-03-31 15:07 ]</font>

ljbrs
2002-Mar-29, 07:49 PM
PAY NO ATTENTION TO ANY OF MY POSTS IN THIS THREAD, BECAUSE MY SCIENCE IS IN ERROR! ljbrs

Hat Monster:

I should have mentioned this before, but I love your illustrations. Also I enjoy and agree with you completely in what you have written. However, all of the professors who lecturer to the general public about physics and/or astronomy at a nearby world-class university use the term *centrifugal force*, so it is not about to go away when people in high places use it regularly.

*Centrifugal force* happens to be one of my favorites and I could not resist. I have physicists going back a long way in my family, so I grew up with an interest and did not follow through, because the extreme male chauvinism in the physics and astronomy departments many, many years ago (when I was of college age) was abysmal. I was not going to spoil their fun (and I fully understood their attitude, because so many women permitted themselves to become silly little things in order to attract men). Women at that time could have spoiled the men's fun by invading their wonderful sanctuary.

I maintained my interest and took physics and calculus courses, in addition to my heavy academic schedule of classes (25 semester hours every semester), but let it stay at that. Physics and calculus are great for the brain. So is astronomy. And I love Phil Plait's book. It is unique in its way of describing astronomy. It is a *must buy* book for everyone who reads or writes anything here on Bad Astronomy. I was only having fun in nitpicking about a favorite topic on my scientific hit list. It is a term most often used (misused?) and difficult to relinquish. Perhaps there are pro-force and anti-force factions in the physics and astronomy departments. We could all start a movement and carry signs, etc., and have sit-ins in the science buildings...

I sometimes have a very silly imagination...

ljbrs /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_rolleyes.gif

_________________
*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 15:09 ]</font>

Hale_Bopp
2002-Mar-29, 08:19 PM
I think a good way of looking at it is Newton's 3rd law...good old action-reaction.

When you are in a car an it acclerates forward, the seat pushes on you and you push back on the seat. The net force is forward, of course, but you do feel the reaction force.

A bad physics joke is to take a fake wooden pear and cut it in half. On the inside of each half, you draw a capital F. You then show it to you students and tell them, "Forces come in pears!" They will never forgive you for it /phpBB/images/smiles/icon_smile.gif

Rob

GrapesOfWrath
2002-Mar-30, 03:28 AM
On 2002-03-29 13:09, SeanF wrote:
In regards to the term "gravitational force", however -- There was an earlier thread (http://www.badastronomy.com/phpBB/viewtopic.php?topic=797&forum=2) 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:

[snip]

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?

Nope, have not changed my mind. Check out this thread (http://www.badastronomy.com/phpBB/viewtopic.php?topic=514&forum=2&start=24) where I said "I'm just concerned that you might have been avoiding the term "centrifugal force" just because it was fictitious. Of course, in this context, gravity is also fictitious." Or this one (http://www.badastronomy.com/phpBB/viewtopic.php?topic=170&forum=4&start=3), where I said "The [physics books] I like list gravity as a fictitious force."

My point is, just because something can be classified as a fictitious force, doesn't mean it is a misuse to use the term. Centrifugal force, Coriolis force, or gravity.

ljbrs
2002-Mar-30, 07:33 PM
PAY NO ATTENTION TO ANY OF MY POSTS IN THIS THREAD, BECAUSE MY SCIENCE IS IN ERROR! ljbrs

My original point was that the term *centrifugal force*, if it were to be found at all in science and/or physics dictionaries, was variously defined as being a *fictitious* force. Some of the dictionaries, while defining *centripetal* force did not have a listing at all for *centrifugal* force. The reason in such cases, obviously, was that there was no meaningful term to describe *centrifugal* force as a force. Now, I have a lot of dictionaries of science, and of astronomy, and of physics. I have a lot of encyclopedias of science, and I have access to the Science and Engineering Library at my alma mater where there are many other such dictionaries.

In *The Facts On File Dictionary of Physics* which I mentioned before for *centrifugal* force, the definition for *centripetal* force begins: *centripetal force A force that causes an object to move in a curved path rather than continuing in a straight line.* I have earlier typed the complete definition in this dictionary for *centrifugal force* so I need not retype it here.

What is called *centrifugal force* always continues in a straight line on a tangent when released and not in a curved line. The definition for *centrifugal force* in that book calles it a *fictitious* force. Therefore, if it is a *fictitious* force, then that would make it an *imaginary* (i.e., non-existent) force at best. When forces are equal in status to non-forces, definitions break down. A lot of people use the term, but I think that people should be aware that *centrifugal force* has never been defined as being a force, at least not in any of the dictionaries I have used up to this time. I will keep looking whenever I am visiting a science library (which is seldom, because of my rather large personal science library at home). It is not really important in the grand sweep of science. However, you know how women like me are -- persistently persistent to the degree of ridiculousness -- not all women, mind you, just women like me!

ljbrs /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_rolleyes.gif

_________________
*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 15:12 ]</font>

David Hall
2002-Mar-31, 02:32 AM
On 2002-03-30 14:33, ljbrs wrote:

The definition for *centrifugal force* in that book calles it a *fictitious* force. Therefore, if it is a *fictitious* force, then that would make it an *imaginary* (i.e., non-existent) force at best.


I wouldn't go so far as to call it "imaginary" or "non-existant" unless I really wanted to be pendantic. When you get on the Tilt-a-Wheel at the amusement park, the thing trying to sling you off of it is anything but imaginary.

Instead it helps to think of it as a fictitious force in that it's actually a combination of two different forces; the object's inertia resisted by the force pulling it at a tangent.

In my view, there's nothing wrong with using the term "centrifugal force" in the vernacular, just so long as you understand what it REALLY is. I mean we still call those nebulosity-surrounded stars "planetary nebulae" even though we know for a fact that they have nothing to do with planets. Save the need for true accuracy for when it's really needed. /phpBB/images/smiles/icon_smile.gif

PS: I'm sure when the Bad Physics book hits the shelves it will have a whole chapter devoted to this one problem. /phpBB/images/smiles/icon_biggrin.gif

The Bad Astronomer
2002-Mar-31, 02:59 AM
On 2002-03-30 21:32, David Hall wrote:
I mean we still call those nebulosity-surrounded stars "planetary nebulae" even though we know for a fact that they have nothing to do with planets.

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. /phpBB/images/smiles/icon_wink.gif

David Hall
2002-Mar-31, 03:11 AM
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. /phpBB/images/smiles/icon_wink.gif


Interesting. Hadn't heard that one.

Ok, "next to nothing". /phpBB/images/smiles/icon_biggrin.gif

GrapesOfWrath
2002-Mar-31, 01:26 PM
On 2002-03-30 14:33, ljbrs wrote:
When forces are equal in status to non-forces, definitions break down.

That, or a new theory of physics is successfully created, as happened a hundred years ago! /phpBB/images/smiles/icon_smile.gif


A lot of people use the term, but I think that people should be aware that *centrifugal force* has never been defined as being a force, at least not in any of the dictionaries I have used up to this time.

As I mentioned before, I found it in the American Heritage Dictionary, which can be found online at www.dictionary.com (http://www.dictionary.com/search?q=Centrifugal%20force).

ljbrs
2002-Mar-31, 03:22 PM
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 /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_wink.gif /phpBB/images/smiles/icon_rolleyes.gif

_________________
*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 15:14 ]</font>

Kaptain K
2002-Mar-31, 04:03 PM
It is my opinion, that this dead horse has been beaten sufficiently./phpBB/images/smiles/icon_smile.gif

David Hall
2002-Mar-31, 04:05 PM
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.

David Hall
2002-Mar-31, 04:12 PM
On 2002-03-31 11:03, Kaptain K wrote:
It is my opinion, that this dead horse has been beaten sufficiently./phpBB/images/smiles/icon_smile.gif


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. /phpBB/images/smiles/icon_razz.gif /phpBB/images/smiles/icon_smile.gif

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

Kaptain K
2002-Mar-31, 04:19 PM
mea culpa

ljbrs
2002-Mar-31, 04:31 PM
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



_________________
*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 15:18 ]</font>

ljbrs
2002-Mar-31, 05:00 PM
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 /phpBB/images/smiles/icon_redface.gif /phpBB/images/smiles/icon_frown.gif /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_eek.gif /phpBB/images/smiles/icon_confused.gif /phpBB/images/smiles/icon_cry.gif

<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>

johnwitts
2002-Mar-31, 09:15 PM
'Borrowing' Hat Monsters diagram...

http://sushi.dbestern.net/forumstuff/forces.png

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.

GrapesOfWrath
2002-Mar-31, 10:36 PM
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.

ljbrs
2002-Apr-02, 01:48 AM
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 /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_confused.gif

ljbrs
2002-Apr-02, 03:35 AM
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 /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_confused.gif /phpBB/images/smiles/icon_wink.gif

GrapesOfWrath
2002-Apr-02, 09:31 AM
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 (http://www.badastronomy.com/phpBB/viewtopic.php?topic=878&forum=9&start=0&start=24)--that gravity itself could be considered a pseudo force. That doesn't mean that we shouldn't use the term "gravity" though.

ljbrs
2002-Apr-03, 02:05 AM
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 /phpBB/images/smiles/icon_biggrin.gif /phpBB/images/smiles/icon_smile.gif /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_wink.gif

SeanF
2002-Apr-03, 02:16 PM
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. /phpBB/images/smiles/icon_smile.gif

GrapesOfWrath
2002-Apr-03, 02:43 PM
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.

SeanF
2002-Apr-03, 03:02 PM
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 (http://www.badastronomy.com/phpBB/viewtopic.php?topic=878&forum=9&start=17) 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."


_________________
SeanF

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

Mr. X
2002-Apr-03, 08:11 PM
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... /phpBB/images/smiles/icon_smile.gif

ljbrs
2002-Apr-04, 12:59 AM
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 /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_confused.gif /phpBB/images/smiles/icon_wink.gif

David Hall
2002-Apr-04, 10:49 AM
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?

DoctorDon
2002-Apr-04, 05:24 PM
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? /phpBB/images/smiles/icon_smile.gif

Don

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

Chip
2002-Apr-04, 06:01 PM
On 2002-03-30 21:59, The Bad Astronomer wrote:
[quote]
"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." /phpBB/images/smiles/icon_wink.gif


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. /phpBB/images/smiles/icon_smile.gif

ljbrs
2002-Apr-04, 11:31 PM
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 /phpBB/images/smiles/icon_frown.gif /phpBB/images/smiles/icon_eek.gif /phpBB/images/smiles/icon_smile.gif

DoctorDon
2002-Apr-05, 02:38 AM
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

The Bad Astronomer
2002-Apr-05, 02:45 AM
On 2002-04-04 13:01, Chip wrote:
[quote]
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. /phpBB/images/smiles/icon_smile.gif


That's different than a planetary nebula. I have page about this here (http://www.badastronomy.com/bitesize/sn87a_threering.html). Expanding spheres are relatively rare in astronomy; you usually get some degree of rotational symmetry (like rings, or barrel shapes).

Chip
2002-Apr-05, 04:21 AM
On 2002-04-04 21:45, The Bad Astronomer wrote: "That's different than a planetary nebula. I have page about this here (http://www.badastronomy.com/bitesize/sn87a_threering.html). 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

ljbrs
2002-Apr-06, 01:27 PM
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 /phpBB/images/smiles/icon_rolleyes.gif /phpBB/images/smiles/icon_confused.gif

The Bad Astronomer
2002-Apr-06, 04:29 PM
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 (http://oposite.stsci.edu/pubinfo/pr/1999/20/index.html). 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>