The following is a discussion of this statement:


Alright, why centrifugal force isn't actually real.

Centrifugal force is the notion that when you are on a spinning object there is some force that is trying to throw you outwards off of it. Like when you're traveling in a car and it makes a sharp rightwards turn you feel like you're being thrown to the left.

this is simply a misconception of what is really happening, in that you are not being thrown any direction, your body is actually just trying to continue in the straight line it was headed in before the car abruptly turned. The CAR is actually applying a rightwards force on you, which is what keeps you inside and going the same direction as the car itself, but most people misconceive of this as YOU pushing out against the car.

Centripetal force is the force of the car holding you inside and keeping your body moving in a circular path instead of flying straight like it wants too.


This conversation came originally from this thread:
http://www.lucasforums.com/showthread.php?s=&postid=1637275#post1637275)

I've created this new thread so as to not de-rail the original.

...anyway - what you seem to be missing here ET, is a basic law of physics:

'For every action, there is an equal and opposite re-action'.
...it doesn't say
'For every action, there is an equal and opposite 'imaginary' re-action!'.

The reaction of centrifugal force to centripital force is just as real as any other force - I assure you. The fact that it happens to be a 're-actionary' force is just a detail.

In your example, the force applied by the car on the person is the centripital force. The person's body is not able to take the course it naturally wants to take because the car is forcing against that person - to deviate their course...

...however if the locking mechanism on the door were to break, or the window smash due to the 'force' of the body pressing against it, it would NOT be centripital forces causing the door to break (centripital forces are only acting on the person in the car -NOT the car itself..) - it would be the centrifugal force acting in opposition to the centripital force. (i.e. the person forcing against the car door to try and keep moving in a straight line..)

...and I know of NO 'imaginary' forces which can break door mechanisms...

I assure you, both forces are as real as each-other. Just because a force is a result of a re-action, doens't mean it's imaginary!

..if this were really the case, rockets would have no 'real' forward thrust (only imaginary), since the forward thrust of a rocket is a 're-action' to the force of the combusted fuel being ejected in the opposite direction...
(or the forward thrust of the rocket is real and the backwards thrust of the combusted fuel is imaginary - if you so choose. Doesn't matter - either way is incorrect, because neither force is 'imaginary')

iamtrip makes a better point than he realises ;)

If you want to find 'imaginary' forces, you'll have to look to the Star Wars films, cos you'll find none in reality :D

To cover some other points you made in the other thread:


That website was wrong about centrifugal force being the third law counter to centripetal force,


No - actually the website (http://www.infoplease.com/ce6/sci/A0811114.html) got it spot on. This isn't some quack webiste, it's a perfectly legitimate source of information...


in the case of someone OUTSIDE of the earth it is gravity that is countering centripetal force, nothing else.


No - not really accurate. When considering the Earth and everything attracted to it because of it's gravitation (including us), the centripital force IS the Earth's gravity, combined with it's rotation. i.e. we feel the effects of this centripital force.

The counter-force (the perfectly real centrifugal force) is felt by the Earth itself, causing it to 'wobble' slightly. Of course any wobble on the Earth caused by our little insignificant selves is not worth mentioning.
...the centrifugal effects of the moon as it rotates in Earth's orbit - however - causes a far more noticible 'wobble' on the Earth...

This same principle can be seen when considering the Sun and all the planets which orbit it.
The planets are kept in orbit by the centripital force of the Sun's gravity. In reaction to this, a centrifugal force acts on the Sun, causing it to 'wobble' back and forth...

...I'm hoping this example helps you understand that both the centripital and centrifugal forces are both legitamate, 'real' forces. One is acting on a planet (keeping it in orbit), and the other is causing a massive star to wobble back and forth several million miles away!


In the case of a car it's a normal force of you pushing back against the car door, not centrifugal force.


The truth is actually the exact opposite of what you just said...

...[the] force of you pushing back against the car door, IS a centrifugal force. (You know, the one you don't believe is really happenning...)

Any force acting on the person by the car is the centripital force (created by the car.)
...but any force acting by the person on the car is the centrifugal force (created 'by the person' as a result of that persons momentum..)

While Renegade is slightly correct in pointing out that centrifugal and centripetal "forces" are terms used to describe the behavior of say... the water in a bucket that's being spun around, and the bucket itself,..

I have to agree with ET, in that calling them "forces" is redundant, since the force keeping the water in the bucket is in fact inertial force. Even physicists agree upon this point, saying that centripetal force is the counter-force to the INERTIAL force operating on the water in the bucket, not "centrifugal FORCE", which they call a "pseudo-force".

And though physicists claim that the act of gripping the bucket and preventing it and the water from flying away is in some way exerting "centripetal force" upon the object, I think this too is redundant, as other forces account for this. And also that the force keeping the water from breaking the bucket and flying free isn't "centripetal force" but is in fact the molecular integrity of the material which the bucket is made of and the molecular forces thereof.

One must remember that these terms were invented and bandied about quite a long time ago. They should be updated to conform more closely with international standard physics terminology IMO. As such I consider centrifugal "force" to be obsolete as a term.

Well - after looking more into the concept of 'pseudo' forces, I must admit I do have a bit of egg on my face, since I didn't even think such a term existed.

So unreserved apologies to you ET. I was certainly wrong to think that the idea of 'imaginary' forces wasn't an established one. I was definalty wrong there...

...however, the more I look into it, the more I think nothing is really settled on this issue - as you have already eluded to Spider.
For example, take a look here:

http://www.physicsnews1.com/question_5.html)

Now - while I'm fairly sure this is reasonably recent, of course I can't give you the immediate credibility of the writer. That would take some investigating.
...but what he's saying is exactly what I've been saying. And I still don't see why I'm wrong - even though there is obviously a fair amount of educated opinion which disagrees with me...

...why aren't the acting and re-acting forces involved in circular motion treated in the same manner as other acting / re-acting forces - like the rocket example I gave earlier...
...or did I get it wrong about the rocket - are one of those forces (the fuel being pushed out the back and the rocket itself being pushed the other way) ... viewed as 'imaginary'?! ... if so, why?

To me (and maybe this is the old-school thinking being mentioned in that link...) a force is something which causes a quantifiable effect. Whether it's to do with something active or something re-active, or whether it's because of the existience of something or the lack of something is just a detail of what causes that force.

...and to me, centrifugal and centripital forces come under the classic heading of 'opposing forces'.

To me, this renaming of things in modern physics doesn't represent a new level of understanding at all. It's just a renaming excerise- specifically of what the word 'force' means...

For instance, Spider - you said that instead of using the term centrifugal force (which apparently is pseudo in nature..), you should more correctly say INERTIAL force.
...so is this INERTAL force you speak of imaginary too? Or is it real?
If it's still imaginary, you seem to have just swapped the term 'centrifugal' for 'inertial'. And since I already knew that centrifugal forces (as I think of them) are caused by inertia, that name tells me nothing new...
However, if this is a 'real' force, then I guess you need to explain to me why changing the name has suddenly made it 'real'?

In short, I'm dubious as to whether this change is actually triggered by (or actually allows) a more fundemental understanding of physics. I suspect this may be more about - as you've hinted towards - merely an exercise in standardisation concerning the word 'force'...

So while I will admit I seem to be a little behind the times, I would like to at least have it recognised that - as far as I'm aware - going by the 'classic' definition of a force, everything I've written thus far is absolutely correct.
...and I think any calculations I ended up peforming would come out just as correct as any performed using 'imaginary' forces..

But I suppose that's like my Dad telling me he could count his money just as well when he used shillings and farthings :)

...man - now I feel old :/

The issue here is that centrifugal force is trying to describe the feeling of you being thrown OUTwards from a rotating object, when in effect you are truly only feeling the effects of the centripetal acceleration..

I shall demonstrate witha diagram.
http://img65.exs.cx/img65/2097/gravity.jpg)

In this picture imagine that we have a person standing on Earth's surface at point A. The arrow up represents the force of the centripetal acceleration acting on the man due to the rotation of the earth. The down arrow is the force of the earths gravity. Now, technically these are not force pairs, because a newtons force pair is denoted as force of B on A is equal and opposite to the force of A on B.

In our case, there are a lot more forces at work in this picture than I have drawn. For example, the force pair to gravity holding me down is my gravity pulling the earth up (from my perspective) The pair to the centripetal acceleration on me from the earths rotation is the centripetal acceleration on the earth by ME. Nowhere in these forces is centrifugal force needed. Now if you wanted to call one of the forces centrifugal force you COULD, though it would be incorrect, because we already have names for the other forces acting on objects.


And it's not really a renaming of physics notions as you imply in your post, but an updated sense of understanding exactly what is occuring between objects, and therefore using new terminology that is more significant and descriptive of what is really going on.


Furthermore, your description of the forces in the solar system are incorrect in that it is gravity and electrical forces that hold the planets to the sun, it is the centripetal force of their rotation that keeps them AWAY from the sun as well.

Edit - the equation for determining Centripetal acceleration is (mass*velocity^2)/radius
This makes sense, because the faster you swing something in a circle the more it feels like it wants to pull away from you.

Yes -I think I'm getting what your saying ET.

...all the forces I know of and understand are present in your explinations. And while it does seem to be a renaming exersise, I think I'm understanding why these naming changes are probably helpful.

...ok - looks like I'm going back to school :) Thx for this heads up. And forgive an old fool like me for not keeping up with the times ;)

RenegadeOfPhunk:

...and to me, centrifugal and centripital forces come under the classic heading of 'opposing forces'.

To me, this renaming of things in modern physics doesn't represent a new level of understanding at all. It's just a renaming excerise- specifically of what the word 'force' means...

For instance, Spider - you said that instead of using the term centrifugal force (which apparently is pseudo in nature..), you should more correctly say INERTIAL force.
...so is this INERTAL force you speak of imaginary too? Or is it real?
If it's still imaginary, you seem to have just swapped the term 'centrifugal' for 'inertial'. And since I already knew that centrifugal forces (as I think of them) are caused by inertia, that name tells me nothing new...
However, if this is a 'real' force, then I guess you need to explain to me why changing the name has suddenly made it 'real'?

In short, I'm dubious as to whether this change is actually triggered by (or actually allows) a more fundemental understanding of physics. I suspect this may be more about - as you've hinted towards - merely an exercise in standardisation concerning the word 'force'...You ask a valid question, and I agree IN THAT I don't think that it matters what we call the force that keeps the water in the bucket...

I mean, you could call it banana force for all the difference it would make...

But A: standardising scientific terms is one of the most important things in science since all scientists have to sing from the same hymn-sheets.

And of course, B: Inertia is a basic force. Calling the effects of angular momentum "centrifugal force" is like calling sea-salt "an element". Salt's made up of two basic building blocks, sodium and chlorine. Likewise the behaviour of the water in the bucket is made up of several basic physical building blocks, that are termed "forces". Such as inertia.

So the update is all good! :D

Well - after looking more into the concept of 'pseudo' forces, I must admit I do have a bit of egg on my face, since I didn't even think such a term existed.

So unreserved apologies to you ET. I was certainly wrong to think that the idea of 'imaginary' forces wasn't an established one. I was definalty wrong there...It takes a big man to say stuff like this, especially on an anonymous internet forum. You certainly have MY respect for doing so.

Originally posted by Spider AL
It takes a big man to say stuff like this, especially on an anonymous internet forum. You certainly have MY respect for doing so.
Deffinately, It's a nice refresh after debating against people who refuse to acknowledge that they may be incorrect, even if it's mostly just an issue of semantics (as in our case it seems)

I will appologize a bit for some of the fervor I brought into my arguments, but I have recently contemplated changing majors into physics, so I'm very outspoken on the subject ;)

It takes a big man to say stuff like this, especially on an anonymous internet forum. You certainly have MY respect for doing so.


Heh -I've seen too many truly hideous examples of people who knowing full well they've got it wrong trying anything they can to squirm out of earlier statements...

...best to just bite the bullet and get on with it... ;)


Well -now that (I hope) were all friends again, maybe one of you can explain a bit more about how this new way of thinking affects Newton's third law? (I'm wondering if it still has any relevance :) )
...from the way you were talking earlier ET, it seemed that you let the opposing forces implied by the third law effectively cancel each-other out in your calculations - in a way that means you can effectively discard them?! ...is that in any way accurate?

..if this is accurate - doesn't this mean that you've effectively negated the need for the third law at all?!!

Specifically, I mean when you said this...


Now, technically these are not force pairs, because a newtons force pair is denoted as force of B on A is equal and opposite to the force of A on B.



I will appologize a bit for some of the fervor I brought into my arguments

No need to apologise. If anybody was acting like a twat, it was me to be honest...


So what is your name for the force which causes the Sun to 'wobble' as the planets orbit it..?

...ahh hang on - think you've already explained it. The wobble on the Sun is simply caused by ... the gravitational force of the planets themselves .... right?

....hmmm - well, as annoyed as I am that I have to relearn a whole load of crap, I must admit that does seem a far simpler way of looking at it...! But I'm still wondering where Newton's third law comes into play here...if at all...

It's not that they cancel each other atall, it's just that the FORCE between them is equal. I am currently exerting the exact same amount of gravitational force on the Earth as the earth is exerting on me. A bizarre concept, but quite true. The reason I am so affected is because of our masses. The force has a more pronounced effect on me and my small mass, while the earth's huge mass is relatively unaffected.

You cannot discard them because the forces they are applying are very real, it's just that they are in effect completely equal and opposite. Just as in a car accident between a Cooper Mini and a Mack truck, both vehicles exert the same amount of force on each other, but the mini gets destroyed because it's small mass is accelerated much more quickly than that of the truck.

I am currently exerting the exact same amount of gravitational force on the Earth as the earth is exerting on me. A bizarre concept, but quite true. The reason I am so affected is because of our masses. The force has a more pronounced effect on me and my small mass, while the earth's huge mass is relatively unaffected.


Yes - this is all suddenly clicking together for me. And I'm finally understanding why your correct in saying that this is more than just a simple renaming exersise.

I mean, I did already understand what you've just said above - in practical terms. I hope this should be obvious from what I posted in my first post.
But the problem is that not only did I have the names of the forces wrong, I also think I had the forces pointing in the wrong directions...

Like you were trying to explain to me earlier - you dont' think of the centripital force as pointing inwards to the centre of the circular movement anymore. The force now points (more logically I must admit) in the direction of the circular movement itself (i.e. at a right-angle to the way I used to think of it).

And this now solves my temporary distress about what i saw as Newton's third law being scrapped!
I only thought the counter-forces had disappeared because I still had my forces (stubbonly ;) ) fixed in the wrong directions..

Ok - so I must say I'm converted. This new way of approaching forces is just plain a better way to approach things...


...I guess I only have one final question - a pedantic one perhaps, but I'll ask it anyway...

Why is centripital force (or I guess the term you were using was centripital 'acceleration' earlier) still called 'centripital' anything, since the force (or the acceleration caused by the force -whatever) is not strictly 'centre-seeking' anymore - as far as I can see...

..it's now at a right-angle to 'centre-seeking'. Is this just carrying a familiar term forward to help us fuddy-duddy's adjust or something?
...if so, let me suggest that it's (well, I was gonna say 'wrong', but I'll say 'unfortunate' instead...) to use the same term for a force that has now 'apparently' changed it's direction at a right-angle from where it used to point...!
Or is this another case where I'm not quite understanding the new terms...?

You're still a little off on Centripetal acceleration, it is not perpendicular to your old perception of it pointing inwards. Centripetal acceleration is outwards. Were the earth to start spinning much faster, eventually our centripetal acceleration would be approximately 9.8 m/s/s and would be a complete counter to gravity, meaning that we would experience weightlessness on earth, because we'd have two forces of equal magnitude pointing in opposite directions, and they would therefore cancel meaning we would experience no NET force.

Centripetal acceleration is outwards.


Why is the word 'centripital' - meaning centre-seeking - used to describe a force acting in a totally opposite direction?! (i.e. 'outwards' from the centre?!)


Were the earth to start spinning much faster, eventually our centripetal acceleration would be approximately 9.8 m/s/s and would be a complete counter to gravity, meaning that we would experience weightlessness on earth, because we'd have two forces of equal magnitude pointing in opposite directions, and they would therefore cancel meaning we would experience no NET force.


I'm not trying to be confruntational here - honestly I am not. But your telling me something I already knew...

I would have worked out what you just said just fine with my old 'understanding' of centripital and centrifugal forces.

You would have said the terms are wrong, and the centrifugal force was 'imaginary', but I would have come to the same conclusion...

So please -you don't need to keep explaining the fact that forces can completly cancel each-other out and what-not. I totally understand and comprehend this...

This is just a matter of understanding the new 'lingo', and the new way of looking at things.


Now that we've covered the 'Earth and us' example, maybe we can cover one of your previous statements in the other thead:


Centripetal force is the force of the car holding you inside and keeping your body moving in a circular path instead of flying straight like it wants too.


Could you do a couple of things for me...

a. Draw a quick diagram showing exactly what direction this centripital force is acting in this case and

b. Describe the counter-force to this centripital force (required to fulfill Newton's third law) and what effects this 'counter' force is having in this senario...

Hopefully this will clear up what I'm missing here...

As I look more and more into various explinations from various sources about why centrifugal force is 'imaginary', I notice a reccuring theme...

They always talk about a very specific type of senario where - for example - a person is travelling in a car and that car changes from a straight path to a curved path.

...they usually say something like this:

'Many people are under the misconception that when someone ends up sliding across the car seat' as the car turns the corner, that this movement is caused by 'centrifugal force'. This is in fact not true. The person sliding may seem to feel like there is a force 'pushing' them towards the side of the car, but this is in fact a force only in their perception. Or in other words, it is an 'imaginary' force. The truth - in fact - is that the person is sliding across that car seat not because a force is causing that movement, but because their existing inertia - built up by the force of the car seat on their body while the car was travelling in a straight line - is now causing that body to continue moving in that original straight line.
...you can see this more clearly if you consider that it is actually the car itself that is changing course. And so rather than the person sliding along the car seat, it is actually the car itself that is changing course 'under' the person, while the person continues to move in a straight line due to their inertia..'

Well fine. I've got no problem with that. And you wanna know why? Because I already knew that before you said anything to me...!
...you don't beleive me? Heh - perhaps you think I'm trying to do exactly what I said I wouldn't do - i.e. try and squirm my way out of earlier statements...?

I really am not. I already completely understood that when someone slides along a car seat when it turns, it is not a force which causes that - centrifugal or otherwise.
...to confirm I'm telling the truth, look back on my previous posts and find any instance where I stated this. i.e. that when someone slides around in a turning car, that's to do with centrifugal forces...

...you won't find any, cos I never said it.

...what I DID talk about, however, was the situation after that person had finished sliding and was in contact with the door - and where the centripital force which starts moving them along with the car starts kicking in.
...only at THIS point was I arguing that centripital and centrifugal forces come into play. And the more I think about it, the more I can't grasp why it is that what I said in the first post of this thread - (thinking specifically about the body in the car senario here) was in fact wrong.

I'm sorry if you think I'm wasting your time here, but I would appreciate it if you could again consider what I said in that first post...


In your example, the force applied by the car on the person is the centripital force. The person's body is not able to take the course it naturally wants to take because the car is forcing against that person - to deviate their course...

...however if the locking mechanism on the door were to break, or the window smash due to the 'force' of the body pressing against it, it would NOT be centripital forces causing the door to break (centripital forces are only acting on the person in the car -NOT the car itself..) - it would be the centrifugal force acting in opposition to the centripital force. (i.e. the person forcing against the car door to try and keep moving in a straight line..)

...and I know of NO 'imaginary' forces which can break door mechanisms...


First of all, just to make it clear once more that I'm not talking about the body sliding around on the car seat..
...I am talking about the body being forced into a curved path by the car door.

Secondly, I want to (hopefully) confirm that the force of the door against that person is called the 'centripital force' (centre-seeking force).

ANd thirdly, I want to confirm that there is an equal and opposite force acting counter to the centripital force.
We both knew it was there, cos we both said so.

Here is what you said in the other thread:

In the case of a car it's a normal force of you pushing back against the car door, not centrifugal force.


Here is what I said concerning that same force:

it would be the centrifugal force acting in opposition to the centripital force. (i.e. the person forcing against the car door to try and keep moving in a straight line..)


You know what, we are both talking about exactly the same bloody force. And we both knew that it was a real, non-fictional, non-imaginary force. The fact I happenned to call it a 'centrifugal force' is but a detail... doesn't mean it suddenly becomes imaginary because of the name I happen to give it - now does it...!!

You can object to the name 'centrifugal' if you like. That's your call. Whatever turns your crank I guess. I could argue that since this force (that we both agree is present AND perfectly real) is acting away from the centre of the circular motion and that centrifugal means 'centre-fleeing', that centrifugal is a perfectly sensible name for this particular force.
...but as we've already agreed -- this is all just semantics.

...what I really want to contest here is that I ever believed in any 'imaginary' forces - cos the fact is I never did.
...I think you heard the word 'centrifugal' and instantly thought 'imaginary'. But I don't think you really took the time to consider the exact force that I was actually talking about. (Which in fairness was in fact real - just as I said it was at the time...)

Now, if you're going to call what I called the centrifugal force, the 'inertial force' of the person (i.e. use the source rather than the direction for the name), why is the term 'centripidal' still being used?!
...why aren't you ditching the term centripital too - and say the 'momentum of the car'..?' (Again, use the source rather than the direction?)

..how is it making things clearer to ditch the directional name for one force, but keep the directional name for it's counterpart?!

...I mean don't get me wrong, I'm sure there is a good reason. But instead of teaching me the evils of imaginary forces I never believed in in the first place, and re-itterating other princpiles of physics to me that I already perfectly understand - just explain to me, clearly and simply, why getting rid of the term 'centrifugal' force (which can be used in the context of a 'real' force - as I've just shown..) suddenly makes things so much clearer?!
...cos right now - to me - it's looking as clear as mud...

Well I honestly cannot tell you WHY the name change, I was taught that there is no centrifugal force, and so I was unaware that it used to be taught that there WAS centrifugal force.

However I DO realize that you understand newtons laws, and I feel a bit ashamed because I think I'm misleading you and quite possibly even giving you straight out WRONG information as far as centripetal force goes.


I'm having a really hard time formulating the discussion about centripetal force into words now, I've sort of flustered myself, so later I might post some text out of my physics book and it may clear things up. :)


Edit - Okay, here is the basic description of Centripetal force from my book

Rounding a curve in a car. You are sitting in the center of the rear seat of a car moving at constant high speed along a flat road. When the driver suddenly turns left, rounding a coerner in a circular arc, you slide across the seat toward the right and then jam against hte car wall for the rest of the turn. What is going on?
While the car moves in the circular arc, it is in uniform circular motion; that is, it has an acceleration that is directed toward the center of the circle. By Newton's second law, a force must cause this acceleration. Moreover, the force must also be directed toward the center of the circle. Thus, it is a centripetal force, where the adjective indicates the direction. In this example, the centripetal force is a frictional force on the tires from the road; it makes the turn possible.



A centripetal force accelerates a body by changing the direction of the body's velocity without changing the body's speed


There, the last quote basically sums up centripetal force, and I have been telling you quite wrongly and misinforming you. And for this I am quite sorry.

No worries ET. I dont' think you should blame yourself. It seems to me that nobody has a straight answer on this - not even the 'professionals'...

Seems to me that this all came about because a lot of people used to use the term 'centrifugal' force inappropiately. (The example of the person sliding around in the car is the most obvious one).

...so I guess someone decided 'OK - let's just scrap this term - cos people aren't using it right.'

OK, and I can understand this to an extent...
But implying that ANY use of the term 'centrifugal' force must be imaginary (which to be fair, many [what you would have thought as] perfectly legitamete sources are saying - in such a way that this is the natural conclusion most sensible people would reach from what is written) is not accurate at all...

...you HAVE to look at the context in which the term is used before calling any instance of centrifugal force imaginary...


And as far as the definition of centripital force:


A centripetal force accelerates a body by changing the direction of the body's velocity without changing the body's speed


...phew! That's a relief. So I don't have to sue my school for teaching me crap in A-level physics then ;) Cos that's how I understand it too :)

Can you just not agree that the force is flowing through us?

A thread about physics? Wow! And I missed it? Damn!

Isn't it already an estabilished fact that all objects going in a certain direction "want to go on in that direction"? What's there to debate?

The debate was over whether or not there is such a thing as centrifugal force.

I've actually come up with a good (at least I think so) description of why physicists use the term Centripetal and not centrifugal.

The reasoning is that applying a force implies exerting something that will cause an acceleration, due to the law of nature Force = mass*acceleration.

So lets look at the car example. The car turns, you fly outwards, the door holds you in and you push the door out.

The door is accelerating you inwards, towards the center of the circle it is moving in. THIS is centripetal force, because it is actually accelerating you.

Your push on the car door, while you ARE exerting a force, you are NOT accelerating the door, and therefore it is NOT centrifugal force :)

The movement outwards in a circle is actually a matter of momentum, and not of force at all. The force that is applied to the door is a normal force, due to Newton's third law. The door is applying a force to you, so you are applying a force of equal magnitude and opposite direction on it.

Yeah, we've already discussed that part of the laws into the dirt :D

Yeah, we've already discussed that part of the laws into the dirt Bluhhh, thread should never have been bumped by that darned ShadowTemplar... :D

If you hit a car door, you transfer part of your energy into it and then use the rest to bounce back (if there's enough energy).

Scientifically, gravity, too, is explained this way in a sense, too: You stay on the floor, which absorbs the energy that would otherwise make you keep falling. Clumsily put, but you know what I mean.

Heh - indeed Spider, this thread probably shouldn't have been ressurected. ;)

...but now that it has, and now that ET has gone and said this:


The door is accelerating you inwards, towards the center of the circle it is moving in. THIS is centripetal force, because it is actually accelerating you.

Your push on the car door, while you ARE exerting a force, you are NOT accelerating the door, and therefore it is NOT centrifugal force


I feel I must retort.
Heh, ok. Nice try ET ;)

Your choosing to imply that because the centrifugal force isn't actually causing 'centre-fleeing' movement, that means you can't actually call it centre-fleeing.

...you choice to see it that way I guess...

If the door mechanism were to fail, or the guy in the car were a particularly fat b**tard, then the door could very well end up being forced away from the centre of movement (by being ripped from it's hinges), and nothing would have changed about the basic nature of the force involved, only it's magnitude or the body the force is being exerted against.

...so why you would suddenly decide this force requires a different name just because the door happens to be bolted to the car is beyond me.
I think you can glean a clue from the word centripital - meaning centre-seeking. Notice the 'seeking' bit. It seeks to get to the centre. Doesn't mean it's going to actually reach the centre, it just means it WANTS to get to the centre.
...and since centrifugal is the opposite of centripital, this means that objects having a centrifugal force applied to them WANT to get away from the centre of movement. (e.g. the door of the car) Whether they can or not is a different matter, but doesn't change the nature of the force, or what you should call that force...

You pull a fridge door. You don't pull hard enough to break the seal and it doesn't move an inch.
You pull again, but harder this time. The seal breaks and the door opens.
...you'd like to propose that instead of calling both instances a 'pulling' force, we call one a non-door opening force, and the other a door-opening force?! How is that making anything simpler, or helping us understand what's going on better?
..the fact is it doesn't. Most sensible people would just call that two instances of pulling a fridge door. One was big enough to open it, one wasn't. But both were instances of the same force...

...I think your really clutching at straws to try and keep this myth of centrifugal force being fundementally 'imaginary' intact...

Consider the example of the Earth orbiting the Sun. This is an example where there is NO good reason why the term 'centrifugal' force cannot be used 'correctly'. (Assuming the term 'centripital' force is still legitamete), since the Sun is INDEED moving in a centre-fleeing fashion as it is being tugged by the Earth along it's orbit...


You can quote your text books all you like. You can even drop the word centrifugal force and called it 'inertial' force if you like.

Potato. Potaado.
..I can choose which termanology I'm confortable with as long as it leads to correct results. Thinking in terms of centripital forces and centrifugal forces leads to correct conclusions, because it's still the same stuff your talking about - it's just using the direction of the force rather than the source. That's it.
Trying to imply there is more to it than that is missing the point entirely.
..I'm with this guy: http://www.physicsnews1.com/question_5.html)

Centrifugal force is only imaginary in particular cases where people misunderstand the basics of physics. When used appropiately and in context, centrifugal force is perfectly real...

I've already swallowed humble pie over not knowing that such a concept as an 'imaginary' force existed.
...I think it's your turn to cut a slice for yourself here. Centrifugal force is only imaginary when the term is used innaccurately. End of story...

The only legitemate reason I can see for ditching the term centrifugal force is to keep termainology consistent, as Spider has mentioned. And I accept this...
...but it's still wrong to call all centrifugal force 'imaginary'. This has got to be one of the most astounding myths in modern physics I've ever come across...!
..and many perfectly intelligent people seem to have swallowed it whole!! :eek:

My explanation was spot on, I take none of your pie :xp:

it is called centripetal force because it is causing an acceleration. It is actually accelerating you inwards. YOU are NOT causing an acceleration on the door by pushing it outwards.

You are exerting a normal force on the door, and the door is exerting a normal force on you. We call the normal force from the door the centripetal force because it's actually accomplishing an acceleration.


But yes, I suppose if you felt that calling your normal force on the door the centrifugal force made you feel warm at night you could always do that and still be pretty much correct in your description of the forces :D

There is no force that goes outward in the circle. There is a force accelerating the object towards the center of a circle. The forward momentum is carrying that object in the direction based on the tangent of its current path. The resultant motion is around in a circle. There is no force that moves an object directly away from the center of rotation.

http://www.geocities.com/jedi_duck_1138/untitleda.GIF)

In the specific instance of the car door, were only debating what deserves to be called 'centripital' or 'centrifugal'. I just want to clarify this has nothing to do with imaginary or real -we both know this force is real, no matter what we call it.
I believe my fridge door analogy is perfectly adequate to back up my view on things, but I do accept you can see it your way too - in THIS particular case...

...but I've gotta be clear, are you STILL claiming that using the term centrifugal force is always going to be describing an 'imaginary' force? (This is the only point I really care about in all this and my only main point of contention with you btw...)
...because if you are, the above debate isn't gonna help you. If the door was ripped from it's hinges and then flies outwards then by your own definition you have a legit, real centrifugal force causing that motion.
And you REALLY need to tackle the point I made about the Sun and the Earth. You have NO right to call that 'centrifugal force' imaginary OR non-centrifugal, since the Sun is indeed being pulled away from the centre of the circular motion...
..sure I can't tempt you with some of this pie? ...quite tasty ;)

Shok_Tinoktin, trust me - your about 5 steps behind us in this debate.
When someone is pressed up against a car door -where the car is travelling in a circular motion, the door is feeling a 'real' force, and it is the EXACT opposite to centre-seeking. THis is what we are debating here.
What you decide to call it isn't relavent to the fact it exists and it's real...

in order for a force to be the direct opposite of a "center-seeking force", it would have to be going directly outward from the center of the circle. as for the car door analogy, the door is the centripedal force, so if there was a centifugal force, and the centripedal force was removed (i.e. the door ripped off its hinges), then you would accelerate outward from the circle. Such is not the case, you would move at the speed and in the direction you were already travelling, thus no force is acting on you, thus there is no centrifugal force.

Shok_Tinoktin,

Again, you are 5 steps behind in this debate...

...please don't make the mistake of taking me for a fool Shok - like ET did at the beginning of this thread... I know physics...

If the door suddenly didn't exist then yes, you would continue on your straight path beause of your already existing momentum, not because of any force being applied on you at that point.
...many people think this is down to centrifugal force. They are wrong. But I already knew this was wrong. This is a 'legitamete' example of an 'imaginary' force.

But imagine if there were an elephant crammed into the car. The elephant could easiely exert enough 'inertial' or 'centrifugal' force (Potato. Potaddo.) on that door to rip it off it's hinges and send it flying outwards.
...what causes that door to fly outwards? Is it the centripital force? Of course not - otherwise the door would fly INTO the car.
Is it like what we just talked about - i.e. it's own momentum? No - of course not. If this were true, the door woudl fly off EVERY TIME the car turned a corner.

...the door woudl be ripped off the car because of the FORCE being exerted by the body inside the car. It's acting away from the centre of the circular movement.
And this force is NOT imaginary!! Imaginary forces don't rip car doors from their hinges!!

QED

...Jesus, this is like pulling teeth... :rolleyes:

IT IS THE MOMENTUM OF THE OBJECT INSIDE AND NOT THE DOOR! An elephant has a greater mass and thus a larger momentum. The law of the conservation of momentum says that when two objects collide, momentum is conserved, but may be transferred. A sufficient momentum being given to the door and in the direction of the movement of the object could tear the door off the hinges purely as a result of the momentum itself.

The law of the conservation of momentum says that when two objects collide, momentum is conserved, but may be transferred. A sufficient momentum being given to the door and in the direction of the movement of the object could tear the door off the hinges purely as a result of the momentum itself.


Well done. You've come up with a fancy way of saying 'One object can apply a force onto another object when it slams into it'.

You don't seem to understand that the only thing being debated here is termaninolgy. Nothing more... So coming up with yet another way of saying the same thing isn't helping anybody...

The fact is my argument would be the same if somebody decided to kick the door off it's hinges (assuming they were strong enough of course). As long as the car was travelling in a circular motion, and the kick was directed away from the centre, then the force applied to the door by the kick would be a 'centrifugal' force -going by the definition of the word centrifugal i.e. 'centre-fleeing'...
..are you gonna tell me that kicking something doesn't exert a force on it? If so, please provide your definition of a force... I'd love to hear it...

...hell, just throw a bloody tennis ball out of the car window away from the centre of motion. Voila. You've just proved centrifugal force exists...

Please read the whole thread from the beginning. Perhaps that will help you understand what the actual point of contention is here...

I am content to agree, that you can CALL it centrifugal if you want to. I simply choose not to since that was the way I was instructed, but your use of it is like if you were talking about physics in german. Sure the words aren't the same but they've all the same meanings behind them :)


maybe a little pie

:)

..tell ya what, I'll have another piece if it makes you feel better mate :) It's pretty tasty stuff ;)

Thanks for at least acknowledging the point I've been trying to make... This isn't really about deciding who's right and who's wrong. It's - like you say - acknowledging you can actually speak the truth in two different 'languages'...

Telling me that all centrifugal force is imaginary is like going up to a Frenchman and saying 'Bonjour isn't correct! Hello is!'

(...waits for a 'Freedom Fries'-like comment ;))

Maybe my statement 'All centrifugal force is imaginary' is a 'myth' was a bit strong - to be fair.

...I just find it suspicious that all the examples I've seen of 'imaginary' centrifugal force involves something similar to sliding along a car-seat!
That doesn't explain anything, because as I've already shown in this thread, that was known to be a misnomer back when the term was accepted anyway!!
...I just don't trust that these people are explaining properly the REAL reasons for completely dismissing the term centrifugal force - in all instances...

First off, I would like to apologise for not reading everything first. All right, now that I have read the entire thread, I think I have a simple explaination of why there is a centripedal force, but not a centrifugal force.

A centripedal force pulls an object in a circle, by pulling it to the center of that circle (or pushing for that matter), but a sideways velocity of some kind prevents it from hitting.

A centrifugal force would thus have to pull an object traveling in a circle directly away from the center of that circle. I can think of no such force, but if anyone else can I will gladly concede.

In examples such as the car, the force that would tear off the door, would not be in the opposite direction of the center of gravity, and I think a diagram can show this:

http://www.geocities.com/jedi_duck_1138/Centrifugal.GIF)

The diagram above is absolutely correct - in my way of thinking...


A centrifugal force would thus have to pull an object traveling in a circle directly away from the center of that circle. I can think of no such force, but if anyone else can I will gladly concede.


I can conclusively PROVE to you that that centrifugal force you have penciled in your diagram HAS to be just as real as the centripital force...
It's because of Newton's third law:

For every action, there is an equal and opposite reaction.

NOTE: It DOESN'T SAY:
For every action, there is an equal and opposite imaginary reaction.
..the re-action is just as 'real' as the action.

What I think your mistaking is that you think the centrifugal force would be acting on the same object that's being affected by the centripital force. THis is incorrect.
Let me try and make it clear.

Person in a car
Centripital force = Door -> Person
Centrifugal force = Person -> Door

Earth orbiting the Sun
Centripital force = Sun's gravity pulling the Earth
Centrifugal force = Earth's gravity pulling the Sun

etc. etc.

If there is a 'real' centripital force acting towards the centre of the circular motion, then according to Newton's third law, there MUST be an equal and opposite 'real' force acting in the opposite direction (The centrifugal force you have marked on your diagram...)

I know ET and perhaps yourself have a different 'way' of explaining the same thing - but here is the important point.

If you ask me to predict what will happen in a given situation involving circular movement using real centrifugal forces, I WILL come to the correct conclusion, assuming I do my sums right...

I'm sure you will also come to the right conclusion with your methods as well - involving 'imaginary' forces.

But I'm happy using mine thanks ;)
..if it ain't broke, don't fix it - as far as I'm concerned...

The bottom line here is you can't tell me that centrifugal force (when refered to properly) is imaginary, because according to my definition of centrifugal force, it's perfectly real.
...I've just proven it using your own diagram...

I think the REAL point here is that thinking too much about it will make your head hurt :D


In Shok's diagram only TWO of those forces are actually forces, the blue and red arrows. The green arrow is just the current velocity of the object.

(lets pretend it's a ball on a string being swung around)

Now the OUTWARD force is the force of the ball pulling the string

The INWARD force is the force of the string pulling the ball.

We call these normal forces.

We call the INWARD force the centripetal force because it is accelerating the ball inwards.

We do NOT call the outward force centrifugal force because it's not actually causing an acceleration on the string.

HOWEVER, if you WANT to call it centrifugal I really think that's up to you, just don't expect me to know what you're talkin about :D


It's not so bad this pie, just a little bitter

We do NOT call the outward force centrifugal force because it's not actually causing an acceleration on the string.


But ET - in your example something IS being accelerated outward - the thing (I assume person) swinging this ball around them!!

The ball (perhaps minutely, perhaps not - depending on it's mass) will be pulling the person AWAY from the centre of motion.
...this is obvious with hammer throwers. They 'wobble' as the hammer pulls them away from centre as they spin...

So now - explain to me exactly why I'm not correct in calling the force which causes the 'swinger' to 'wobble' centrifugal

* It is a 'real' force
* It causes 'real' motion
* It is acting away from the centre of the circular movement...

You can call it whatever you like. I don't give a rats-arse. Just please tell me why the word 'centrifugal' doesn't make sense in this instance so that I shouldn't use it...
...you haven't come CLOSE to coming up with a reasonable argument on THIS specific point I'm afraid...


..at some point you HAVE to admit (without saying it and then trying to take it back again in your next post) that this is JUST terminology and NOTHING MORE.
By the definition of centrifugal force (centre-fleeing), when a hammer thrower wobbles, you can classify the force which causes that wobble as centrifugal force.
...the only way to make that not true is to alter the definition of centrifugal force (i.e. make a new language). There IS no other way...

a net force (vector) in a certain direction acting on a mass (scalar) will have a resultant acceleration (vector) in that same direction. If you have a heavy object on a rope, and you swing it in circles, then let go, it will continue travelling in a straight line given the tangent of the circle at the point where the object was when the rope was released. The direction of the acceleration will be in the direction of the inertia arrow on my diagram. So, working backwards, it can be seen that the force acting on it was not travelling in the direction of the centrifugal force arrow.

If you have a heavy object on a rope, and you swing it in circles, then let go, it will continue travelling in a straight line given the tangent of the circle at the point where the object was when the rope was released.


Your not keeping up with the conversation here Shok. I've already covered things like sliding across a car seat when a car turns, or when something flies off when you let go of it whilst spinning it round.

It has nothing to do with ANY force. It's only existing momentum generated from a previous force. I already know this perfectly well...

This is DIFFERENT to the REAL centrifugal force I'm talking about. Belevie me, it's perfectly real. You just have a different name for it. That's all.

I just believe my name is perfectly valid. I have not heard one argument to suggest my name (centrifugal force) doesn't make sense, or isn't 'real'. (When used properly of course - but that goes without saying...)

ok, ok I think I may have a solution. I haven't thought this out to much, but bear with me.

When drawing a force diagram, you have to consider the objects one at a time.

The forces acting on the person: Normal/centripedal force.

The forces acting on the car: Friction/centripedal force.

The forces acting on the ground: Applied force.

The forces acting on the door: Applied force, normal/centripedal force.

If I am not mistaken, you are claiming that the "centrifugal" force is being applied to the door. In actuality, the applied force is in the direction of the momentum of the person, and the normal/centripedal force is a result of the car, pulling the door in a circle.

If I am not mistaken, you are claiming that the "centrifugal" force is being applied to the door. In actuality, the applied force is in the direction of the momentum of the person, and the normal/centripedal force is a result of the car, pulling the door in a circle.


Right - NOW your starting to catch up Shok So maybe you can now move on to the real question...

Since we are agreed that this force applied by the 'momentum of the person' is a real force (not imaginary) - and not only that, but it is acting in opposition to the centre-seeking force, (as predicted by the Third Law and which by definition makes it the centre-fleeing force...)

...and since centrifugal means (for the umphtempth time) 'centre-fleeing' - then why for the love of God can't I call it centrifugal force?!
*rasies arms to the sky* ..why can't I call a spade a spade for crying out loud...!! :D

You must understand Shok, that teaching that all instances of centrigual force are 'imaginary' is a relatively modern teaching. I did A-level physics, and we were taught very differntly.
..so far, I see no reason to pick up this idea of imaginary forces. To me, it solves nothing.


Here, I'll prove my point. Here is an explination of centripital and centrifugal forces in the 'old school' (and as far as I'm concerned CLEARER school) of thought...
http://www.infoplease.com/ce6/sci/A0811114.html)
..notice how centrifugal force IS perfectly real (no mention of it being 'imaginary' in any way...) It is the force-pair to centripital force...
..and again - if you missed this link earlier:
http://www.physicsnews1.com/question_5.html)




ok, ok I think I may have a solution. I haven't thought this out to much, but bear with me.


Believe me Shok, I already have a pefectly sound solution...
...it's to accept that centrifugal force (if you accept the literal definition of the word centrifugal of course) is perfectly real... :D

I don't need your solution thanks. No offense, but your welcome to it ;)

Originally posted by RenegadeOfPhunk
Since we are agreed that this force applied by the 'momentum of the person' is a real force (not imaginary) - and not only that, but it is acting in opposition to the centre-seeking force, (as predicted by the Third Law and which by definition makes it the centre-fleeing force...)

We do not agree. The momentum of the person is not in and of itself a force, and not in opposition to the center-seeking force. The momentum is in the direction that is labeled as "inertia" on my previous diagram. It is pushes the door in that direction. The door, in turn, pushes back in the opposite direction. These forces are not centripedal or centrifugal forces.

Shok, if you did not agree with my previous statement, then you do not fully understand the implications of Newton's third law, and as such, were not gonna get very far.

Your confusion on this matter is evident in the way you are contridicting yourself. You correctly stated that the force of the door pushing against the person is the centripital force, and yet now you say:


The door, in turn, pushes back in the opposite direction. These forces are not centripedal or centrifugal forces.


The force of the door pushing against the person IS the centripital force, and yet is NOT the centripital force?!
...huh?
...or do you believe that 2 seperate forces are acting from the door on the person?! (One centripital and one - ermm - re-actionary?!)

Either way, your dead wrong...

There is only 1 force acting from door against person. It is the centripital force caused by the movement of the car.
There is also only 1 counter-force acting in the opposite direction (predicted by Newton's law). It is acting between the person and the door caused by the person's momentum.
I call it 'centrifugal force'. You call it the force caused by their momentum.

You using the 'source' of the force to define it's name. I'm using the 'direction' of the force to define it's name.
..and here you are thinking you know more than I do :rolleyes: I can only pity you at this point...

But it's ok. Believe centrifugal force isn't real.
I'm truly past caring now. You can only bash your head against a brick wall so many times before you find something better to do... :rolleyes:

The car is what is exerting the centripedal force, via friction. The door supplies a normal force in the direction opposite of the momentum, which is not directly away from or towards the center of the circle. However, I too am beyond caring, but I find it hard to walk away form a challenge, so unless you stop trying or convince me, I'll probably keep at it.

The door supplies a normal force in the direction opposite of the momentum, which is not directly away from or towards the center of the circle.


Aha - so you think there is a re-actionary force - opposed to the direction of momentum! which is acting perpendicular or at some other offset to directly to OR away from the centre...

I understand where your coming from now. THe only problem is your wrong. What your probably getting confused with is the direction of the initial inertia
...but of course this isn't a force :) ..neither can it create a re-action force I'm afraid... Only a real force (not inertia or acceleration or blah blah) can be matched with a real counter-force.

This is basic stuff concerning Newton's third law, and stuff you need to grasp before you will understand what's going on here.
...and if ET is gonna be objective, I hope he'll correct you on this too...

Read up some more on the details of circular motion. In fact look at your own diagram ;)
The only thing on your diagram (which is correct btw - I guess you copied it out of a textbook, even though evidently you didn't quite understand it...) which is not pointing either directly towards the centre of motion or away from the centre of motion is the intial 'inertia'. (If this initial inertia wasn't present, there would be no circular motion - the object would just travel straight towards the 'source')

But surely you should know that inertia isn't a force. (It is created by force and can create a force if it impacts into something else, but not while it is happily trundling freely along it's circular path... ;) )

You've spent several posts thinking you had to teach me this (with your letting-go of swinging objects and what-not), when I already knew it! Now it looks like I have to remind you of this fact.
...in your diagram of 'classic' circular motion, the only forces at work are pointing into or away from the centre. There are NONE, either action OR re-action that are pointing in any other direction...

Re-read your text book. And more carefully this time...

It came out of my head. I was not drawing it as a force, it was only the direction that the momentum was moving. The force involved is an effect of the collision, in which an applied force is pushing the door in the direction of the momentum. I only labeled that as inertia, because that was a term that was being used, and I thought I could keep it simples like that. Perhaps a diagram of the forces between the door and the person is in order:

http://www.geocities.com/jedi_duck_1138/Door.GIF)

Since the car is pulling both the person and the door together around the circle, neither one is exerting a centripedal force on the other. Relative to each other, this is a static problem. Therefore, the net force equals zero. The only force that the person exerts on the door is an applied force that is a result of, and in the direction of its momentum. The door applies a force on the person that is perpendicular to its surface, and equal in magnitude to the component of the force that is pushing against it. Depending on the angle, there may be more unnaccounted for force, which is opposed by a force of friction.

Well, thanks for making what you believe is happenning clearer.
Unfortunately, what you beleive is happenning doesn't match up with reality.

Please read up in your physics books. Once you come to the realisation that the only forces involved in circular motion are directed exactly towards (and away from -the counter-force) the exact centre of the motion (and in NO other direction) then we can try and talk this through further. Till then, anything I say isn't gonna make any sense to you I'm afraid...

What I am saying, is that what you are saying is not what is causing the circular motion. I was describing what is going on between the door and the person, which has nothing to do with the circular motion. Surely you agree that there can be circular motion without you flying into the door. That is because the car is pulling you in a circle. If the door were to come off its hinges, it would be because of the collision between the person and the door. This transfer of momentum could be calculated without involving circular motion (assuming you know the momentum that has resulted from the person being accelerated by the car), because the only objects involved are the door and the object. Relative to each other, what I drew is all that is happening. When calculating Newton's third law, you can break it down into components, and calculate it as I did.

I would like to make a correction though. I said there were no centripedal or centrifugal forces involved between these two. In hindsight, the normal force that the door exerts on the person is a centripedal force (in addition to the car), that is opposed under Newton's third law by a component of the force that the person exerts.