Centripidal and centrifugal forces

[RenegadeOfPhunk]

What I am saying, is that what you are saying is not what is causing the circular motion.

 

No Shok, I am absolutely right. 100%.

What you are failing to take into account here is the initial inertia facing perpundicular to the centripital force.

 

THIS is what enables a force pointing to the centre of a circle from an object (and nothing else BUT that one force - no others are needed) to cause movement along that circles circumfrence.

 

Seriously, I'm right about this. You really do need to read up. Your now pretty much making this stuff up as you go along...

[RenegadeOfPhunk]

Here - check the example of the car out about 1/6th of the way down this page. THis might help you:

 

http://www.physicsclassroom.com/Class/circles/U6L2a.html

 

Notice that all the forces acting on the car are identified, but the ONLY force making the car travel in a circular motion is caused by friction (the centripital force). That's it. ONE force to move in a circular direction.

..and yes, that ONE friction force is pointing directly at the CENTRE of the circular movement. No other forces are causing it -as the text clearly states...

 

It is the friction force which supplies the centripetal force requirement for the car to move in a horizontal circle.

[Shok_Tinoktin]

You can make a turn without hitting the side of the car. The seat is usually enough to pull you in a circle. When you do hit the door, then you are being pulled to the center of rotation by the seat, and you are being pushed by the door. The person and the door, relative to each other, can be thought of as not moving in a circle, since they are both moving together. If you did not think of it like this, you would have to consider the expansion of the universe, etc. in any calculation. Relative to each other (at the moment of impact and after) the two are not moving. Thus, the net force is equal to zero. Since you have momentum that is going forward, then your momentum does not apply a force that directly opposes the centripedal force. What opposes the centripedal force is a component of that applied force. The additional applied force is opposed by friction.

 

Edit: just read your last post, are we talking about the car or the person/door, it is a huge difference.

[RenegadeOfPhunk]

Edit: just read your last post, are we talking about the car or the person/door, it is a huge difference.

 

Ermm, nope. Not any huge difference at all.

Friction between the car and the road causes the centripital force which makes the car take a circular path -as CLEARLY described in the site I just linked you to...

In turn, the force of the door pressing against you (it's got nada to do with the seat, unless you in a car with really wierdly positioned seating arrangements ) forces the person in the car into a circular motion as well.

 

Both these forces are acting towards the centre of the circular motion. The road acts against the car, and then in turn the car acts against the person inside.

...it's not hard really...

 

 

You kinda getting to the stage where trying to babble on about net forces and - heh - the expansion of the universe - is somehow gonna get you out of this. It's not.

Regardless of how you try and break up the forces involved, the 'overall', combined force is ALWAYS going to point straight towards the centre of the circular motion.

...ALWAYS...

 

But anyway, your not listenting to me, or official physics sites for that matter. So feel free to choose your own reality.

...maybe if someone else can confirm this to you, you'll be more willing to listen - hopefully...

[Shok_Tinoktin]

Are you really trying to tell me that you can not make a turn in a car without crashing into the door? I will tell you what the difference between the car/ground example and the person/door example is. The difference is, the car is moving in a circle relative to the ground. The person is not moving in a circle relative to the door, the two are moving in a circle together.

 

In the car example that you linked to, look at the forces involved. There is no centrifugal force. That is the topic right? The opposing force is the ground being pushed by the car. You aren't trying to claim that this is a centrifugal force are you?

 

If you look at one of my previous posts, you will see that I already claimed that the only relevant force on the car was frictional/centripedal.

[RenegadeOfPhunk]

Are you really trying to tell me that you can not make a turn in a car without crashing into the door?

 

If the movement is truly circular (not just slightly curved - you need to keep up ), your travelling with any kind of real speed, and your in an average car (with no seatbelts on) - yes, that's EXACTLY what I'm saying! Jeez - you have BEEN in a car right?

 

I will tell you what the difference between the car/ground example and the person/door example is. The difference is, the car is moving in a circle relative to the ground. The person is not moving in a circle relative to the door, the two are moving in a circle together.

 

Ermm, I haven't mentioned anything about what these circular movements are relative to because they are a given assuming you at least 8 years old...

If you have to clarify this for yourself though, go right ahead. I'll just wait for you to catch up...

 

I could start over-complexing things by saying that the gravity acting on the car is in fact a centripital force if you consider the spin of the Earth itself, but that's getting away from the actual issue.

...it's not my style to try and hide my inadequacies by changing the subject

 

In the car example that you linked to, look at the forces involved. There is no centrifugal force. That is the topic right? The opposing force is the ground being pushed by the car. You aren't trying to claim that this is a centrifugal force are you?

 

Ahh - now we come to it

 

Yes, indeed there WILL be a counter force from the frictional centripital force. The article doesn't show it, presumebly to keep things simple since modern physics is almost trying to 'brush over' this particular kind of counter-force - but we know it is there without question because of Newton's third law. Just as the gravitational force pulling the car towards the Earth causes a resultant normal force from the Earth keeping the car 'up', so it is that the frictional force of the road against the car results in the car exerting a force back against the road.

This will cause a little bit of extra wear and tear on the road over time, but other than that, assuming it's not run down, the road is big enough and bad enough to not be too affected by this counter-force

 

...and yes - surprise surprise, I call this centrifugal force! Why? What direction will this force be facing pray-tell? Opposite to the centripital force of course!

(...man, I'm getting real de-ja-vu here... I swear I've been over this before...)

 

If you write to the creator of this article and ask about where the counter-force to the centripital force is (demanded by the third law) he will be forced to admit he just didn't mention it...

...I'll let him make his exuses at to why...

 

..he won't call it centrifugal force, he'll call it something else - I'll grant you that. That's what this whole thread is about. But he WILL conceed a counter-force to the centripital force between the car and the road WILL be present.

I'ts basic physics. If you knew some I woudlnt' have had to spell it out to you

 

Go ahead, write an e-mail to this site. Consider it homework

 

If you look at one of my previous posts, you will see that I already claimed that the only relevant force on the car was frictional/centripedal.

 

...and so if you accept that the meaning of centripital is centre-seeking, and that no other forces other than centre-seeking are involved in keeping a circular path, haven't you just proved my point for me?

[Shok_Tinoktin]

If the movement is truly circular (not just slightly curved - you need to keep up ), your travelling with any kind of real speed, and your in an average car, yes, that's EXACTLY what I'm saying! Jeez - you have BEEN in a car right?

What about in the event you aren't driving with any kind of real speed (i wont get into detail about how much speed that is, but lets just say i dont remember the last time i crashed into my car door), do you just keep going straight without your car?

 

If you are talking about the relation of forces between the door and the person, then relative motion EXTREMELY important! When comparing the person to the door, you could easily say that the person is not moving, and the same can be said about the door. When you compare the car to the ground, you cannot say that both are not moving.

 

I can tell you right now what the force is that is opposite to the friction that the ground exerts on the car, according to Newton's third law. The opposite of the force of the ground on the car, is the car on the ground. That is, an applied force due to the momentum of the car. The car does not need an opposite force acting on it, it needs to exert an equal force back.

 

As for proving your point, I thought you said that there is a centrifugal force acting on the car. That doesn't sound like only a centripedal force to me.

[RenegadeOfPhunk]

What about in the event you aren't driving with any kind of real speed (i wont get into detail about how much speed that is, but lets just say i dont remember the last time i crashed into my car door), do you just keep going straight without your car?

 

If you are talking about the relation of forces between the door and the person, then relative motion EXTREMELY important! When comparing the person to the door, you could easily say that the person is not moving, and the same can be said about the door. When you compare the car to the ground, you cannot say that both are not moving.

 

Wow - your trying REALLY hard to change the subject now aren't you

 

We all know what is relative to what. It's plain common sense. This kind of tactic may work with others more gulliable, but not me I'm afraid

And were not considing a breaking car. Were considering a car moving in a circular motion with a 'constant' speed.

...please try and stay on topic...

 

The car does not need an opposite force acting on it, it needs to exert an equal force back.

 

Ermm, when did I ever say the opposite force would act back 'on the car'.

...have you not been reading a word I've been saying! I mean come on. The least you can do is actually read my arguments if you want to contiinue this.

 

A force acts from object A to object B.

The counter force will act from B to A.

 

Door -> Person.

Person -> Door.

 

Sun -> Earth

Earth -> Sun.

 

Even if you don't agree with it, please have the common desency to actually read what I type

...this is what I've said from the beginning!

...so of course I would expect the counter force to exert back from the car to the road, if the inital force was from the road to the car.

Jeez.... this is hard work. I think I need a lie down...

 

As for proving your point, I thought you said that there is a centrifugal force acting on the car. That doesn't sound like only a centripedal force to me

 

OK - and this proves my point beyond doubt!

Please quote me where I said the centrifugal force in this case acts ON the car.

...I said it acts ON the road -the exact opposite!!!!!

*swoon*

 

I don't think I can keep this up much longer mate. If I disappear for a while, it's because I've gotten dizzy with the circular arguments and fainted...

[Shok_Tinoktin]

You're exhausting me. And I'm not trying to change the subject, I'm responding directly to what you say.

 

You don't agree that a centrifugal force would by definition have to be a force that push/pulls an object that is going in a circle directly away from the center of rotation?

[RenegadeOfPhunk]

You don't agree that a centrifugal force would by definition have to be a force that push/pulls an object that is going in a circle directly away from the center of rotation?

 

Yes, but you have to be clear on which object your talking about.

OK - I'm gonna calm down now. I'm sorry I got worked up and I apologise if I've given you offense. You've handled yourself surprisingly well - kudos for that.

 

...but I need you to listen to me now. Cos I think what you've just said above should allow us to sort this once and for all - if your willing to really listen to me...

I dont' expect you to magically agree with me, but I do EXPECT you to TRULY understand what I am saying, and you also have to accept I'm not wrong just because a few modern physicsts have decided they dont' like the word 'centrifugal'.

I have to be wrong based on the definitions of the words 'centrifugal' and 'force'. (Which as I will clearly show you, I am not)

 

...OK - here goes...

I think the main reason why you've figured this whole centrifugal force stuff is crap is because a lot of the time you've been applying it to the wrong object.

 

I'll demonstrate this with an example. When I was first learning about these forces, I learned that it is the centripital force of the Sun's gravity which keeps the Earth in orbit around it.

..now - according to the way I was taught physics (and PLEASE have some respect for the fact that I did well at A-level physics and this WAS the correct teachings at the time - and I still beleive they are right now of course...) meant there was a counter-force to this force.

 

At first I assumed this counter force would also be affecting the Earth somehow. Maybe it affected the tides or something. But when I was told the actual answer, it was obvious.

I was expecting the counter-force to also affect the Earth, when actually it affects the SUN.

 

The Sun is pulled off-centre by the Earth as it orbits.

Now, you can just call this 'the Earth pulling the Sun with it's gravity' if you like. THat's fine. It's correct. It's fact. You don't HAVE to use the term centrifugal if you don't want to...

...but if I choose to use the term centrifugal force to describe it (the force-pair with centripital force), and I'll defend it with the following:

 

* Centrifugal force means 'centre-fleeing'

* The Sun, when it is pulled by the Earth is pulled in a circular motion round the center of Earth's orbit. i.e. the Sun 'wobbles'. The Sun is continually being pulled away from the centre of the orbit of the Earth - hence 'centre-fleeing'

* This force is real and not imaginary. There are no imaginary forces that can make starts wobble. (In the case of Jupiter, the Sun wobbles enough that it displaces it's entire diamiter towards Jupiter!)

 

So - with these facts, I have no problem with calling the Earth's (or other planets) pull(s) on the Sun as they go round in orbit [a] or [the] centrifugal force - the force-pair with centripital force (with acts on the EARTH.)

 

Now, modern physicsts seem to want to get rid of this way of looking at circular motion that has worked for 100's of years.

..why? THis is exactly what I want to know. I see no reason to ditch it. If you want to follow what modern physics teaches, then fine - be my guest.

...but don't try and make out I don't know what I'm talking about -

 

We can carry this on a bit more civily if you are willing to concide this is a matter of termanology and nothing more. It's taken ET quite a few posts to finally come to that realisation.

 

Let's hope you don't take more...

[Shok_Tinoktin]

All right, trying to end this once and for all, and if someone digs this up again, i will hunt that person down.

 

I would first like to say that I intended absolutely no disrespect. I was trying to stick to the topic, and not get caught up in formalities. It seemed at times that you were not respecting me, but I assumed that you were just misunderstanding me, and want to say I took no offense, it was purely about trying to get a solid answer we can agree on.

 

That being said, let me try to make sure we understand each other perfectly. The reason that I do not use the term centrifugal, is that i do not consider the sun to be in circular motion. That is debateable, and I have no intention to get into that, it seems to be purely a matter of opinion. I do not accuse the force that you describe as "centrifugal" of being imaginary. My opinion is that in order to have a "centrifugal" force, the object that is in rotation has to have a force pulling it directly away from the center of the circle. And I assume that you agree that no such force exists. I think it is safe to say that we agree on the physical properties involved, so there is no way that either of us will be able to convince the other.

 

So basically what I am trying to say, no hard feelings, and lets try to get on with our lives.

[RenegadeOfPhunk]

is that i do not consider the sun to be in circular motion.

 

Well, let's be clear - the Sun is not in a circular motion because it has 9-odd planets, a few of them big f**kers - yanking it all over the place in different directions.

...it doesn't so much make a circular motion as dance back and forth a bit irratically!

 

...but if there was only one big planet like Jupiter, and all the other planets didn't exist - then yes, the Sun would infact follow a (rather noticable - depending on your viewpoint obviously ) consistent circular path round the centre of Jupiter's orbit.

 

[edit]

If you need actual real-world examples, consider the Earth and the Moon. The Earth in fact DOES follow a circular path around the centre of the Moon's orbit around the Earth as the Moon tugs at it.

It's not a very big circular path cos the moon isn't that big compared with the Earth. In fact the centre of this circular motion is still under the Earth's surface, but most certainly NOT the Earth's centre.

 

It's known as the Earth-Moon 'barycentre'

From http://www.synapses.co.uk/astro/moon1.html

 

Where's the Earth-Moon barycenter?

 

It's in the Earth but not at its center.

 

Imagine a line drawn from the Earth's center to the Moon's center. The barycenter is along that line. The balancing of the two masses places it at all times within the Earth (below the Earth's surface), but never at its center.

 

... The Earth and the Moon revolve around the Earth-Moon barycenter!

 

There in black and white, the Earth relolves around a central point (follows a circular path) because of the moon...

Other sources (since your bound to not accept this very easiely)...

 

THis one shows it VERY clearly - including a frighfully simple diagram to explain it all.

It also clarify's the radius of the circular movement the Earth is following because of the moon - approx. 2,900 miles...

http://baby.indstate.edu/gga/gga_cart/gecar279.htm

It is not just the moon that is revolving about the earth but that the moon has enough mass to cause the earth, in turn, to move in a circular orbit

 

http://www.crewdog.net/lobsterpage/tides/forces.html

The moon does not orbit the earth. The earth and moon actually orbit each other with the center of the orbit at a point about 1,100 miles inside the earth.

 

[/edit]

 

But let's not fight over this now. You go and ask someone you trust is this is not indeed the case. and what I've posted above is perfectly true...

...then you come back and we'll talk some more.

 

...till then, I'm getting some sleep! Man, I've been up typing longer than I realised

 

Perhaps we'll talk again soon.

[ShadowTemplar]

OK, I'll wrap this up, as well as correcting a single few slight mistakes I noticed while skimming through the thread...

 

The difference between centrifugal and centripetal forces is one of perception. Which one you need to use is based entirely on your choice of frame of referance. Normally, you would like to use an inertial frame of referance, although this is not always the case. In an inertial frame of referance objects behave according to Newton's Second Law (F = dp/dt). A coordinate system fixed on a point on the surface of the Earth is (approximately) an inertial system (in fact it is non-inertial, due to the rotation of the Earth). In an inertial system there is no such thing as a centrifugal force. We shall se below why this is the case.

 

A non-inertial frame of referance is one where N2 is not obeyed, and hence objects behave in a manner dissimiliar to that predicted by classical dynamics. We don't like that, so we introduce fictional forces which we define to be the forces that would have to be applied to a body moving in the non-inertial frame of referance for it to obey N2. In other words, they are the "missing force" in your force diagram.

 

How does this relate to the issue of centrifugal vs. centripetal motion? Consider a body moving in a circle relative to an inertial frame of referance. This object must be affected by a net external force applied by physical object(s) and/or field(s) directed towards the centre of the motion (the centripetal - "centre-seeking" - force).

 

Now consider the same situation as viewed from the frame of referance whose origin describes the same circle as the body. As we have already established, the body is subject to a net external force. But as the frame of referance moves along with the body, the body is at rest in this frame of referance. Thus this frame of referance is non-inertial, because if it was inertial, there would have to be no net external force acting on the body in order for it to be at rest. Enter the fictional force. By introducing a fictional force of the same magnitude and opposite direction to the external force, we arrive at a situation where the net force is zero, which is what we would like, because we can then tackle the problem in the same way that we would normally tackle a problem in an inertial frame of referance. The non-inertial approach to analysing the situation is perfectly equivalent to the inertial. Anyway, this should be an outline of the use of the words centripetal force and centrifugal force in physics. But whether centrifugal force 'exists' is really a philosophical discussion.

 

And now for the corrections:

 

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.

 

This actually has nothing whatsoever to do with centrifugal force, because the equal and opposite reaction force acts on the object excerting the other force.

 

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)

 

Wrong. The important concept involved here is called net external force. The net external force is indeed directed radially inwards in the case of circular motion at constant speed. If the speed is changing, then you have a tangential component of the force as well, obviously.

 

Also, someone somewhere (can't find it right now) mentioned an exchange of (mechanical) energy in a static situtation. No energy is exchanged without motion unless a body is emitting radiation, transferring heat etc. A person standing on the ground does not transfer energy to the ground, and nor does the ground transfer energy to him (assuming of course that they have the same temperture and radiation emissions is ignored).

 

Anyway, let's wrap this up and get back to bashing Bush.

[RenegadeOfPhunk]

Well, this is the most important point you've made to this particular 'argument':

 

The non-inertial approach to analysing the situation is perfectly equivalent to the inertial. Anyway, this should be an outline of the use of the words centripetal force and centrifugal force in physics. But whether centrifugal force 'exists' is really a philosophical discussion.

 

Look - I already accept the heart of this problem is only to do with how you choose to view circular motion. (Your 'frame of referance' as you choose to call it)

So please dont' tell me this like it's something I didn't know, or didn't already accept

...this point has been established and agreed by most of us right back at the beginning of this thread. But thanks for the info *sigh*

 

...perhaps you CAN explain what ET and others have failed to thus far. just explain to me very clearly how your 'philosophy' gives you a more clear understanding of the universe than my 'philosophy'.

i.e. PROVE to me this is more than just a word game

 

..I put it to you that it really doesn't make a difference...

..the only difference so far I can admit to noticing is the fact that a lot (if not most) people seem to think this way nowadays, and so in the interests of following the crowd like a Sheep, you have to consider it in this way too.

 

You can say I'm wrong according to your 'philosophy', but what do I care? I perform my calculations, I get the correct results.

 

...save your 'here is how you SHOULD think about it' for someone else thanks...

 

...dont' just telll me 'I look at this the wrong way, cos -well, it's just wrong..'.

THis would be like a Linux user telling a Windows user it's wrong for them to use Windows and they should use Linux - but not even bothering to give them any practical reasons why Linux is going to allow them to do their work any more efficiently, or in any way better!!

 

Not only arrogant, but also a bit daft, if you don't mind me saying so...

SHOW me clearly why the way I look at it is gonna come to incorrect conclusions... If you can't do that, then get back to your Bush bashing (I'll happily join you in that )

 

This actually has nothing whatsoever to do with centrifugal force, because the equal and opposite reaction force acts on the object excerting the other force.

 

I take the overall point your trying to make. If this opposing-force is not involved in actually 'creating' the circular motion being described, then why bother showing it on the diagram...

But with all due respect ST, your making the same mistake Shok did. When did I ever specify which object this force HAS to apply to for it to be classified as centrifugal force?!

...all I was pointing out is that the counter-force MUST exist and that it will be opposite to the centripital force.

 

The defintion of the word centrifugal is 'centre-fleeing'. Therfore, if there is any force which is acting away from the centre of a circular motion irrelavent to which particular object it's acting on, then by the very defintion of the word, you can call it 'centrifugal force'.

...ET, Shok, ST - none of you can alter this fact, no matter how much you rant and rave on like moden physics bible bashers

 

 

...I know very well centrifugal forces don't always cause a circular motion. But, they are always a by-product of circular motion... (Again, according to my way of viewing this, which I still see no reason to drop or discard..)

...this is only about what you do and don't bother showing, and what viewpoint you take on the situation.

 

I don't accept you are in a position to correct me on anything. Your showing the same kind of mindless arrogance many others on this board are. Just because you look at it a different way, you feel you have the right to tell me my way of looking at it is wrong - instead of being realistic (and polite also btw) and admitting it's just different and in NO way inferior.

 

Spider is the only one who has properly stated this clearly. And for that he deserves credit.

The rest of you are just drones who think because this is the modern way of thinking, it has to be better without even having to come up with a reason

 

I truly do think some of you silly boys see the dismissal of centrifugal forces as 'imaginary' as something akin to general relativity supersceeding Newton's 'classic' laws of gravitation. i.e. it demonstrates a deeper understanding of how the universe works.

....heh, if you do, I can only feel pity for you

 

General relativity teaches us something more fundemental about the universe and quantifiably helps us describe the universe better (case in point, Mercury's slightly altered orbit around the Sun from classic gravitation)

Dismissing centrifugal forces , however, is nothing more than a word game and a mental exersice.

...if you disagree, then bloody well stop pissing about and prove it!

 

Again - will I come to the wrong conclusions in certain situations...? If so, how?!

Please give me a specific example where I'm going to not predict circular motion properly with my 'classic' viewpoint of the centripital and centrifugal force-pair. Good luck - you won't find one...

 

Wrong. The important concept involved here is called net external force. The net external force is indeed directed radially inwards in the case of circular motion at constant speed.

 

..this is exactly my point. Why do you think this proves me wrong?!

I already stated quite a few posts ago that we are considering a circular motion at constant speed. And in the case of circular motion, the acting force is absolutely only pointing towards the centre of the circular motion. (Centripital force)

If it was only slightly curved motion, then of course the situation is different, but of course it wouldn't be circular motion anymore now would it

Here's hoping you don't need me to explain to you what the difference is between a circle and a curve...

 

Anyway, let's wrap this up and get back to bashing Bush.

 

No, your gonna have to do quite a bit better than that ST. Sorry...

[ShadowTemplar]

From what I've gathered from your post, what you want to call centrifugal force is the reaction force on the object that's causing the centripetal force. If so, your ternimology is wrong. Centrifugal force is a name reserved for the fictional force acting on objects in a frame of referance that moves in a circle relative to an inertial frame of referance. Period. That's convention. Why? I don't know. But it's darn practical that we all speak the same language. There is indeed a reaction force. But that's not called centrifugal force.

 

And now there are a few comments that I would like to make regarding the rest of your post:

 

the heart of this problem is only to do with how you choose to view circular motion. (Your 'frame of referance' as you choose to call it)

 

Actually, that's not what I mean when I say 'frame of referance'. Frame of referance is a technical term - you could replace it with 'observer' or 'co-ordinate system'.

 

...perhaps you CAN explain what ET and others have failed to thus far. just explain to me very clearly how your 'philosophy' gives you a more clear understanding of the universe than my 'philosophy'.

i.e. PROVE to me this is more than just a word game

 

Here's a tricky one. As I said above, it is largely a matter of definition. The scientific community has decided to limit the use of the term 'centrifugal force' to the fictional force acting on a body in a frame of referance that is moving in a circle. This is probably because this force is used far more often than the outwards-pointing reaction force you talk about. Thus, when you say 'centrifugal force', you know that you are observing the movement from a non-inertial frame of referance. This means that the term carries a lot more information than simply 'force directed radially outwards'.

 

this is exactly my point. Why do you think this proves me wrong?!

 

Well, you were saying that the centripetal force was the only force involved in creating the circular motion. But the centripetal force doesn't even need to be a force in its own right - rather it is a sum of forces. I simply didn't think that you were being specific enough. And being specific is absolutely nessecary in physics (as I have learned the hard way a couple of times).