Is the concept of reactive centrifugal force valid?

[harrylin]

[..] This is NOT what I said. You made this up and it is wrong. Those two forces act on two different objects. So they cannot balance each other. [..]

Yes, sorry I misunderstood what you meant with "balance" (but no I do not misunderstand Newton). Harald

 

[harrylin]

When you draw a free body diagram you will not show something labeled as "centripetal force". [..]

Perhaps you think that centripetal force, because it is not an "active" force, is somehow not an "actual" force?
- http://dictionary.reference.com/browse/actual
Just attach a force meter to the rope, and you will see how real it is. :tongue2:

The use of noninertial frames is essential in order to treat certain kinds of problems. Physicists have no problem using them without confusion.

Indeed I have no problem using non-inertial frames; and neither did I ever need any fictitious force. Do you think that Newton was unable to solve some classical mechanics problems with rotating frames?

 

[harrylin]

It is not always possible to find a frame of reference that is inertial everywhere. Often, it's simply inconvenient to do so. Ability to work in accelerated frames of reference, and by extension with fictitious forces, is a big part of understanding physics. It by no way goes against Newtonian philosophy, and any confusion stems from receiving poor education in classical mechanics.

Yes any confusion stems from receiving poor education in classical mechanics. However, Newton did not need (or want) to use fictitious forces - even when handling rotating frames - and his understanding of mechanics was excellent. I'm grateful to Alonso and Finn that they also did not introduce fictitious forces in their mechanics book; and I think it's safe to say that also that book is very good.

 

[K^2]

Newton also concluded that an object with insufficient velocity to stay in circular orbit would spiral down, and got rather annoyed when he got corrected. Nor did he have to deal with many-body systems or curved space-time. So take from that what you will.

 

[harrylin]

Newton also concluded that an object with insufficient velocity to stay in circular orbit would spiral down, and got rather annoyed when he got corrected. [..] So take from that what you will.

What I take from that: It would be useful to see how the introduction of fictitious forces helps to solve that problem.

 

[Doc Al]

Perhaps you think that centripetal force, because it is not an "active" force, is somehow not an "actual" force?

Nope, nothing to do with "active" or not. All of the individual forces that may produce the net force that we label 'centripetal' are active forces (in most cases).

You are acting like there is a new type of force called 'centripetal force', as distinct from gravity, electromagnetic force, etc.

 

[Doc Al]

I'm grateful to Alonso and Finn that they also did not introduce fictitious forces in their mechanics book; and I think it's safe to say that also that book is very good.

That's a freshman physics text. There's no reason to introduce fictitious forces in physics 101. Wait until next year when you study classical mechanics.

 

[Dale]

Indeed I have no problem using non-inertial frames; and neither did I ever need any fictitious force.

If you worked in non-inertial frames then you used fictitious forces, regardless of whether you called them fictitious forces and put them on the left side or called them modifications to the second law and put them on the right side.

 

[Andrew Mason]

It is not always possible to find a frame of reference that is inertial everywhere. Often, it's simply inconvenient to do so. Ability to work in accelerated frames of reference, and by extension with fictitious forces, is a big part of understanding physics. It by no way goes against Newtonian philosophy, and any confusion stems from receiving poor education in classical mechanics.

This is a good point. The reaction force to the centripetal force on a rotating object tethered to the Earth accelerates the Earth so one cannot analyse the reaction force if one assumes the Earth is an inertial frame.

AM

 

[A.T.]

This is a good point. The reaction force to the centripetal force on a rotating object tethered to the Earth accelerates the Earth so one cannot analyse the reaction force if one assumes the Earth is an inertial frame.

Of course you can analyze the reaction force to the centripetal force in the rest frame of the Earth. It is a real force that can be measured at the attachment, and appears in every frame.

What you cannot do in the rest frame of the Earth, is to assume momentum conservation for the Earth. But that is completely irrelevant to the naming of the above force.

 

[Andrew Mason]

Of course you can analyze the reaction force to the centripetal force in the rest frame of the Earth. It is a real force that can be measured at the attachment, and appears in every frame.

What you cannot do in the rest frame of the Earth, is to assume momentum conservation for the Earth. But that is completely irrelevant to the naming of the above force.

If one analyses the forces in the rest frame of the earth, and make the assumption that the Earth is not accelerating, there will be fictitious forces, such as the "reactive centrifugal force".

AM

 

[harrylin]

That's a freshman physics text. There's no reason to introduce fictitious forces in physics 101. Wait until next year when you study classical mechanics.

That's classical mechanics incl. rotating frames, Coriolis acceleration etc. Do you really think that Newton needed fictitious forces for his calculations? As a matter of fact, why do you think that you need any fiction at all?

 

[harrylin]

If one analyses the forces in the rest frame of the earth, and make the assumption that the Earth is not accelerating, there will be fictitious forces, such as the "reactive centrifugal force". AM

The reaction force to a real force is not a fictitious force; thus what you write is a contradiction in terms. About everyone who answered you here has tried to explain that to you, to no avail. Thus, I now start to wonder if your question if you are missing something was genuine...

 

[Hootenanny]

That's classical mechanics incl. rotating frames, Coriolis acceleration etc. Do you really think that Newton needed fictitious forces for his calculations? As a matter of fact, why do you think that you need any fiction at all?

Because sometimes it is more convient to work in non-inertial frames. Working in non-inertial frames introduces psuedo forces. End of.

This has been explained in detail many many times in this thread.

 

[A.T.]

If one analyses the forces in the rest frame of the earth, and make the assumption that the Earth is not accelerating, there will be fictitious forces, ...

Wrong. If one assumes an inertial frame, then there are no inertial forces, per definition. Whether the inertial frame assumption is valid is another question, and depends what you want to investigate.

...such as the "reactive centrifugal force".

Wrong. The "reactive centrifugal force" is a consequence of Newtons 3rd Law which does not apply to inertial forces.

 

[JeffKoch]

Like Newton I do not use fictitious forces but only real forces; I do favour a ban on confused "rotating observers" with their fictitious forces. Newton did not use such fictions, instead he used reference systems in rectilinear motion.

This sounds like the quaint antiquated view that "fictitious" forces like centrifugal force are in some way illusions, they aren't real forces and we should dispense with them because they don't fit a rigid Newtonian view - our high-school physics teachers probably told us that centrifugal force isn't a real force, and some of us continue to carry along this baggage. But centrifugal force is very real to the rotating observer - and it can do work - so it's not helpful to convince the observer that the force that is trying to push him off his merry-go-round horse, or is squashing him into his roller-coaster seat, is not real and that an enlightened rotating observer should think, "Ah, I feel this apparent force, but it's just a consequence of my non-inertial frame of reference so I should ignore it".

 

[harrylin]

Because sometimes it is more convient to work in non-inertial frames. Working in non-inertial frames introduces psuedo forces. End of.
This has been explained in detail many many times in this thread.

Repetition of error doesn't make it correct and I already explained that that is a misconception: good mechanics textbooks explain how to correctly work (and calculate) with non-inertial frames without introducing pseudo forces.

 

[Dale]

That's classical mechanics incl. rotating frames, Coriolis acceleration etc. Do you really think that Newton needed fictitious forces for his calculations?

Yes, they are needed in non inertial frames.

Tell me, what is the expression for the path of an arbitrary inertially moving object in a rotating reference frame?

 

[harrylin]

[..] it's not helpful to convince the observer that the force that is trying to push him off his merry-go-round horse, or is squashing him into his roller-coaster seat, is not real and that an enlightened rotating observer should think, "Ah, I feel this apparent force, but it's just a consequence of my non-inertial frame of reference so I should ignore it".

Exactly! My advice: start from the start of this thread and read the corresponding Wikipedia articles.

 

[Andrew Mason]

The reaction force to a real force is not a fictitious force;

Exactly. So the reaction to a centripetal force is a real force. Centrifugal "forces" are not real forces. So the reaction to a centripetal force cannot be a centrifugal force.

thus what you write is a contradiction in terms. About everyone who answered you here has tried to explain that to you, to no avail. Thus, I now start to wonder if your question if you are missing something was genuine...

You seem to have misunderstood the point I was making.

AM

 

[Hootenanny]

Repetition of error doesn't make it correct and I already explained that that is a misconception: good mechanics textbooks explain how to correctly work (and calculate) with non-inertial frames without introducing pseudo forces.

Repetition of what error precisely?

Are you saying that it is incorrect to work with non-inertial reference frames? (I'm looking forward to your answer to this)

Just to clarify, I didn't say that we couldn't transform the problem into an inertial frame - just that it is sometimes more convient to work in a non-inertial frame. Suppose we want the "answer" given with respect to a non-inertial frame. It would be crazy to transform the problem from a non-inertial frame, to an inertial frame, get "the answer" and then transform back to a non-inertial frame. Don't you agree?

 

[A.T.]

So the reaction to a centripetal force is a real force.

Correct.

Centrifugal "forces" are not real forces.

Wrong. The term "centrifugal" has nothing to do with real vs. inertial. It simply means "away from the center".

So the reaction to a centripetal force cannot be a centrifugal force.

It can. But it doesn't have to.

 

[harrylin]

Repetition of what error precisely?

Are you saying that it is incorrect to work with non-inertial reference frames? (I'm looking forward to your answer to this) [..] It would be crazy to transform the problem from a non-inertial frame, to an inertial frame, get "the answer" and then transform back to a non-inertial frame. Don't you agree?

There is no need to jump all the time between frames, that would indeed be crazy.
It's an error to think that in order to make use of non-inertial frames such as of the surface of the Earth, one would be condemned to make use of fictitious forces. One merely has to take into account (not forget) the state of motion of the frame to which one refers.

Yes, they are needed in non inertial frames.
Tell me, how do you determine the path of an arbitrary inertially moving object in a rotating reference frame?

That's physics 101: by means of a coordinate transformation I can derive the acceleration wrt that frame - just as standard textbooks do (indeed it has already been done, no need to reinvent the wheel). From that I would next calculate the path by integration; and note that no force comes into play here. How do you derive the path with the use of fictitious forces?

 

[JeffKoch]

Are you saying that it is incorrect to work with non-inertial reference frames? (I'm looking forward to your answer to this)

I also look forward to his refutation of Einstein's modern view.

 

[Doc Al]

The term "centrifugal" has nothing to do with real vs. inertial. It simply means "away from the center".

I think this is partly a semantic problem. True, the etymological meaning of "centrifugal" is "away from the center". But in standard physics usage "centrifugal force" has a more specific meaning, namely the inertial pseudoforce. Use it to mean something else at the risk of creating confusion. (I would never refer to "reactive centrifugal force" as merely "centrifugal force".)

 

[Andrew Mason]

Wrong. If one assumes an inertial frame, then there are no inertial forces, per definition. Whether the inertial frame assumption is valid is another question, and depends what you want to investigate.


Wrong. The "reactive centrifugal force" is a consequence of Newtons 3rd Law which does not apply to inertial forces.

If you assume that a non-inertial frame of reference is inertial you will observe what you will interpret to be forces but which are simply inertial effects.

AM

 

[Dale]

That's physics 101: by means of a coordinate transformation I can derive the acceleration wrt that frame

Right, and what is the expression for the acceleration in that frame?

 

[harrylin]

Exactly. So the reaction to a centripetal force is a real force. Centrifugal "forces" are not real forces. So the reaction to a centripetal force cannot be a centrifugal force.

You seem to have misunderstood the point I was making.

AM

The point that I tried to make: you pretended to ask a question. However, instead you seem to be trying to make a point, and all our efforts to explain you that a centrifuge creates a real force of the clothes on the tub are wasted on you.

Here's my last try, centrifugal:
http://dictionary.reference.com/browse/centrifugal

Good luck
Harald

 

[harrylin]

I also look forward to his refutation of Einstein's modern view.

Einstein's view is not modern anymore. See the physics FAQ: "modern usage demotes the uniform "gravitational" field back to its old status as a pseudo-field"
http://www.desy.de/user/projects/Physics/Relativity/SR/TwinParadox/twin_gr.html

 

[A.T.]

I think this is partly a semantic problem.

I agree. Some seem totally stuck at this "centrifugal" specifier in "reactive centrifugal force", which is not important at all, but rather a trivial additional bit of information. But it seems to cause more confusion than it's worth.

 

[A.T.]

If you assume that a non-inertial frame of reference is inertial you will observe what you will interpret to be forces but which are simply inertial effects.

Yes, but the "reactive centrifugal force" is not such a force. It appears in all reference frames. If the reaction to centripetal force is not acting away from the rotation center, then the term "reactive centrifugal force" is simply not used, or at least not with the "centrifugal" specifier.

 

[harrylin]

I agree. Some seem totally stuck at this "centrifugal" specifier in "reactive centrifugal force", which is not important at all, but rather a trivial additional bit of information. But it seems to cause more confusion than it's worth.

Yes, it seems to be a debate between language purists ("centrifugal" has a precise generic meaning) and jargon purists ("centrifugal force" can only mean what is in vogue).

 

[harrylin]

Right, and what is the expression for the acceleration in that frame?

See Wikipedia*; I asked you how you derive the path of your inertial object with the use of fictive forces.

*http://en.wikipedia.org/wiki/Fictitious_forces#Rotating_coordinate_systems

 

[Andrew Mason]

The point that I tried to make: you pretended to ask a question. However, instead you seem to be trying to make a point, and all our efforts to explain you that a centrifuge creates a real force of the clothes on the tub are wasted on you.

My question was: am I missing something? You are persuading me that I wasn't.

If you put the washing machine in space and put it in spin cycle to spin the clothes, what happens? (let's assume there is electrical power somehow and let's assume that the machine is much heavier than the drum and clothes). The drum containing the clothes spins. But the rotating drum and clothes causes the washing machine to prescribe a circular motion about a point at or very close to the centre of the post on which the drum is centred (depending on how evenly the mass is distributed). In fact, both the drum and the rest of the machine are rotating (differently - at different angular speeds) about a common point. There is no centrifugal reaction force. There are only centripetal forces. The centripetal forces all sum to zero, since there is no net force on the machine as a whole. Where is the centrifugal force (I did not say centrifugal effect)?

Now keep increasing the mass of the washing machine until it has a mass of 6 x 10^24 kg. What, fundamentally, changes? If nothing changes fundamentally, then where does a "real" centrifugal reaction force arise?

AM

 

[K^2]

Andrew, in this case, the opposing centripetal forces are each-other's reaction forces. Reaction force is just another name for a constraint force. You constrain the clothes to move along the curve. Hence the force on whatever provides the constraint. Nothing else to it.