Is the concept of reactive centrifugal force valid?

[A.T.]

You mean that the force due to inertia may not be called "inertial force" because it is used for fictitious force?

Inertial forces appear only in non-inertial frames. The centripetal force on the child appears in every frame, so it is not an inertial force.

One really should ban all that pseudo-force nonsense!

Stick to inertial frames and you will never see them.

In the non-rotating frame the centripetal force acting on the child in not balanced by any force.

That is erroneous as I demonstrated + cited and as you actually summarized next: The centripetal force on the child, by the seat is exactly balanced by the centrifugal force on the seat, by the child.

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. You still don't quite grasp Newtons 3rd Law.

Here is my full quote again. Read it again carefully and note that I talk about two different frames:

In the non-rotating frame the centripetal force acting on the child in not balanced by any force. That's why the child goes in circles and not straight. The reactive centrifugal force is not acting on the child.

In the rotating frame where the child is at rest, the centripetal force acting on the child is balanced by the inertial centrifugal force acting on the child.

 

[harrylin]

What exactly are you talking about? The passage you quoted was talking about centripetal forces, not fictitious forces.

You wrote:
"the 'centripetal force' is just the net force in the radial direction producing the centripetal acceleration--not an actual force itself."
So, what are you talking about with "not an actual force" if not a fictitious force?

What do you mean? Are you saying that 'centrifugal forces' are somehow in conflict with Newtonian physics?

To the contrary: only real forces in any direction (incl. centripetal and centrifugal) belong to Newtonian physics. Once more:
"This is the centrifugal force, with which the body impels the circle; and to which the contrary force, wherewith the circle continually repels the body towards the centre, is equal."
- http://gravitee.tripod.com/booki2.htm

 

[K^2]

Note that the OP's confusion was caused by the introduction of fictitious forces - a concept that is in conflict with Newton's philosophy and approach to physics. That may have some use as a calculation trick or shortcut, but the confusions it caused resulted (and still result) in much more loss of time than the few seconds or minutes that may be gained by it in special cases.

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.

 

[A.T.]

I see you have rephrased your statement:

In the non-rotating frame the centripetal force acting on the child in not balanced by any force. [..]

To the contrary, as I demonstrated + cited and as you actually summarized yourself (those forces are by Newton's 3d law equal and in opposite direction):

Interaction forces that obey Newtons 3rd law:
- centripetal force on the child, by the seat
- centrifugal force on the seat, by the child

What is your point here? Where is the contradiction to my quote above? Force pairs in Newtons 3rd Law are NOT balancing each other. They act on different objects. I thought you realized this a few posts up.

 

[Doc Al]

You wrote:
"the 'centripetal force' is just the net force in the radial direction producing the centripetal acceleration--not an actual force itself."
So, what are you talking about with "not an actual force" if not a fictitious force?

When you draw a free body diagram you will not show something labeled as "centripetal force". (Although that is a common physics 101 error.) Nothing to do with fictitious forces.

To the contrary: only real forces in any direction (incl. centripetal and centrifugal) belong to Newtonian physics. Once more:
"This is the centrifugal force, with which the body impels the circle; and to which the contrary force, wherewith the circle continually repels the body towards the centre, is equal."
- http://gravitee.tripod.com/booki2.htm

Crack open a modern (within the past century) book on classical mechanics.

 

[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.

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".)