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Author Topic: Are frames of reference even more misunderstood than centripetal force?  (Read 30456 times)

Offline ∆thelwulf

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"...gravity is a psuedoforce of types..."
Why is it so important to keep refering to gravity as a"psuedoforce of types"?


Because if you study the quality of Geezers arguement, you essentially keep finding the same thing. He keeps making a mention that the centrifugal force is a psuedoforce, that it is not ''real''.

Well, neither is gravity then, but we still take it as being within the framework of field theories. See, originally, Geezer said, ''there is no such force'' --- to ''there is only the centripetal force.'' Now, gravity is well-understood of as the curvature of space, but no one hangs about to say, ''well gravity isn't real because it's a psuedoforce.''

That kind of thinking is never heard of... only from cranks who can't accept that gravity is something we all come to experience in.
 

Offline JP

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Not to put words in Geezer's mouth, but I don't think he's saying that a pseudoforce can't have real effects in some reference frame.  I think he's saying (quite reasonably) that you shouldn't group "centrifugal force" together with non-inertial forces by labeling it a force. 

It's certainly quite easy to look back at the idea of pseudoforces once you've had advanced courses in mechanics and say that it's obvious where pseudoforces come from and what they physically mean.  It's incredibly confusing to have your instructor in physics 101 tell you that there are reference frame dependent forces which vanish depending on the reference frame you choose: it's hard enough learning to apply Newton's laws in inertial reference frames.  It makes a lot of pedagogical sense to break "force" into two categories: one which has the properties of "real" forces and one which has the properties of pseudoforce--simply because they behave differently in physics, and the latter category requires a lot more mathematical sophistication to tackle properly.
 

Offline Geezer

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Quote from: ∆thelwulf link
"...gravity is a psuedoforce of types..."
Why is it so important to keep refering to gravity as a"psuedoforce of types"?

If we take a case that we are probably all familiar with - the case where we are spinning around on a roundabout, or a carousel, our senses tell us that a force is acting on us in an outward direction radial to the rotation of the device. This is commonly referred to as "centrifugal force".
 
However, if you accept that Newton's laws of motion have not yet been repealed, there is no actual force acting in that direction. There is a force acting in the opposite direction - the centripetal force, and without that force, we would not be rotating at all. The apparent "centrifugal force" is only a reaction to the real centripetal force. That's why it is referred to as a pseudoforce, a fictional force, etc., etc.
 
It's actually impossible to experience any "centrifugal force" in the absence of centripetal force, because, without a centripetal force, there would be no rotation at all.
 
I do know there are other methods, like Hamiltonian Mechanics, that can be used to describe these phenomena, but they go to great lengths to avoid the discussion of any forces, so it's not very likely they will help to resolve the issue.
 
The so-called "centrifugal force" is a consequence of Newton's Laws of Motion. I really don't have any objection to alternative scientific interpretations, but we may not use an aberrant interpretation of Newton's Laws of Motion to disprove Newton's Laws of Motion.
 

Offline ∆thelwulf

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Not to put words in Geezer's mouth, but I don't think he's saying that a pseudoforce can't have real effects in some reference frame.  I think he's saying (quite reasonably) that you shouldn't group "centrifugal force" together with non-inertial forces by labeling it a force. 

The discussion has became so semantic and technical, I've almost lost track of what people are really intending in their discussions.
 

Offline Geezer

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The discussion has became so semantic and technical, I've almost lost track of what people are really intending in their discussions.

It's very simple.
 
If you can't explain the physical phenomenon that produces a force, it's not a real force. It's an artifact.
 

Offline ∆thelwulf

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If someone asked me, ''is the centrifugal force real?''

I'd reply with the question ''is the force of gravity just as real as the centrifugal force?''

If the answer is yes, then that is all I need.
 

Offline ∆thelwulf

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Let me remind you why you created this thread. It was to denounce what I asked... if an elevator was going straight to the heavens and into space, what would stop it from being ripped apart by the centrifugal forces?

You said, ''that's easy. It won't because the centifugal force does not exist.''

Since that remark, we have made some headway. I have shown that black holes are pivotal to understanding centrifugal forces (taken seriously by physicists) whether it is or not a real force created by physical mediators. We've also established time and time again, that gravity is a psuedoforce, but in light of this we do not go around saying it does not exist.

And what are we stuck on here? Semantics.
 

Offline yor_on

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The interesting thing to me is that we presume two frames of reference to proof a force :)
And it doesn't really seem to ah, matter what I call it, or deem it to be.
 

Offline JP

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I think we all agree on the physics, but we disagree on what terms should be used to describe different parts of the physics.  The problem with writing this agreement off as semantics is that you're indicating that semantics isn't important in physics.  But the precision of the definitions we give to terms is absolutely critical to science!
 

Offline ∆thelwulf

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I think we all agree on the physics, but we disagree on what terms should be used to describe different parts of the physics.  The problem with writing this agreement off as semantics is that you're indicating that semantics isn't important in physics.  But the precision of the definitions we give to terms is absolutely critical to science!

I'm not saying semantics is really not that important, only that it has muddled this conversation to tenebrous levels :P
 

Offline Geezer

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Let me try to dispel the muck with a simple experiment:

We have a rotatable circular platform, say, 4 meters in diameter. We step onto the platform, and the platform starts to slowly rotate. We continue standing on the platform as it slowly rotates.

Question: Why do we rotate with the platform?
 

Offline Pmb

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Quote from: Geezer
Let me try to dispel the muck with a simple experiment:

We have a rotatable circular platform, say, 4 meters in diameter. We step onto the platform, and the platform starts to slowly rotate. We continue standing on the platform as it slowly rotates.

Question: Why do we rotate with the platform?

To be extremely general, because as the platform starts to rotate it forces the muck to exert a force between the platform and us. This causes a force on us which causes us to rotate.
 

Offline Geezer

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Quote from: Geezer
Let me try to dispel the muck with a simple experiment:

We have a rotatable circular platform, say, 4 meters in diameter. We step onto the platform, and the platform starts to slowly rotate. We continue standing on the platform as it slowly rotates.

Question: Why do we rotate with the platform?

To be extremely general, because as the platform starts to rotate it forces the muck to exert a force between the platform and us. This causes a force on us which causes us to rotate.

Is the force is conveyed by friction between our shoes and the platform?
 
EDIT: Wait a minute. I thought when you apply a force to something it is supposed to go in a straight line. Weren't we just taught that?
 
Why doesn't the force make us go in a straight line until we fall off the platform?
« Last Edit: 27/04/2012 23:05:45 by Geezer »
 

Offline wolfekeeper

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I find this thread to be not very good at all.

Centrifugal force (and coriolis force) are pseudo forces due to a combination of rotation of the reference frame and momentum; and they are needed when you are analysing a situation in a rotating reference frame to get the correct movements; together they explain why the stars spin around the Earth (for example).

You use rotating reference frames when it's more convenient to use them; for example a space elevator would rotate with the Earth, and are thus are particularly easy to analyse in the rotating reference frame.

Another example is the Lagrange points; these are stationary, but only in a rotating reference frame, they are balanced between gravity and the centrifugal (pseudo)force.
 

Offline wolfekeeper

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Why doesn't the force make us go in a straight line until we fall off the platform?
Because you're not moving at all in the rotating reference frame that rotates with the platform, at least until the centrifugal force exceeds the friction of your shoes.
 

Offline Pmb

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I find this thread to be not very good at all.
The quality of a thread is determined by it's usefulnes to address/answer a person's query/questions. If Geezer gets his question answered to his satisfaction then the thead was a very good thread.
 

Offline Pmb

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Quote from: Geezer
Is the force is conveyed by friction between our shoes and the platform?
Yes. So long as you have a good understanding of kinetic coefficient of friction, static coefficient of friction and how to apply them to a rotating platform, shoes and muck.
 
Quote from: Geezer
EDIT: Wait a minute. I thought when you apply a force to something it is supposed to go in a straight line. Weren't we just taught that?
Itís a bit more complicated than that. When an object is sitting on a rotating platform there is a force of friction, which is perpendicular to the direction of motion, which then makes the object move in a circular motion. Eventually, as you increase the rotational motion of the platform the kinetic coefficient of friction increases so as to make the kinetic coefficient of friction becomes greater than the static coefficient of friction and the object then moves in a straight line.

The dynamics are similar to that of a charged particle moving in a magnetic field. See http://home.comcast.net/~peter.m.brown/sr/cyclotron.htm

This is a tricky thing to describe so you may want to rethink it until you get it right since I probably got something wrong in the explanation.  :-'(
 

Offline Geezer

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Itís a bit more complicated than that. When an object is sitting on a rotating platform there is a force of friction, which is perpendicular to the direction of motion, which then makes the object move in a circular motion. Eventually, as you increase the rotational motion of the platform the kinetic coefficient of friction increases so as to make the kinetic coefficient of friction becomes greater than the static coefficient of friction and the object then moves in a straight line.


Ah right, but there can only really be one force acting on our hypothetical person via their shoes. Presumably it must act in a direction that continuously changes as the platform rotates?

(It seems Wolfkeeper is unaware of Newton's First Law. The so called "rotating reference frame"* could not exist in the absence of centripetal force.)

*What?! Reference frames do not rotate. Everything else around them might, but they don't. If they did, they would hardly be a reference.
 

Offline wolfekeeper

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Nope, no centripetal force is necessarily involved.

The whole point is to give the same results as a non rotating reference frame. As in, you can take any situation in an inertial reference frame and model the same situation in any rotating reference frame, and the same thing must happen, otherwise you're doing it wrong.

The motion won't usually look the same, but if you do the rotation mapping back to the inertial frame, everything must be the same as if you did it all in the inertial frame.
« Last Edit: 02/05/2012 02:49:48 by wolfekeeper »
 

Offline Pmb

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Nope, no centripetal force is necessarily involved.
You're wrong on that point.
 

Offline Pmb

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Ah right, but there can only really be one force acting on our hypothetical person via their shoes.
That is not quite true. There is the force due to the centripital force which is due to choice in frame of reference, there is the force of friction and there is the force of friction. The total force is the sum of all the forces acting on the shoes and it is that total force which is responsiple for explanation of the motion of the shoes.
Presumably it must act in a direction that continuously changes as the platform rotates?
I agree with that.
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*What?! Reference frames do not rotate. Everything else around them might, but they don't. If they did, they would hardly be a reference.
In Newtonian Dynamics that would be incorrect. In General Relativity that is incorrect.
 

Offline wolfekeeper

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Nope, no centripetal force is necessarily involved.
You're wrong on that point.
Nope.

What you do to derive a rotating reference frame is take the normal inertial frame of reference and calculate a coordinate transformation to transfer everything, all the Newtonian mechanics, to that rotating frame.

When you do that, all the normal physics still works, but two pseudo accelerations/pseudo forces appear, the coriolis and centrifugal, if you apply them both, then (somewhat counterintuitively) Newton's laws still work perfectly, they sort of cancel out the effects of the rotating reference frame.

There is no centripetal that appears at all, although if there was one in the original situation in the inertial frame of reference, then there will be one afterwards, but if there wasn't, then there won't be one afterwards.
 

Offline Geezer

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Nope, no centripetal force is necessarily involved.
You're wrong on that point.
Nope.

What you do to derive a rotating reference frame is take the normal inertial frame of reference and calculate a coordinate transformation to transfer everything, all the Newtonian mechanics, to that rotating frame.

When you do that, all the normal physics still works, but two pseudo accelerations/pseudo forces appear, the coriolis and centrifugal, if you apply them both, then (somewhat counterintuitively) Newton's laws still work perfectly, they sort of cancel out the effects of the rotating reference frame.

There is no centripetal that appears at all, although if there was one in the original situation in the inertial frame of reference, then there will be one afterwards, but if there wasn't, then there won't be one afterwards.


But that's my point.

The above is all very well, and mathematically correct, but it only obfuscates this particular problem.

Unless you instantaneously jump into a new frame, there must be a centripetal force, and why would you feel the need to jump into a different frame half-way through a problem?

If we have to instantaneously "change our frame of reference" when we stand on a rotating platform, do we also have to "change our frame of reference" when we step onto a linear moving platform like an escalator, or moving sidewalk, and then describe all the dynamics in terms of pseudo forces? Likewise when we are walking, or rollerblading down the street?

I suppose we could do that, but it seems like an academic exercise.

Of course there are complex situations where these treatments are useful, but from a teaching perspective, if you introduce them too early, you're only going to confuse a lot of students, or other TNS members.
 

Offline wolfekeeper

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What you do to derive a rotating reference frame is take the normal inertial frame of reference and calculate a coordinate transformation to transfer everything, all the Newtonian mechanics, to that rotating frame.

When you do that, all the normal physics still works, but two pseudo accelerations/pseudo forces appear, the coriolis and centrifugal, if you apply them both, then (somewhat counterintuitively) Newton's laws still work perfectly, they sort of cancel out the effects of the rotating reference frame.

There is no centripetal that appears at all, although if there was one in the original situation in the inertial frame of reference, then there will be one afterwards, but if there wasn't, then there won't be one afterwards.

But that's my point.

The above is all very well, and mathematically correct, but it only obfuscates this particular problem.

Unless you instantaneously jump into a new frame, there must be a centripetal force,
Nope. It turns out when you do the maths there's a steady 'force' that is proportional to the distance from the axis that points away from the axis, that is proportional to the distance (the centrifugal force), and there's a coriolis force that acts when something moves* (where "moves" means relative to the rotating reference frame), which acts at 90 degrees to the movement and the axis.


Quote
and why would you feel the need to jump into a different frame half-way through a problem?
You don't "jump" in, you just do it all as if it wasn't rotating, and then add on those two fixup force to deal with the fact that you're working in a rotating frame. In many cases either the centrifugal or coriolis cancel out anyway. On the surface of the Earth, the centrifugal mostly cancels for example.
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If we have to instantaneously "change our frame of reference" when we stand on a rotating platform, do we also have to "change our frame of reference" when we step onto a linear moving platform like an escalator, or moving sidewalk, and then describe all the dynamics in terms of pseudo forces? Likewise when we are walking, or rollerblading down the street?
You can, but you wouldn't normally. You only use accelerated reference frames when it's convenient.
Quote
I suppose we could do that, but it seems like an academic exercise.
Using rotating reference frames is what the weather forecast people do. If they didn't the maths gets even more horrible. They pretty much have to use rotating reference frames.
 

Offline Geezer

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Nope. It turns out when you do the maths there's a steady 'force' that is proportional to the distance from the axis that points away from the axis, that is proportional to the distance (the centrifugal force), and there's a coriolis force that acts when something moves* (where "moves" means relative to the rotating reference frame), which acts at 90 degrees to the movement and the axis.


You can do the math all you like (and I agree with your math) but it does not serve to provide a good explanation for what actually happened. It's only complicating things unnecessarily.

In this example, our subject experienced a force that continuously changed direction, otherwise he would have traveled in a straight line. "Centrifugal force" does not help to explain the phenomenon. In fact, it's downright misleading.
 

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