On another forum I seem to have got myself into a debate on whether an inertial reaction force is a real or “imaginary” force.
We discussed this at length here, many years ago.
What is the consensus these days?
The Rev Dodgson said:
On another forum I seem to have got myself into a debate on whether an inertial reaction force is a real or “imaginary” force.We discussed this at length here, many years ago.
What is the consensus these days?
To get the ball rolling, can I get you to provide a definition of the IRF so that we not arguing at cross purposes?
dv said:
The Rev Dodgson said:
On another forum I seem to have got myself into a debate on whether an inertial reaction force is a real or “imaginary” force.We discussed this at length here, many years ago.
What is the consensus these days?
To get the ball rolling, can I get you to provide a definition of the IRF so that we not arguing at cross purposes?
Where’s the fun in that?
I mean that when a massive body is accelerated by an external force, there is an equal and opposite reaction force due to the inertia of the body. For instance, if we sit in an accelerating car we feel a force that might be interpreted as a force pushing us back into the car seat.
That is what I am calling the inertial reaction force.
The Rev Dodgson said:
dv said:
The Rev Dodgson said:
On another forum I seem to have got myself into a debate on whether an inertial reaction force is a real or “imaginary” force.We discussed this at length here, many years ago.
What is the consensus these days?
To get the ball rolling, can I get you to provide a definition of the IRF so that we not arguing at cross purposes?
Where’s the fun in that?
I mean that when a massive body is accelerated by an external force, there is an equal and opposite reaction force due to the inertia of the body. For instance, if we sit in an accelerating car we feel a force that might be interpreted as a force pushing us back into the car seat.
That is what I am calling the inertial reaction force.
Given that you are not in fact accelerating backwards, the force is fictitious.
dv said:
The Rev Dodgson said:
dv said:To get the ball rolling, can I get you to provide a definition of the IRF so that we not arguing at cross purposes?
Where’s the fun in that?
I mean that when a massive body is accelerated by an external force, there is an equal and opposite reaction force due to the inertia of the body. For instance, if we sit in an accelerating car we feel a force that might be interpreted as a force pushing us back into the car seat.
That is what I am calling the inertial reaction force.
Given that you are not in fact accelerating backwards, the force is fictitious.
OK, that’s the position I don’t accept.
The Rev Dodgson said:
dv said:
The Rev Dodgson said:Where’s the fun in that?
I mean that when a massive body is accelerated by an external force, there is an equal and opposite reaction force due to the inertia of the body. For instance, if we sit in an accelerating car we feel a force that might be interpreted as a force pushing us back into the car seat.
That is what I am calling the inertial reaction force.
Given that you are not in fact accelerating backwards, the force is fictitious.
OK, that’s the position I don’t accept.
Then I think probably we would get in a non-edifying argument about the meaning of “fictitious force”, since that is basically the according-to-Hoyle way to work out whether something is a fictitious force.
At the risk of unedifying arguments:
It seems to me that if a body moving at constant speed (and thus with zero nett external force) appears to be accelerating, then it is reasonable to call the force that appears to be doing the accelerating a fictitious force, because there is in fact no force.
On the other hand if a body really is accelerating, then there is an external applied force, and an equal and opposite inertial reaction force, and it seems to me confusing to call this inertial reaction force a fictitious force, when it is in fact a real force with real effects, such as being felt by the body being accelerated.
The Rev Dodgson said:
At the risk of unedifying arguments:It seems to me that if a body moving at constant speed (and thus with zero nett external force) appears to be accelerating, then it is reasonable to call the force that appears to be doing the accelerating a fictitious force, because there is in fact no force.
On the other hand if a body really is accelerating, then there is an external applied force, and an equal and opposite inertial reaction force, and it seems to me confusing to call this inertial reaction force a fictitious force, when it is in fact a real force with real effects, such as being felt by the body being accelerated.
The force that is having those “real effects” is the force that is actually doing the accelerating. You are being squashed by the force that is accelerating you forward.
Might have the wrong end of the stick but if it can be measured and needs to be accommodated in manufacturing wouldn’t that make it real?
AwesomeO said:
Might have the wrong end of the stick but if it can be measured and needs to be accommodated in manufacturing wouldn’t that make it real?
You can’t measure it: you can measure the positive acceleration .
You don’t need to accommodate it in manufacturing: you accommodate for the actual forces caused by the real acceleration.
dv said:
The Rev Dodgson said:
At the risk of unedifying arguments:It seems to me that if a body moving at constant speed (and thus with zero nett external force) appears to be accelerating, then it is reasonable to call the force that appears to be doing the accelerating a fictitious force, because there is in fact no force.
On the other hand if a body really is accelerating, then there is an external applied force, and an equal and opposite inertial reaction force, and it seems to me confusing to call this inertial reaction force a fictitious force, when it is in fact a real force with real effects, such as being felt by the body being accelerated.
The force that is having those “real effects” is the force that is actually doing the accelerating. You are being squashed by the force that is accelerating you forward.
No, you are being squashed by the interaction of the forward force and the opposite reaction forces.
dv said:
AwesomeO said:
Might have the wrong end of the stick but if it can be measured and needs to be accommodated in manufacturing wouldn’t that make it real?
You can’t measure it: you can measure the positive acceleration .
You don’t need to accommodate it in manufacturing: you accommodate for the actual forces caused by the real acceleration.
Which are in the case of a car seat a forward force at the base of the seat, and a rearward force distributed over the volume of the seat plus another rearward force from the person sitting in the seat.
The Rev Dodgson said:
dv said:
The Rev Dodgson said:
At the risk of unedifying arguments:It seems to me that if a body moving at constant speed (and thus with zero nett external force) appears to be accelerating, then it is reasonable to call the force that appears to be doing the accelerating a fictitious force, because there is in fact no force.
On the other hand if a body really is accelerating, then there is an external applied force, and an equal and opposite inertial reaction force, and it seems to me confusing to call this inertial reaction force a fictitious force, when it is in fact a real force with real effects, such as being felt by the body being accelerated.
The force that is having those “real effects” is the force that is actually doing the accelerating. You are being squashed by the force that is accelerating you forward.
No, you are being squashed by the interaction of the forward force and the opposite reaction forces.
That is not right. Taking the simple example of a rocket, with an astronaut on a comfy seat. The reaction chamber accelerates forward, there are forces in the materials connecting the reaction chamber to the seat, and the back of the seat is squashed and compressed because it is being accelerated into the astronaut’s bottom.
dv said:
The Rev Dodgson said:
dv said:The force that is having those “real effects” is the force that is actually doing the accelerating. You are being squashed by the force that is accelerating you forward.
No, you are being squashed by the interaction of the forward force and the opposite reaction forces.
That is not right. Taking the simple example of a rocket, with an astronaut on a comfy seat. The reaction chamber accelerates forward, there are forces in the materials connecting the reaction chamber to the seat, and the back of the seat is squashed and compressed because it is being accelerated into the astronaut’s bottom.
whereas in order for there to be some force in other direction, you’d need a second rocket (or something like it) on the front end of the astronaut, pushing backwards.
dv said:
The Rev Dodgson said:
dv said:The force that is having those “real effects” is the force that is actually doing the accelerating. You are being squashed by the force that is accelerating you forward.
No, you are being squashed by the interaction of the forward force and the opposite reaction forces.
That is not right. Taking the simple example of a rocket, with an astronaut on a comfy seat. The reaction chamber accelerates forward, there are forces in the materials connecting the reaction chamber to the seat, and the back of the seat is squashed and compressed because it is being accelerated into the astronaut’s bottom.
If the astronaut was massless there wouldn’t be any squashing.
The squashing is due to the combination of the accelerating force and the inertial reaction force.
In a body that only appears to be accelerating (due to a non-inertial FoR) then there is no such squashing, so it is perfectly reasonable to call such a force “imaginary”.
dv said:
dv said:
The Rev Dodgson said:No, you are being squashed by the interaction of the forward force and the opposite reaction forces.
That is not right. Taking the simple example of a rocket, with an astronaut on a comfy seat. The reaction chamber accelerates forward, there are forces in the materials connecting the reaction chamber to the seat, and the back of the seat is squashed and compressed because it is being accelerated into the astronaut’s bottom.
whereas in order for there to be some force in other direction, you’d need a second rocket (or something like it) on the front end of the astronaut, pushing backwards.
In that case you would have a reduced inertial reaction force and increased external reaction force.
Why would you call one a real force, and not the other?
The Rev Dodgson said:
dv said:
The Rev Dodgson said:No, you are being squashed by the interaction of the forward force and the opposite reaction forces.
That is not right. Taking the simple example of a rocket, with an astronaut on a comfy seat. The reaction chamber accelerates forward, there are forces in the materials connecting the reaction chamber to the seat, and the back of the seat is squashed and compressed because it is being accelerated into the astronaut’s bottom.
If the astronaut was massless there wouldn’t be any squashing.
The squashing is due to the combination of the accelerating force and the inertial reaction force.
In a body that only appears to be accelerating (due to a non-inertial FoR) then there is no such squashing, so it is perfectly reasonable to call such a force “imaginary”.
I don’t think I’ve got anything further to add. There aren’t inertial forces, under Newtonian or Einsteinian relativity. The effects are due to the forward acceleration, and the term for an interpretation of such effects when viewed in an accelerating frame is conventionally called a “fictitious force”. I got nothing else.
dv said:
I don’t think I’ve got anything further to add. There aren’t inertial forces, under Newtonian or Einsteinian relativity. The effects are due to the forward acceleration, and the term for an interpretation of such effects when viewed in an accelerating frame is conventionally called a “fictitious force”. I got nothing else.
So how would you differentiate between the sort of fictitious force that causes strain in a non-rigid medium, and the sort that doesn’t?
The Rev Dodgson said:
dv said:I don’t think I’ve got anything further to add. There aren’t inertial forces, under Newtonian or Einsteinian relativity. The effects are due to the forward acceleration, and the term for an interpretation of such effects when viewed in an accelerating frame is conventionally called a “fictitious force”. I got nothing else.
So how would you differentiate between the sort of fictitious force that causes strain in a non-rigid medium, and the sort that doesn’t?
What kind of fictitious force is not associated with strain in a non-rigid medium?
dv said:
The Rev Dodgson said:
dv said:I don’t think I’ve got anything further to add. There aren’t inertial forces, under Newtonian or Einsteinian relativity. The effects are due to the forward acceleration, and the term for an interpretation of such effects when viewed in an accelerating frame is conventionally called a “fictitious force”. I got nothing else.
So how would you differentiate between the sort of fictitious force that causes strain in a non-rigid medium, and the sort that doesn’t?
What kind of fictitious force is not associated with strain in a non-rigid medium?
The speed force or the rings The Lantern Corps possess
dv said:
The Rev Dodgson said:
dv said:I don’t think I’ve got anything further to add. There aren’t inertial forces, under Newtonian or Einsteinian relativity. The effects are due to the forward acceleration, and the term for an interpretation of such effects when viewed in an accelerating frame is conventionally called a “fictitious force”. I got nothing else.
So how would you differentiate between the sort of fictitious force that causes strain in a non-rigid medium, and the sort that doesn’t?
What kind of fictitious force is not associated with strain in a non-rigid medium?
The fictitious force you get when you divide the apparent acceleration of a non-accelerating body by its mass.
The Rev Dodgson said:
dv said:
The Rev Dodgson said:So how would you differentiate between the sort of fictitious force that causes strain in a non-rigid medium, and the sort that doesn’t?
What kind of fictitious force is not associated with strain in a non-rigid medium?
The fictitious force you get when you divide the apparent acceleration of a non-accelerating body by its mass.
Give an example of apparent acceleration of a non-accelerating body.
I’m not sure I understand but if you put a pressure sensor between your back and the vertical part of the seat of a car and then accelerate the car I’ll bet my left swinger that it will register a reading and go back to zero when the cars velocity becomes stable or better put when the acceleration stops.
Peak Warming Man said:
I’m not sure I understand but if you put a pressure sensor between your back and the vertical part of the seat of a car and then accelerate the car I’ll bet my left swinger that it will register a reading and go back to zero when the cars velocity becomes stable or better put when the acceleration stops.
Yes. That’s the direct effect of the forward acceleration.
dv said:
Peak Warming Man said:
I’m not sure I understand but if you put a pressure sensor between your back and the vertical part of the seat of a car and then accelerate the car I’ll bet my left swinger that it will register a reading and go back to zero when the cars velocity becomes stable or better put when the acceleration stops.
Yes. That’s the direct effect of the forward acceleration.
But if I was strapped to the seat or bound to it as a rigid body that force would not be there, is that the crux of the argument?
Peak Warming Man said:
dv said:
Peak Warming Man said:
I’m not sure I understand but if you put a pressure sensor between your back and the vertical part of the seat of a car and then accelerate the car I’ll bet my left swinger that it will register a reading and go back to zero when the cars velocity becomes stable or better put when the acceleration stops.
Yes. That’s the direct effect of the forward acceleration.
But if I was strapped to the seat or bound to it as a rigid body that force would not be there, is that the crux of the argument?
The force would still be there. You have mass. You’re in contact with the seat. The seat is accelerating. To do this, it applies a forward force on your body. There isn’t any way around it. In the accelerating frame of reference, the effect is as though there is a force being applied by your body onto a seat which (in the accelerating frame of the car) is at rest. The term used to describe these forces that only exist when considered in an accelerating frame is “fictitious force”.
Colorado U: Frames with Linear Acceleration
Background Physics: Motion and Forces: Fictitious Forces
dv said:
Peak Warming Man said:
dv said:Yes. That’s the direct effect of the forward acceleration.
But if I was strapped to the seat or bound to it as a rigid body that force would not be there, is that the crux of the argument?
The force would still be there. You have mass. You’re in contact with the seat. The seat is accelerating. To do this, it applies a forward force on your body. There isn’t any way around it. In the accelerating frame of reference, the effect is as though there is a force being applied by your body onto a seat which (in the accelerating frame of the car) is at rest. The term used to describe these forces that only exist when considered in an accelerating frame is “fictitious force”.
Colorado U: Frames with Linear Acceleration
Background Physics: Motion and Forces: Fictitious Forces
I’m at the redoubt so going to links is a pain.
Carry on then.
Not into golf then?
furious said:
- I’m at the redoubt so going to links is a pain.
Not into golf then?
He told you. He is in the rough.
The Rev Dodgson said:
dv said:
The Rev Dodgson said:Where’s the fun in that?
I mean that when a massive body is accelerated by an external force, there is an equal and opposite reaction force due to the inertia of the body. For instance, if we sit in an accelerating car we feel a force that might be interpreted as a force pushing us back into the car seat.
That is what I am calling the inertial reaction force.
Given that you are not in fact accelerating backwards, the force is fictitious.
OK, that’s the position I don’t accept.
Ditto.
dv said:
The Rev Dodgson said:
dv said:What kind of fictitious force is not associated with strain in a non-rigid medium?
The fictitious force you get when you divide the apparent acceleration of a non-accelerating body by its mass.
Give an example of apparent acceleration of a non-accelerating body.
Frictionless parcel sliding across your frictionless back seat when you drive round a bend.
Ball on floor of bus rolling backwards when bus accelerates forwards
Any inertial motion observed from a non-inertial frame of reference.
The Rev Dodgson said:
dv said:
The Rev Dodgson said:The fictitious force you get when you divide the apparent acceleration of a non-accelerating body by its mass.
Give an example of apparent acceleration of a non-accelerating body.
Frictionless parcel sliding across your frictionless back seat when you drive round a bend.
Ball on floor of bus rolling backwards when bus accelerates forwards
Any inertial motion observed from a non-inertial frame of reference.
I don’t see a special need for different categories of fictitious forces on that basis. Compare to the gravity force on a falling object v gravitational force on a cushion on the ground. The nature of the “force” is the same
i suspect i agree with dv
Under Einstein…gravity is fictitious
Under Einstein…gravity is fictitious
Under Einstein…gravity is fictitious
Under Einstein…gravity is fictitious
Under Einstein…gravity is fictitious
i suspect i continue to agree with dv
Great sage, equal of heaven…
i could say that again
Though, that post is very real…
The Rev Dodgson said:
dv said:Given that you are not in fact accelerating backwards, the force is fictitious.
OK, that’s the position I don’t accept.
Ditto. Mechanically, what you’re calling “inertial reaction force” is mechanically identical to the force exerted by a spring.
The body that is being pushed is being compressed elastically, and that elastic compression pushes back on the body doing the original pushing, just like a compressed spring pushes back on whatever is pushing it.
Sorry about that
dv said:
Sorry about that
Are echoes related to inertial reaction forces?
Mollwollfumble , there are compression forces in a compressed spring no matter the cause even in a non-accelerating frame.
dv said:
Mollwollfumble , there are compression forces in a compressed spring no matter the cause even in a non-accelerating frame.
Well duh.
dv said:
The Rev Dodgson said:
dv said:Give an example of apparent acceleration of a non-accelerating body.
Frictionless parcel sliding across your frictionless back seat when you drive round a bend.
Ball on floor of bus rolling backwards when bus accelerates forwards
Any inertial motion observed from a non-inertial frame of reference.
I don’t see a special need for different categories of fictitious forces on that basis. Compare to the gravity force on a falling object v gravitational force on a cushion on the ground. The nature of the “force” is the same
But those are real forces.
I’m talking about fictitious forces in that there is no force because the body is not accelerating.
i thought an object in freefall is said to be not accelerating
>opposite reaction forces
probably related to the .5MV(^2)
that non-linear beast
SCIENCE said:
i thought an object in freefall is said to be not accelerating
Einsteinially, yes
actually to wind the clock back a bit, rather than travelling at some complex speed i offer reexamination of this
/* I mean that when a massive body is accelerated by an external force, there is an equal and opposite reaction force due to the inertia of the body. For instance, if we sit in an accelerating car we feel a force that might be interpreted as a force pushing us back into the car seat. That is what I am calling the inertial reaction force. */
my added <emphasis>
I disagree that we feel such a force at all, except in the sense that “intuitive” thinking can be called “feeling”.
When i sit on my chair, i feel the chair pushing up on my cheeks. I do not feel gravity pulling me down. Indeed, now that we mention it, and i have occasion to think of it, i wonder whether this might be part of how it took centuries for a coherent theory of gravity to emerge — gravity is of no feeling.
When i sit in a car as it accelerates, i feel a force that is pushing me forwards — it arises from the car seat.
I argue that rather than /* we feel a force that might be interpreted as a force pushing us back */, we interpret the fact that we are squashing the seat, as an expectation (not feeling) that there must be a force pushing us back.
Indeed, the failing in the analysis is clear when you read the phrase again: /* we feel a force that might be interpreted as a force */. What The Force?
rather than “feeling”, maybe call it “seeming”
as a bonus, i shall post some of the bullfaeces our NSW high school science books are offering in my next contribution here…
Here we go, i loaded them to here: https://www.flickr.com/photos/58669456@N00/
There are 8 screen grabs from a presentation i compiled them into; i apologise for some of the bizarre references which have become unclear through slides being taken out of context. Let me know if any need further explanation / elaboration.
ROFL, almost makes me wish you were a real person
sometimes i wish the same thing
Pinocchio?
please note, i am also aware that they phuqt up the unit of force, the newton (they wrote Newton)
however the slide i had for that was simply a whole bunch of French (pardon mine) so i didn’t bother posting the image here
for those interested, the relevant passage:
“Les noms des unités sont imprimés en caractères romains (droits), et sont considérés comme des noms ordinaires. En français, et en anglais, les noms d’unités commencent par une minuscule (même si le symbole de l’unité commence par une majuscule), sauf s’ils se trouvent placés au début d’une phrase ou dans un titre en majuscules.” http://www.bipm.org/utils/common/pdf/si_brochure_8.pdf
never realised there was a connection between Talking Heads and Tom Tom Club
Of the car and occupant example there’s coupling, and deformation.
If you separate those masses (occupant in a seat from the composite mass), the coupling of the accupant (by the seat) can be seen as creating a delay or lag.
The deformations involve energy conversions.
So the question/s might be…
1. Do the forces holding the mass/es together resist deformations (answer’s yes).
2. Are the two masses, seen separately, gaining KE non-linearly at different rates (answer’s yes I guess)
3. Are the forces resisting deformation a reaction force (probably can be seen so).
But, my physics and math is not good.
JudgeMental said:
never realised there was a connection between Talking Heads and Tom Tom Club
it was the Force that made me do it.
dv said:
ROFL, almost makes me wish you were a real person
What sort of person is he/she/it?
JudgeMental said:
JudgeMental said:
never realised there was a connection between Talking Heads and Tom Tom Club
it was the Force that made me do it.
roll the numbered cube, or die
SCIENCE said:
JudgeMental said:
JudgeMental said:
never realised there was a connection between Talking Heads and Tom Tom Club
it was the Force that made me do it.
roll the numbered cube, or die
LTJTB, MZL.
;-)
The Rev Dodgson said:
dv said:
ROFL, almost makes me wish you were a real person
What sort of person is he/she/it?
Fictitious
We don’t seem to be making a lot of progress, but I have learned something: a N is written as newton when we spell the whole name.
The logic of that escapes me, but better go with the flow I suppose.
The Rev Dodgson said:
We don’t seem to be making a lot of progress,
As I suspected in my very first post, this basically comes down to definitions. The forces you are talking about are fictitious force by the definition of “fictitious forces”. If you don’t like it, call them d’Alembert forces.
but I have learned something: a N is written as newton when we spell the whole name. The logic of that escapes me, but better go with the flow I suppose.
It is the same for all units. It’s actually pretty normal to apply common case-logic to words derived from people’s name. You don’t have to say Galvanise or Mesmerise or Fuchsia or Maverick. They are just ordinary words now.
dv said:
Given that you are not in fact accelerating backwards, the force is fictitious.
The force is real. The reason you don’t accelerate backwards is the force the ground exerts on you as you push against the ground to accelerate the object.
From Newton’s second law, where all forces are real, one can derive Newton’s third law. In this derivation, both the force and its reaction are necessarily real.
KJW said:
From Newton’s second law, where all forces are real, one can derive Newton’s third law. In this derivation, both the force and its reaction are necessarily real.
We’re not discussing whether they are real. We are discussing whether they are fictitious forces.
Isn’t that two sides of the same coin?
dv said:
KJW said:
From Newton’s second law, where all forces are real, one can derive Newton’s third law. In this derivation, both the force and its reaction are necessarily real.
We’re not discussing whether they are real. We are discussing whether they are fictitious forces.
Well that clears things up.
AwesomeO said:
dv said:
KJW said:
From Newton’s second law, where all forces are real, one can derive Newton’s third law. In this derivation, both the force and its reaction are necessarily real.
We’re not discussing whether they are real. We are discussing whether they are fictitious forces.
Well that clears things up.
We’ve moved on from the force.
We’re now in the realms of terminology, Jones.
dv said:
The Rev Dodgson said:
We don’t seem to be making a lot of progress,
As I suspected in my very first post, this basically comes down to definitions. The forces you are talking about are fictitious force by the definition of “fictitious forces”. If you don’t like it, call them d’Alembert forces.
I don’t think it is just a question of definitions.
In the case of the parcel sliding over the frictionless seat we are agreed (I think): the force is fictitious and is invented to generate the acceleration if we pretend the FoR is inertial.
In the case of the person sitting in an accelerating chair, viewed from the FoR of the person, there are actually three forces:
1. A real forward force
2. A real equal and opposite inertial reaction force, that compresses the face of the chair and the back of the person
3. An imaginary rearward reaction force, to remove the acceleration, form the FoR of the person.
I agree that force 3 is entirely imaginary, virtual, or whatever you want to call it. The problem is that the force that the person feels “pushing them into the chair” is often called the imaginary force, when it is in fact force 2, the inertial reaction force.
KJW said:
From Newton’s second law, where all forces are real, one can derive Newton’s third law. In this derivation, both the force and its reaction are necessarily real.
Yes, that is where I’m coming from.
furious said:
- We’re not discussing whether they are real. We are discussing whether they are fictitious
Isn’t that two sides of the same coin?
Not at all. “Fictitious Force” is a term of art in the biz.
/* I agree that force 3 is entirely imaginary, virtual, or whatever you want to call it. The problem is that the force that the person feels “pushing them into the chair” is often called the imaginary force, when it is in fact force 2, the inertial reaction force. */
i disagree
force 2 does not push anyone into any chair, it is the person pushing the chair
the force that the person imagines “pushing them into the chair” is force 3 and at that point we agree, it is entirely imaginary
dv said:
KJW said:
From Newton’s second law, where all forces are real, one can derive Newton’s third law. In this derivation, both the force and its reaction are necessarily real.
We’re not discussing whether they are real. We are discussing whether they are fictitious forces.
Fictitious forces, according to the definition, are called fictitious because they are not real.
To put it another way, if the astronaut is exerting a force on the chair, why is he not tending to accelerate it?
Also: sorry to pass the buck but the two links I provided earlier probably make a more convincing case than I do.
¿who curates official definitions of things like “fictitious force”, is it BIPM or IUPAC or some other consensus organisation?
SCIENCE said:
/* I agree that force 3 is entirely imaginary, virtual, or whatever you want to call it. The problem is that the force that the person feels “pushing them into the chair” is often called the imaginary force, when it is in fact force 2, the inertial reaction force. */i disagree
force 2 does not push anyone into any chair, it is the person pushing the chair
the force that the person imagines “pushing them into the chair” is force 3 and at that point we agree, it is entirely imaginary
How can you feel an entirely imaginary force?
dv said:
To put it another way, if the astronaut is exerting a force on the chair, why is he not tending to accelerate it?
Because it is being balanced by other forces, such as the exhaust.
Wikipedia..
SCIENCE said:
¿who curates official definitions of things like “fictitious force”, is it BIPM or IUPAC or some other consensus organisation?
Isn’t it you?
The point is that one needs to consider all the forces in the system to determine why this or that isn’t accelerating.
The Rev Dodgson said:
How can you feel an entirely imaginary force?
I don’t, i feel the chair pushing me up and forward, and that’s it.
Or do you mean this?
doi:10.1162/jocn_a_00393
http://www.mitpressjournals.org/doi/abs/10.1162/jocn_a_00393#.WO9is2j-u00
KJW said:
dv said:
To put it another way, if the astronaut is exerting a force on the chair, why is he not tending to accelerate it?
Because it is being balanced by other forces, such as the exhaust.
flatulencewillgetyounowhere
SCIENCE said:
KJW said:
dv said:
To put it another way, if the astronaut is exerting a force on the chair, why is he not tending to accelerate it?
Because it is being balanced by other forces, such as the exhaust.
flatulencewillgetyounowhere
But it’s not balanced by the exhaust. The system is accelerating forward exactly as much as you’d expect from the force generated by the rocket. If you consider the whole system in an inertial frame you can completely ignore the fictitious forces.
Here is another similar article.
DOI: 10.1038/35784
https://www.ncbi.nlm.nih.gov/pubmed/9486643
Botvinick M, Cohen J. Rubber hands ‘feel’ touch that eyes see. Nature. 1998 Feb 19;391(6669):756.
… ten subjects … seated with their left arm resting upon a small table. A standing screen was positioned beside the arm to hide it from the subject’s view and a life-sized rubber model of a left hand and arm was placed on the table directly in front of the subject. The subject sat with eyes fixed on the artificial hand while we … stroke the rubber hand and the subject’s hidden hand, synchronising the
timing … subjects experienced an illusion in which they seemed to feel the touch not of the hidden brush but that of the viewed brush, as if the rubber hand had sensed the touch.
(not quite the same as completely imagining the feeling of force, but getting there)
dv said:
SCIENCE said:
¿who curates official definitions of things like “fictitious force”, is it BIPM or IUPAC or some other consensus organisation?
Isn’t it you?
Correct, it isn’t…
or actually, no, it is, but only for the terms within my own scope of use.
dv said:
Also: sorry to pass the buck but the two links I provided earlier probably make a more convincing case than I do.
Missed those before.
I think the second link is consistent with what I am saying, although it gets a little unclear at times.
I’ll have a look at the first one.
dv said:
SCIENCE said:
KJW said:Because it is being balanced by other forces, such as the exhaust.
flatulencewillgetyounowhere
But it’s not balanced by the exhaust. The system is accelerating forward exactly as much as you’d expect from the force generated by the rocket. If you consider the whole system in an inertial frame you can completely ignore the fictitious forces.
The exhaust accelerates the mass of the rocket plus the astronaut. The seat is accelerating the astronaut only. The astronaut is countering the exhaust enough that the rocket alone is accelerating. This demands that the force of the seat on the astronaut be opposite to the force of the astronaut on the seat.
OK, the first one is an excellent example of applying the term “imaginary force” both to forces that really are imaginary, and to real forces.
DOI: 10.3389/neuro.09.006.2008
http://journal.frontiersin.org/article/10.3389/neuro.09.006.2008/full
… a virtual limb can be made to feel part of your body if … invoked through tactile stimulation on a person’s hidden real right hand with synchronous virtual visual stimulation on an aligned 3D stereo virtual arm … After 5 min of stimulation the virtual arm rotated … the extent of the illusion was also correlated with … muscle activity onset in the right arm … during this period that the arm was rotating …
DOI: 10.1371/journal.pbio.0040069
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.0040069
… fMRI … correlates of a robust somatosensory illusion that can dissociate tactile perception from physical stimulation … stimulation at the wrist, then near the elbow, can create the illusion of touches at intervening locations along the arm, as if a rabbit hopped along it … illusory sequences activated contralateral primary somatosensory cortex, at a somatotopic location corresponding to the filled-in illusory perception on the forearm … the amplitude of this somatosensory activation was comparable to that for veridical stimulation including the intervening position on the arm … provide direct evidence that illusory somatosensory percepts can affect primary somatosensory cortex in a manner that corresponds somatotopically to the illusory percept.
Commentary at
http://www.apa.org/monitor/jan07/pain.aspx
Tap people’s arms rapidly at the wrist and then at the elbow, and they will feel a phantom tap right in the middle … participants … reported feeling the illusory touch and the real one equally strongly, and their brains agreed-the S1 area registered both sensations at the same location in the brain and with a similar amount of neural activity … traditionally we thought S1 formed a map of the body that faithfully represents the actual touch on the skin, but our results suggest … S1 seems to be representing what we feel-not what is actually there …
The Rev Dodgson said:
OK, the first one is an excellent example of applying the term “imaginary force” both to forces that really are imaginary, and to real forces.
The accelerated pendulum is a good example.
From the PoV of the inertial FoR there are two real horizontal forces: the horizontal component of the string tension forwards, and the inertial reaction force of the pendulum backwards.
From the PoV of the accelerated FoR there are two real horizontal forces as above plus an imaginary horizontal force to keep the pendulum at its angle.
But the article describes it as though the real force 2 was the imaginary force, which obviously it isn’t.
The Rev Dodgson said:
The Rev Dodgson said:
OK, the first one is an excellent example of applying the term “imaginary force” both to forces that really are imaginary, and to real forces.
The accelerated pendulum is a good example.
From the PoV of the inertial FoR there are two real horizontal forces: the horizontal component of the string tension forwards, and the inertial reaction force of the pendulum backwards.
From the PoV of the accelerated FoR there are two real horizontal forces as above plus an imaginary horizontal force to keep the pendulum at its angle.
But the article describes it as though the real force 2 was the imaginary force, which obviously it isn’t.
Now you see it, then you don’t.
Final inertia for tonight:
Consider the frictionless parcel apparently sliding over the back seat of the car.
We are agreed that from the PoV of the car there is a fictitious force to account for this apparent acceleration.
What happens when the parcel hits the door?
There is now a real force from the door on the parcel, and a real reaction force from the parcel on the door.
The fictitious force doesn’t go away, it is still required to explain why the parcel now appears to be stationary, but it is not the force that the parcel “feels” (and causes it to deform). That is a real action/reaction, not a fictitious force.
Applies imaginary force.
The Rev Dodgson said:
Applies imaginary force.
Did you have to think about it? This be the question.
roughbarked said:
The Rev Dodgson said:
Applies imaginary force.
Did you have to think about it? This be the question.
Yes, I gave the imaginary force real thought.
>I mean that when a massive body is accelerated by an external force, there is an equal and opposite reaction force due to the inertia of the body. For instance, if we sit in an accelerating car we feel a force that might be interpreted as a force pushing us back into the car seat.
That is what I am calling the inertial reaction force.
you feel the coupling of the force, the energy transfer (causing change)
Imagine a large circular maglev platform floating above the ground, with negligible resistance to horizontal movement. The platform has a car on top driving around the edge in a circle. The platform and the car have the same mass.
What path does the centre of the platform follow?
What path does the car follow, relative to the ground?
What is the radial force between the car and the platform, as a proportion of the force for the car travelling with the same radius and velocity on the ground?
imagine a binary star system
The Rev Dodgson said:
they’re correct — there is such a thing as fiction
Basically, a fictitious force occurs when one treats an accelerated frame of reference as if it were an inertial frame of reference, or an inertial frame of reference as if it were an accelerated frame of reference. Reaction forces associated with Newton’s third law of motion are unrelated to this and are therefore real (non-fictitious) forces.
In the case of a merry-go-round, it is obvious from the viewpoint of a person standing next to it that a rider is in an accelerated frame of reference. And knowing this, the rider is also aware of being in an accelerated frame of reference. But in the case of gravitation, it is far less obvious which is the inertial frame of reference and which is the accelerated frame of reference, leading to erroneous views about the nature of gravitation.
The Rev Dodgson said:
It should be noted that the centripetal force, the inward force required to maintain the stationary trajectory within the rotating frame of reference, is a real force.
dv said:
The Rev Dodgson said:
dv said:Given that you are not in fact accelerating backwards, the force is fictitious.
OK, that’s the position I don’t accept.
Then I think probably we would get in a non-edifying argument about the meaning of “fictitious force”, since that is basically the according-to-Hoyle way to work out whether something is a fictitious force.
Having given this due consideration, I am happy to report that dv is wrong.
A “fictitious force” (in the context of this discussion) is a force applied in the equations of motion so that they will give the right answer when velocities are measured with respect to an accelerating and/or rotating frame of reference. Because it is a fictitious force it has no real effect on the body to which it is applied, and so it cannot be felt.
In the case of a person sitting in the seat of an accelerating car there is a real forwards force from the seat on the person, and a real backwards force from the person on the seat, the inertial reaction force. It is the combination of these forces that the person feels, regardless of what frame of reference they choose to determine their motion. Summing the external applied forces, there is a nett forwards force, so the person will accelerate forwards, relative to any non-accelerating frame of reference.
If they choose to use the seat as the frame of reference, then they need to introduce an imaginary backwards force to balance the real forwards force, but this is not the inertial reaction force, which is entirely independent of the frame of reference used for measuring motion.
All this is consistent with the 2nd of dv’s links, and also with KJW’s posts.
It is however not consistent with Wikipedia, and much else on the Internet, which insists that the inertial reaction force is the imaginary force, in spite of the fact that it has real effects and can be measured.
It would be good to see Wikipedia corrected, but I won’t hold my breath.
The Rev Dodgson said:
It is however not consistent with Wikipedia,
In fact Wikipedia is not consistent with Wikipedia, since this article:
Reactive_centrifugal_force
says the same as I said in the previous post.
The Rev Dodgson said:
The Rev Dodgson said:
It is however not consistent with Wikipedia,In fact Wikipedia is not consistent with Wikipedia, since this article:
Reactive_centrifugal_force
says the same as I said in the previous post.
Actually it looks like dv didn’t actually link to Wikipedia, it must have been someone in the other forum discussion.
But here is a Wikipedia quote that is inconsistent with their Reactive_centrifugal_force article:
Figure 1 (top) shows an accelerating car. When a car accelerates, a passenger feels like they’re being pushed back into the seat. In an inertial frame of reference attached to the road, there is no physical force moving the rider backward. However, in the rider’s non-inertial reference frame attached to the accelerating car, there is a backward fictitious force. We mention two possible reasons for the force to clarify its (the force’s) existence:
1.Figure 1 (center panel). To an observer at rest on an inertial reference frame (like the ground), the car will seem to accelerate. In order for the passenger to stay inside the car, a force must be exerted on the passenger. This force is exerted by the seat, which has started to move forward with the car and is compressed against the passenger until it transmits the full force to keep the passenger moving with the car. Thus, the forces exerted by the seat are unbalanced,so the passenger is accelerating in this frame.
2.Figure 1 (bottom panel). From the point of view of the interior of the car, an accelerating reference frame, there is a fictitious force pushing the passenger backwards, with magnitude equal to the mass of the passenger times the acceleration of the car. This force pushes the passenger back into the seat, until the seat compresses and provides an equal and opposite force. Thereafter, the passenger is stationary in this frame, because the fictitious force and the real force of the seat are balanced.
so does that mean dv and semantics were actually right after all
SCIENCE said:
so does that mean dv and semantics were actually right after all
No.
Not dv anyway.
Since there is not post in tis thread headed semantics, I don’t know if he she or it was right or not.
