| Topic: |
Science > Physics |
| User: |
"Jon Slaughter" |
| Date: |
25 Jan 2007 08:45:26 PM |
| Object: |
fictitious forces |
I'm trying to understand fictitious forces and I'm a little confused.
Suppose we take the car example where the car accelerates and the person
inside feels a force pushing them back.
The problem I have with that is that what a human feels is subjective. Just
cause we feel a force pushing us back doesn't mean there is a force pushing
us back. Ok, maybe thats why its fictitious but heres what I am wondering
about.
Suppose we have a platform where we can accelerate it at different rates and
gravity is always normal to it. We attach different matterials to the
platform.
Now when the platform accelerates the force causing this acceleration will
be "transmitted" through the platform onto the material by the frictional
forces holding the material down onto the platform.
But the force will take time to propagate through the material.
Suppose we have have a table with a book on it on the platform. If we push
the platform the book may or may not fall off depending on how fast we push
it(really, I suppose, how fast the acceleration is).
Now if, say, it was a human standing on the platform then there feet will
move first and then there knee's and so on. But it will take time for this
to happen and it seems that the time it takes dependents on how fast the
acceleration happens.
If, say, we could accelerate the platform fast enough we might be able to
rip the torso off the waist in a similar fashion that the book can slide off
the table.
My question is, is really this fictitious force one of that exists because
the forces are not transfered instantaneously? I suppose it has to do with
all the atomic forces existing and trying to transfer the force through the
material.
That is, the acceleration of the platform sorta works against the attractive
forces in the material that hold the material together.
So maybe in reality the force we feel in such a situation is the internal
forces trying to counteract the acceleration of the platform to keep our
body together?
I think we might be able to model this using a series of connect springs.
|
+
|
+
|
+
|
-------
the | are springs, + are connections that can break if to great a force is
applied, and the ------ is the platform.
Now if we apply a force to the platform the spring will seem to bend to an
outside observer. Now the stronger the springs the less the bend but it
depends on the acceleration. To great of an acceleration and the connectors
will break.
My reasoning though is why it bends is because of the atomic forces inside
the springs is not distributed along the entire spring in a uniform way. At
the bottom the the force is ~= to the force on the platform but as we move
up to the tip of the string the force decreases due, I suppose, to a sorta
dispersion of the force from all the atoms.
Now the spring itself might feel a force acting on it but what it feels is
really the internal force not being distributed evenly. The problem of him
feeling the platform as not accellerating and the force being on the type is
probably one of psychology rather than physics?
Anyways, It seems interesting because the forces seem to propagate at a
finite speed through the body depending on the change of acceleration with a
sorta of dissipation of the forces in the body.
I suppose that if the platform accelerates fast enough it will compress in
the direction of the acceleration?
Is any of this making sense? I've been out of school for a few years and
forgot everything I've learned(unfortunately at an amazing rate) and I'm
trying to understand some of these things. Sorry if I'm not explaining
things well but I don't have a good understanding of it ;/
Basically to sum it up I'm wondering if the fictitious force's acting on the
body in a non-inertial reference frame are due to an uneven distribution of
the force on the body that exists because of the bodies properties. It just
seems to me that the internal attractive forces are what play a part since
depending on there strength they will exhib this frictional force more or
less.
If you put a piece of grass on the platform it will bend much more than a
piece of metal. Both should experience the same forces which is entirely due
to the acceleration of the platform. But in the first case the grass's
internal forces are much weaker than the metals. there might be an
elongation of two materials in the direction normal to the acceleration
though to counterbalance these forces. (they elongate due to the
acceleration of the platform but the internal forces limit this to some
degree and depending on the strength of those forces the material might
break).
Thanks for any comments. Take it easy on me as I'm just trying to understand
some basic stuff.
Jon
.
|
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| User: "Pmb" |
|
| Title: Re: fictitious forces |
26 Jan 2007 04:45:18 AM |
|
|
"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:aXduh.66472$qO4.40875@newssvr13.news.prodigy.net...
I'm trying to understand fictitious forces and I'm a little confused.
Suppose we take the car example where the car accelerates and the person
inside feels a force pushing them back.
The problem I have with that is that what a human feels is subjective.
Just cause we feel a force pushing us back doesn't mean there is a force
pushing us back. Ok, maybe thats why its fictitious but heres what I am
wondering about.
Then don't use a human. Use a device which measures the forces exerted on
it. What will tell you if a force is exerted on something? In this case
there is a "real" force pushing on the driver and that's the force exerted
on the driver.
Suppose we have a platform where we can accelerate it at different rates
and gravity is always normal to it. We attach different matterials to the
platform.
Now when the platform accelerates the force causing this acceleration will
be "transmitted" through the platform onto the material by the frictional
forces holding the material down onto the platform.
But the force will take time to propagate through the material.
Suppose we have have a table with a book on it on the platform. If we push
the platform the book may or may not fall off depending on how fast we
push it(really, I suppose, how fast the acceleration is).
Now if, say, it was a human standing on the platform then there feet will
move first and then there knee's and so on. But it will take time for this
to happen and it seems that the time it takes dependents on how fast the
acceleration happens.
If, say, we could accelerate the platform fast enough we might be able to
rip the torso off the waist in a similar fashion that the book can slide
off the table.
That would be impossible. His feet would have to be nailed to the platform,
otherwise his feet will slip on the platform before it rips something off.
Its more likely the feet would rip off at the ankles if the acceleration was
great enough and his shoes were bolted to the platform.
My question is, is really this fictitious force one of that exists because
the forces are not transfered instantaneously? I suppose it has to do with
all the atomic forces existing and trying to transfer the force through
the material.
What you call "fictitous force" is normall known as an "inertial force." The
reason we don't normally experience inertial forces is that all of our
molecules are accelerated at the same time so that the person feels nothing
until something stops him from accelerating. Here's a few nice quotes on the
subject
http://www.geocities.com/physics_world/gr/inertial_force.htm
Pete
That is, the acceleration of the platform sorta works against the
attractive forces in the material that hold the material together.
So maybe in reality the force we feel in such a situation is the internal
forces trying to counteract the acceleration of the platform to keep our
body together?
I think we might be able to model this using a series of connect springs.
|
+
|
+
|
+
|
-------
the | are springs, + are connections that can break if to great a force is
applied, and the ------ is the platform.
Now if we apply a force to the platform the spring will seem to bend to an
outside observer. Now the stronger the springs the less the bend but it
depends on the acceleration. To great of an acceleration and the
connectors will break.
My reasoning though is why it bends is because of the atomic forces inside
the springs is not distributed along the entire spring in a uniform way.
At the bottom the the force is ~= to the force on the platform but as we
move up to the tip of the string the force decreases due, I suppose, to a
sorta dispersion of the force from all the atoms.
Now the spring itself might feel a force acting on it but what it feels is
really the internal force not being distributed evenly. The problem of him
feeling the platform as not accellerating and the force being on the type
is probably one of psychology rather than physics?
Anyways, It seems interesting because the forces seem to propagate at a
finite speed through the body depending on the change of acceleration with
a sorta of dissipation of the forces in the body.
I suppose that if the platform accelerates fast enough it will compress in
the direction of the acceleration?
Is any of this making sense? I've been out of school for a few years and
forgot everything I've learned(unfortunately at an amazing rate) and I'm
trying to understand some of these things. Sorry if I'm not explaining
things well but I don't have a good understanding of it ;/
Basically to sum it up I'm wondering if the fictitious force's acting on
the body in a non-inertial reference frame are due to an uneven
distribution of the force on the body that exists because of the bodies
properties. It just seems to me that the internal attractive forces are
what play a part since depending on there strength they will exhib this
frictional force more or less.
If you put a piece of grass on the platform it will bend much more than a
piece of metal. Both should experience the same forces which is entirely
due to the acceleration of the platform. But in the first case the
grass's internal forces are much weaker than the metals. there might be
an elongation of two materials in the direction normal to the acceleration
though to counterbalance these forces. (they elongate due to the
acceleration of the platform but the internal forces limit this to some
degree and depending on the strength of those forces the material might
break).
Thanks for any comments. Take it easy on me as I'm just trying to
understand some basic stuff.
Jon
.
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| User: "" |
|
| Title: Re: fictitious forces |
26 Jan 2007 05:46:40 AM |
|
|
I am also confused
I think they don't exist because the name itself suggests that they are
just made up to fullfill the laws.
It is due to them that in some frame of refrences important physical
laws still stand true but does that mean that without them all laws all
are not true in all frame of refrence
.
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| User: "Pmb" |
|
| Title: Re: fictitious forces |
26 Jan 2007 06:21:46 AM |
|
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<Apeksha.Jain15@gmail.com> wrote in message
news:1169812000.011772.170440@l53g2000cwa.googlegroups.com...
I am also confused
I think they don't exist because the name itself suggests that they are
just made up to fullfill the laws.
That is why theire is pressure to change the name from "fictitious force" to
"inertial force".
Pete
.
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| User: "Jon Slaughter" |
|
| Title: Re: fictitious forces |
26 Jan 2007 07:27:19 AM |
|
|
"Pmb" <peter102560_nospam@comcast.net> wrote in message
news:quudnW_-0Z38byTYnZ2dnUVZ_uejnZ2d@comcast.com...
<Apeksha.Jain15@gmail.com> wrote in message
news:1169812000.011772.170440@l53g2000cwa.googlegroups.com...
I am also confused
I think they don't exist because the name itself suggests that they are
just made up to fullfill the laws.
That is why theire is pressure to change the name from "fictitious force"
to "inertial force".
Now, I think what they mean is that these forces are fictitious in an
"absolute" frame of reference. An observer in an inerial frame of reference
does not see these as forces tha the observer in the non-inertial frame of
reference.
These forces are due to the fact that the frame of reference is non-inerial
and therefor exist only because of that. In essence they are manifestations
of the force that is causing the frame of reference to be non-inertial and
not real forces that outside that frame.
This supposes that the forces inside an inertial frame are somehow more real
then the ones in non-inertial frames.
Wikki says simple that these forces are used to describe behavior inside a
non-inertial frame. The earth spins and creates a coriolis force if we treat
it as an inertial frame. i.e., we somehow have to compensate for treating a
non-interial reference frame as being inertial and we do this by creating
these fictitious forces.
I do agree though that the name is bad. I wouldn't say they are fictitious
but more like manifestations of a non-interial frame of reference.
Jon
.
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| User: "Dirk Van de moortel" |
|
| Title: Re: fictitious forces |
26 Jan 2007 04:27:07 AM |
|
|
"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message news:aXduh.66472$qO4.40875@newssvr13.news.prodigy.net...
I'm trying to understand fictitious forces and I'm a little confused.
Suppose we take the car example where the car accelerates and the person
inside feels a force pushing them back.
He doesn't feel a force pushing him back.
Let him close his eyes and ask him what he feels.
He feels the car seat push him forward.
The problem I have with that is that what a human feels is subjective. Just
cause we feel a force pushing us back doesn't mean there is a force pushing
us back. Ok, maybe thats why its fictitious but heres what I am wondering
about.
Suppose we have a platform where we can accelerate it at different rates and
gravity is always normal to it. We attach different matterials to the
platform.
Now when the platform accelerates the force causing this acceleration will
be "transmitted" through the platform onto the material by the frictional
forces holding the material down onto the platform.
But the force will take time to propagate through the material.
Suppose we have have a table with a book on it on the platform. If we push
the platform the book may or may not fall off depending on how fast we push
it(really, I suppose, how fast the acceleration is).
Now if, say, it was a human standing on the platform then there feet will
move first and then there knee's and so on. But it will take time for this
to happen and it seems that the time it takes dependents on how fast the
acceleration happens.
If, say, we could accelerate the platform fast enough we might be able to
rip the torso off the waist in a similar fashion that the book can slide off
the table.
My question is, is really this fictitious force one of that exists because
the forces are not transfered instantaneously? I suppose it has to do with
all the atomic forces existing and trying to transfer the force through the
material.
Yes
That is, the acceleration of the platform sorta works against the attractive
forces in the material that hold the material together.
So maybe in reality the force we feel in such a situation is the internal
forces trying to counteract the acceleration of the platform to keep our
body together?
Of course. What we feel ultimately depends on the molecular
processes in our muscles and nerves.
I think we might be able to model this using a series of connect springs.
|
+
|
+
|
+
|
-------
the | are springs, + are connections that can break if to great a force is
applied, and the ------ is the platform.
Now if we apply a force to the platform the spring will seem to bend to an
outside observer. Now the stronger the springs the less the bend but it
depends on the acceleration. To great of an acceleration and the connectors
will break.
My reasoning though is why it bends is because of the atomic forces inside
the springs is not distributed along the entire spring in a uniform way. At
the bottom the the force is ~= to the force on the platform but as we move
up to the tip of the string the force decreases due, I suppose, to a sorta
dispersion of the force from all the atoms.
Now the spring itself might feel a force acting on it but what it feels is
really the internal force not being distributed evenly. The problem of him
feeling the platform as not accellerating and the force being on the type is
probably one of psychology rather than physics?
Anyways, It seems interesting because the forces seem to propagate at a
finite speed through the body depending on the change of acceleration with a
sorta of dissipation of the forces in the body.
I suppose that if the platform accelerates fast enough it will compress in
the direction of the acceleration?
Yes
Is any of this making sense?
Yes, everything.
I've been out of school for a few years and
forgot everything I've learned(unfortunately at an amazing rate) and I'm
trying to understand some of these things. Sorry if I'm not explaining
things well but I don't have a good understanding of it ;/
Don't be so modest ;-)
Basically to sum it up I'm wondering if the fictitious force's acting on the
body in a non-inertial reference frame are due to an uneven distribution of
the force on the body that exists because of the bodies properties. It just
seems to me that the internal attractive forces are what play a part since
depending on there strength they will exhib this frictional force more or
less.
If you put a piece of grass on the platform it will bend much more than a
piece of metal. Both should experience the same forces which is entirely due
to the acceleration of the platform. But in the first case the grass's
internal forces are much weaker than the metals. there might be an
elongation of two materials in the direction normal to the acceleration
though to counterbalance these forces. (they elongate due to the
acceleration of the platform but the internal forces limit this to some
degree and depending on the strength of those forces the material might
break).
Thanks for any comments. Take it easy on me as I'm just trying to understand
some basic stuff.
Very nice analysis.
Dirk Vdm
.
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| User: "Dumbledore_" |
|
| Title: Re: fictitious forces |
26 Jan 2007 08:25:52 AM |
|
|
"Dork Van de merde" aka
"Dork Van de psychopath",
"Dork Van de psychotic fumble mumbler",
"Dork Van de fuckhead",
"Dirk Van de moortel" <dirkvandemoortel@ThankS-NO-SperM.hotmail.com> =
wrote in message news:%Hkuh.500$U12.475@news.cpqcorp.net...
[anip]
http://users.telenet.be/vdmoortel/dirk/Physics/TwinsEvents.html
"We use 3 inertial reference frames" [because Dorks can't get the result =
they want in two].=20
"In neither of these frames any form of acceleration is felt" [neither =
one of
all three].
"In order for the travelling twin to make HIS trip, SHE must be in frame =
S'
while going away".
"if T =3D 5 years and v =3D 0.8c, then the stay at home twin will have =
aged=20
10 years".
Belgium is where the farts blow.
"Your conclusion is dead wrong.
Start over, but skip the first part and the conclusion." -- Dork Van de=20
fuckhead.=20
"You made a mistake" -- Dork Van de psychotic fumble mumbler.
ASSistant professor Paul B. Andersen, tusseladd:
"That is, we can reverse the directions of the frames
which is the same as interchanging the frames,
which - as I have told you a LOT of times,
OBVIOUSLY will lead to the transform:
t =3D (tau-xi*v/c^2)/sqrt(1-v^2/c^2)
x =3D (xi - v*tau)/sqrt(1-v^2/c^2)
or:
tau =3D (t+xv/c^2)/sqrt(1-v^2/c^2)
xi =3D (x + vt)/sqrt(1-v^2/c^2)"=20
.
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| User: "Jon Slaughter" |
|
| Title: Re: fictitious forces |
26 Jan 2007 07:27:52 AM |
|
|
Thanks,
Glad to know I'm not to far off ;) Maybe my brain didn't loose as many brain
cells as I thought.
Thanks,
Jon
"Dirk Van de moortel" <dirkvandemoortel@ThankS-NO-SperM.hotmail.com> wrote
in message news:%Hkuh.500$U12.475@news.cpqcorp.net...
"Jon Slaughter" <Jon_Slaughter@Hotmail.com> wrote in message
news:aXduh.66472$qO4.40875@newssvr13.news.prodigy.net...
I'm trying to understand fictitious forces and I'm a little confused.
Suppose we take the car example where the car accelerates and the person
inside feels a force pushing them back.
He doesn't feel a force pushing him back.
Let him close his eyes and ask him what he feels.
He feels the car seat push him forward.
The problem I have with that is that what a human feels is subjective.
Just cause we feel a force pushing us back doesn't mean there is a force
pushing us back. Ok, maybe thats why its fictitious but heres what I am
wondering about.
Suppose we have a platform where we can accelerate it at different rates
and gravity is always normal to it. We attach different matterials to the
platform.
Now when the platform accelerates the force causing this acceleration
will be "transmitted" through the platform onto the material by the
frictional forces holding the material down onto the platform.
But the force will take time to propagate through the material.
Suppose we have have a table with a book on it on the platform. If we
push the platform the book may or may not fall off depending on how fast
we push it(really, I suppose, how fast the acceleration is).
Now if, say, it was a human standing on the platform then there feet will
move first and then there knee's and so on. But it will take time for
this to happen and it seems that the time it takes dependents on how fast
the acceleration happens.
If, say, we could accelerate the platform fast enough we might be able to
rip the torso off the waist in a similar fashion that the book can slide
off the table.
My question is, is really this fictitious force one of that exists
because the forces are not transfered instantaneously? I suppose it has
to do with all the atomic forces existing and trying to transfer the
force through the material.
Yes
That is, the acceleration of the platform sorta works against the
attractive forces in the material that hold the material together.
So maybe in reality the force we feel in such a situation is the internal
forces trying to counteract the acceleration of the platform to keep our
body together?
Of course. What we feel ultimately depends on the molecular processes in
our muscles and nerves.
I think we might be able to model this using a series of connect springs.
|
+
|
+
|
+
|
-------
the | are springs, + are connections that can break if to great a force
is applied, and the ------ is the platform.
Now if we apply a force to the platform the spring will seem to bend to
an outside observer. Now the stronger the springs the less the bend but
it depends on the acceleration. To great of an acceleration and the
connectors will break.
My reasoning though is why it bends is because of the atomic forces
inside the springs is not distributed along the entire spring in a
uniform way. At the bottom the the force is ~= to the force on the
platform but as we move up to the tip of the string the force decreases
due, I suppose, to a sorta dispersion of the force from all the atoms.
Now the spring itself might feel a force acting on it but what it feels
is really the internal force not being distributed evenly. The problem of
him feeling the platform as not accellerating and the force being on the
type is probably one of psychology rather than physics?
Anyways, It seems interesting because the forces seem to propagate at a
finite speed through the body depending on the change of acceleration
with a sorta of dissipation of the forces in the body.
I suppose that if the platform accelerates fast enough it will compress
in the direction of the acceleration?
Yes
Is any of this making sense?
Yes, everything.
I've been out of school for a few years and forgot everything I've
learned(unfortunately at an amazing rate) and I'm trying to understand
some of these things. Sorry if I'm not explaining things well but I
don't have a good understanding of it ;/
Don't be so modest ;-)
Basically to sum it up I'm wondering if the fictitious force's acting on
the body in a non-inertial reference frame are due to an uneven
distribution of the force on the body that exists because of the bodies
properties. It just seems to me that the internal attractive forces are
what play a part since depending on there strength they will exhib this
frictional force more or less.
If you put a piece of grass on the platform it will bend much more than a
piece of metal. Both should experience the same forces which is entirely
due to the acceleration of the platform. But in the first case the
grass's internal forces are much weaker than the metals. there might be
an elongation of two materials in the direction normal to the
acceleration though to counterbalance these forces. (they elongate due to
the acceleration of the platform but the internal forces limit this to
some degree and depending on the strength of those forces the material
might break).
Thanks for any comments. Take it easy on me as I'm just trying to
understand some basic stuff.
Very nice analysis.
Dirk Vdm
.
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| User: "Andy Resnick" |
|
| Title: Re: fictitious forces |
26 Jan 2007 08:29:52 AM |
|
|
Jon Slaughter wrote:
<snip>
My question is, is really this fictitious force one of that exists because
the forces are not transfered instantaneously? I suppose it has to do with
all the atomic forces existing and trying to transfer the force through the
material.
I was following you until this paragraph- can you explain what you are
trying to say again?
That is, the acceleration of the platform sorta works against the attractive
forces in the material that hold the material together.
So maybe in reality the force we feel in such a situation is the internal
forces trying to counteract the acceleration of the platform to keep our
body together?
Are you referring to mechansensation (i.e. the otoliths and inner ear
mechanism) or are you using the term "feel" in a less literal sense?
<snip>
Anyways, It seems interesting because the forces seem to propagate at a
finite speed through the body depending on the change of acceleration with a
sorta of dissipation of the forces in the body.
The mass-sping model of elastic media are perfectly valid in the linear
limit. Real materials don't behave like this- there is a viscous
component (modeled as a dashpot... has anyone ever actually *seen* a
dashpot?)
And the theory of elastic wave propogation is well-developed. It's
messy, but well developed.
I suppose that if the platform accelerates fast enough it will compress in
the direction of the acceleration?
Is any of this making sense? I've been out of school for a few years and
forgot everything I've learned(unfortunately at an amazing rate) and I'm
trying to understand some of these things. Sorry if I'm not explaining
things well but I don't have a good understanding of it ;/
Basically to sum it up I'm wondering if the fictitious force's acting on the
body in a non-inertial reference frame are due to an uneven distribution of
the force on the body that exists because of the bodies properties. It just
seems to me that the internal attractive forces are what play a part since
depending on there strength they will exhib this frictional force more or
less.
Oh boy... non-uniform stress distributions over a heterogeneous body.
There's not much we can say as a general statement. Again, the theory
is well-developed and does not require knowledge of the microstructure
of the body- rather, a constitutive relation is required relating the
stress to the deformation (or deformation gradient).
Also, "fictional forces" have a clear maning in physics that may not be
what you mean. In mechanics, the force is independent of the properties
of the body- it's the difference between kinematics (the result of
applying a force) and dynamics (the origin of the force).
If you put a piece of grass on the platform it will bend much more than a
piece of metal. Both should experience the same forces which is entirely due
to the acceleration of the platform. But in the first case the grass's
internal forces are much weaker than the metals. there might be an
elongation of two materials in the direction normal to the acceleration
though to counterbalance these forces. (they elongate due to the
acceleration of the platform but the internal forces limit this to some
degree and depending on the strength of those forces the material might
break).
Ok, what you are talking about here is simply the physics of elastic
bodies, and comparing two bodies with different consitutive
relationships (Young's modulus).
Thanks for any comments. Take it easy on me as I'm just trying to understand
some basic stuff.
Jon
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
.
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| User: "Jon Slaughter" |
|
| Title: Re: fictitious forces |
26 Jan 2007 09:17:47 AM |
|
|
"Andy Resnick" <andy.resnick@op.case.edu> wrote in message
news:epd3d0$jr1$1@eeyore.INS.cwru.edu...
Jon Slaughter wrote:
<snip>
My question is, is really this fictitious force one of that exists
because the forces are not transfered instantaneously? I suppose it has
to do with all the atomic forces existing and trying to transfer the
force through the material.
I was following you until this paragraph- can you explain what you are
trying to say again?
What I mean is that a "body" is made up of a collection of atoms. The reason
why we group these atoms together as a "body" is because of the attractive
forces these atoms have for each other. If there no attraction then there is
no reason for them to "stick" together and any motion applied will make them
seperate(well, not in general but I think you get my point).
So what I mean is that if we apply a force to say a piece of a long metal
bar then we can cause it to want to rotate about some center. But if say it
was water that we applied the force to then we would just "tear" it apart.
In the first case the metal bar had some forced transmitted to the other end
but obviously the distribution of the forces along the bar is not uniform or
the bar would not have rotated in the first place. It does happen to water
to some degree but not nearly as much.
But how was the forces transmitted to the other end of the bar? It must have
been due to these attractive forces.
But now there is a differential in force between the two ends of the bar
which cause it to rotate. We can either see the one end as recieving a force
and the other end not, or the opposite, or both ends recieving 1/2 the force
but in opposite directions. I think this is called a couple or something.
Mathematically we see the resultant of net forces and can create any set of
forces that sum up to the net force and that is a possible scenario.
That is if Fn = sum(F_k) where F_k is a force and if we just know Fn then we
can find many solutions that work.
Now we know the true solution because we created the force but if you were
the bar and you didn't know then you could come up with anything that would
solve the above equation to explain the forces you are experiencing. I think
this is sorta what it means to be a fictitious force.
Does that make more sense?
That is, the acceleration of the platform sorta works against the
attractive forces in the material that hold the material together.
So maybe in reality the force we feel in such a situation is the internal
forces trying to counteract the acceleration of the platform to keep our
body together?
Are you referring to mechansensation (i.e. the otoliths and inner ear
mechanism) or are you using the term "feel" in a less literal sense?
No, the psychological aspect of feeling. Similar to how we experience heat.
We might say something is hot only because it is hotter than something else
but in reality it might not be hot at all. Ofcourse I think that doesn't
make to much sense but...
<snip>
Anyways, It seems interesting because the forces seem to propagate at a
finite speed through the body depending on the change of acceleration
with a sorta of dissipation of the forces in the body.
The mass-sping model of elastic media are perfectly valid in the linear
limit. Real materials don't behave like this- there is a viscous
component (modeled as a dashpot... has anyone ever actually *seen* a
dashpot?)
Sure. I was just using it as an example. I was't even think about how
springs actually behave but just trying to simplify it a little.
And the theory of elastic wave propogation is well-developed. It's messy,
but well developed.
I suppose that if the platform accelerates fast enough it will compress
in the direction of the acceleration?
Is any of this making sense? I've been out of school for a few years and
forgot everything I've learned(unfortunately at an amazing rate) and I'm
trying to understand some of these things. Sorry if I'm not explaining
things well but I don't have a good understanding of it ;/
Basically to sum it up I'm wondering if the fictitious force's acting on
the body in a non-inertial reference frame are due to an uneven
distribution of the force on the body that exists because of the bodies
properties. It just seems to me that the internal attractive forces are
what play a part since depending on there strength they will exhib this
frictional force more or less.
Oh boy... non-uniform stress distributions over a heterogeneous body.
There's not much we can say as a general statement. Again, the theory is
well-developed and does not require knowledge of the microstructure of the
body- rather, a constitutive relation is required relating the stress to
the deformation (or deformation gradient).
Yes, but this doesn't tell you what is actually is happening inside but only
how to predict the outcome. Its kind like saying that the odds of flipping a
coin and landing on heads is 1/2 but this tells you nothing about the actual
mechanics that produce a coin landing on heads.
Also, "fictional forces" have a clear maning in physics that may not be
what you mean. In mechanics, the force is independent of the properties of
the body- it's the difference between kinematics (the result of applying a
force) and dynamics (the origin of the force).
Well, probably not exactly. What I mean is the attractive forces between
atoms. This is similar to frictional forces in that friction is due to
attractions but I sorta mean the forces inside a body that keep the body
together and transfer forces within that body. If you hit a nail with a
hammer the forces that drives the tip into the wood isn't magically created.
It must be due to the the particles that consist of the nail and hold the
nail together.
Maybe one can think that there exists a microscopic affinty that holds a
body together. Maybe p(x,y,z,t). Here p represents the affinity that the
point at (x,y,z,t) for other points near it. so if p = 0 then it means that
there is no affinty for any material of the body at that point to be
attracted to other points of the body near it. So if p is small then it
means its weak there and if p is large then it means the material at that
point is attracted to the other points near it a great deal.
But p would be related to how the forces are transfered from the nail head
to the point. We could think that an ideal nail would have, say, p = 1
inside the nail and 0 outside. Then if the force is not to great it will be
transmitted through the atoms almost perfectly to the head. The speed at
which it does this will be very fast. I suppose it will be at c. I suppose
we could also think of a very long nail and then it it with a hammer and
then see how long it takes the tip to recieve the force.
I guess your right about the elasticity but I'm curious I suppose to how it
happens. I guess they probably explain this stuff in a book on the theory of
elasticity though. I'll try to check it out.
If you put a piece of grass on the platform it will bend much more than a
piece of metal. Both should experience the same forces which is entirely
due to the acceleration of the platform. But in the first case the
grass's internal forces are much weaker than the metals. there might be
an elongation of two materials in the direction normal to the
acceleration though to counterbalance these forces. (they elongate due to
the acceleration of the platform but the internal forces limit this to
some degree and depending on the strength of those forces the material
might break).
Ok, what you are talking about here is simply the physics of elastic
bodies, and comparing two bodies with different consitutive relationships
(Young's modulus).
Yeah, I think so. I'll have to look over the theory and see what it says. I
know they talk about a lot of global aspects but I'm interested in the
mechanism's. It has to do with the electronic forces that exist and I'm
trying to see how these things manifest themselfs on a macroscopic scale.
Its way to say that if we push on a paddlewheel that it will cause it to
rotate, but why? What happens when we apply the force to the small piece of
area on the edge of one part of the wheel? How does that cause the other
end to recieve a force? and this force is no like someone pushing on the
other end but due to the material wanting to stay the same material(the
affinity of material).
The way I am visualizing it is that when I push on the edge I setup a chain
reaction where all the atoms are transmitting this force down the body and
it eventually reaches the other side. But because the forces are not
perfectly transmitted the resultant force is not exactly the same and its
not in the same direction but sorta split up. Actually the force turn sinto
many smaller forces in many different directions and many being canceled out
to keep the body from tearing apart.
Its hard to explain. I just know if I push on water that my finger will go
through. If I push on metal I can make the whole bar move(well, it probably
will rotate). Why? Seems to me it has to do with that "affinity" thing I
talked about. Its the same interactions that make a body a body but when we
apply a force at one point it causes some atoms to move farther apart than
normal, but they have an attraction and then try and counter this force. Its
sorta like a wave that is propagated along the bar but its a force and its
propagated by the need for the body to maintain a body(its really an innate
probably of the body, if it didn' thave this then it wouldn't be a "body").
I know I'm not being to clear but hopefully you get my idea.
Thanks,
Jon
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| User: "Andy Resnick" |
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| Title: Re: fictitious forces |
29 Jan 2007 09:14:05 AM |
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Jon Slaughter wrote:
<snip>
Its hard to explain. I just know if I push on water that my finger will go
through. If I push on metal I can make the whole bar move(well, it probably
will rotate). Why? Seems to me it has to do with that "affinity" thing I
talked about. Its the same interactions that make a body a body but when we
apply a force at one point it causes some atoms to move farther apart than
normal, but they have an attraction and then try and counter this force. Its
sorta like a wave that is propagated along the bar but its a force and its
propagated by the need for the body to maintain a body(its really an innate
probably of the body, if it didn' thave this then it wouldn't be a "body").
I know I'm not being to clear but hopefully you get my idea.
Ok, I think I have a better idea- you are asking how macroscopic
properties arise from microscopic properties. Fortunately (or
unfortunately, depending on your point of view), this is an active area
of research becuase there is no theory for constitutive relations. We
have theoretical derivations for only the simplest materials- perfect
crystals and perfectly homogeneous dielectrics. For real materials,
there is no theory. There are many constitutive relations out there,
but all of them are little more than curve-fitting.
It is tempting to claim that it is all reducible, in the end, to
electrostatic/electrodynamic interactions between electron clouds. This
should be resisted, for the same reason that detailed knowledge of a
flipped coin trajectory is required to "prove" that it lands, on
average, face side up 50% of the time.
--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
Case Western Reserve University
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| User: "Sue..." |
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| Title: Re: fictitious forces |
28 Jan 2007 05:29:51 PM |
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On Jan 25, 9:45 pm, "Jon Slaughter" <Jon_Slaugh...@Hotmail.com> wrote:
I'm trying to understand fictitious forces and I'm a little confused.
[...]
Basically to sum it up I'm wondering if the fictitious force's acting on the
body in a non-inertial reference frame are due to an uneven distribution of
the force on the body that exists because of the bodies properties. It just
seems to me that the internal attractive forces are what play a part since
depending on there strength they will exhib this frictional force more or
less.
Induction forces are *evenly* distributed. They ~penetrate~
matter because they induce their neighors into participating
in the path.
http://www.chem.purdue.edu/gchelp/liquids/inddip.html
http://www.research.ibm.com/grape/grape_ewald.htm
<<... Newton recognized that the
law of inertia is unsatisfactory
in a context so far unmentioned in this
exposition, namely that it gives no
real cause for the special physical
position of the states of motion of the
inertial frames relative to all other
states of motion. It makes the observable
material bodies responsible for the
gravitational behaviour of a material
point, yet indicates no material cause
for the inertial behaviour of the material
point but devises the cause for it
(absolute space or inertial ether). This
is not logically inadmissible although
it is unsatisfactory. For this reason
E. Mach demanded a modification of the
law of inertia in the sense that the
inertia should be interpreted as an
acceleration resistance of the bodies
against one another and not against "space".
This interpretation governs the expectation
that accelerated bodies have concordant
accelerating action in the same
sense on other bodies (acceleration induction).
This interpretation is even more
plausible according to general relativity
which eliminates the distinction between
inertial and gravitational effects.
It amounts to stipulating that, apart
from the arbitrariness governed by the
free choice of coordinates, the
gm v -field shall be completely determined
by the matter. Mach's stipulation is favoured
in general relativity by the circumstance
that acceleration induction in accordance
with the gravitational field equations really
exists, although of such slight intensity
that direct detection by mechanical experiments
is out of the question. >>
http://nobelprize.org/nobel_prizes/physics/laureates/1921/einstein-
lecture.html
Sue...
http://www.esa.int/SPECIALS/GSP/SEM0L6OVGJE_0.html
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| User: "Matthew Lybanon" |
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| Title: Re: fictitious forces |
26 Jan 2007 11:18:14 AM |
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in article aXduh.66472$qO4.40875@newssvr13.news.prodigy.net, Jon Slaughter
at wrote on 1/25/07 8:45 PM:
I'm trying to understand fictitious forces and I'm a little confused.
Suppose we take the car example where the car accelerates and the person
inside feels a force pushing them back.
The problem I have with that is that what a human feels is subjective. Just
cause we feel a force pushing us back doesn't mean there is a force pushing
us back.
An inanimate object would be affected in the same way as a human being. The
human's subjective interpretation (what he "feels") is not the point--except
that it may coincide with what is happening.
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| User: "G=EMC^2 Glazier" |
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| Title: Re: fictitious forces |
28 Jan 2007 01:49:21 PM |
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Jon Best to keep in mind inertia and gravity are the same Bert
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| User: "Sam Wormley" |
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| Title: Re: fictitious forces |
28 Jan 2007 01:54:19 PM |
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G=EMC^2 Glazier wrote:
Jon Best to keep in mind inertia and gravity are the same Bert
You mean inertia makes the apple fall?
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| User: "The Ghost In The Machine" |
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| Title: Re: fictitious forces |
28 Jan 2007 04:02:08 PM |
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In sci.physics, Sam Wormley
<swormley1@mchsi.com>
wrote
on Sun, 28 Jan 2007 19:54:19 GMT
<Lb7vh.370465$1i1.49502@attbi_s72>:
G=EMC^2 Glazier wrote:
Jon Best to keep in mind inertia and gravity are the same Bert
You mean inertia makes the apple fall?
Maybe in a rotating spacewheel a la _2001_. :-)
Certainly not around here.
--
#191,
Linux. Because Windows' Blue Screen Of Death is just
way too frightening to novice users.
--
Posted via a free Usenet account from http://www.teranews.com
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| User: "=?UTF-8?Q?Jeff=E2=80=A6Relf?=" |
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| Title: Gravity is inertia ( and vice versa ) but it's modeled differently. |
28 Jan 2007 02:07:23 PM |
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Hi Bert, Gravity is inertia ( and vice versa )
in the same sense energy is mass ( and vice versa ).
Intrinsically, they are truly the same;
but, thanks to differences in the --> Prior Knowns <-- ,
they have to be --> Modeled <-- differently.
Special Relativity:
e = m * c^2
General Relativity ( which includes Special Relativity ):
4D_Geodesic [ i.e. General Relativity's gravitational energy field ]
= 8 * pi * G * T_αβ / c^4 [ i.e. Pressure and Density ]
.
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