Is centrifugal force a real force ? You know centripetal force is inward, while centrifugal force causes things to go out.

I heard it is fictitious !

:)

In a strict sense, "centrifugal force" is a fictitious force. Objects will continue in a straight line unless a force is applied. If you are swinging a weight around your head, in order to get the weight to move in a circle rather than a straight line, you must apply force. Your muscles don't know physics and interpret this as working against some force trying to make the weight move in a straight line!

Centrifugal force is as real as Hollywood special effects.
For a sufficiently large (radius)/(budget), it is indistinguishable from a real (force)/(shot) until you step outside the (accelerating system)/(dark cinema).

mmmh, don't be so sure...

In certain situations I agree it is ficticious or inertial but in others it may not.

:)

Note that as far as the movie experience goes, it really doesn't matter if a particular shot was made using real action or using special effects. The experience is the same.

The same reasoning applies to centrifugal force and gravity.

Consider this,

You are standing in one of those rotating rides you see at theme parks (the ones where they spin around and you get stuck to the sides of the inside walls! (gravitrons or something stoopid like that)). Anyway, when you are inside you 'feel' an outward push as some interpret as a 'force' that is pushing you outwards, pressing you on the inside of the wall. Now if I was to stick you to the outside of the gravitron and start it up. You will hang on with all your might until you eventually succumb to the 'outward force' that you experienced on the inside. You let go and you... fly off at a tangent!!!! NOT straight outwards!!! So centrifugal force is fictional.

>> You are standing in one of those rotating rides you see at theme parks (the ones where they spin around and you get stuck to the sides of the inside walls!

OK, the outward force pushing you against the cylinder is the result of inertial action. But there is a real force pushing onto the drum.. This can be seen in ball bearings, the outside case is often cracked if the balls spin too fast.

This real force is away from the centre, ... we have a term for forces towards the centre..centripetal force..

Away from the centre is centrifugal force, and in the above case it becomes a real force resulting from an inertial action..

WE are used to non inertial action being produced by a force, centrifugal force is the opposite.

IMO

:)

Centrifugal force is a misnomer. An object does not flee from the center of rotation (Id est: It is not centerfleeing).

Think of spinning a weight on a string, causing it to follow a circular path. If the string breaks, the weight does not move away from the center of rotation. It moves in a straight line tangent to its circular path.

When David slew Goliath, he released the stone when it was moving toward Goliath, not when it was exactly on the line between him and Goliath (At that time, the stone was moving perpendicular to the line between them).

BTW: The story of David and Goliath seems to me to be one of the more believable stories in the Bible. A teenager considers himself indestructible and has the nerve to face a giant. He also has the fast relexes required to get away with the foolishness. The big dumb jerk cannot conceive of losing to the little guy and does not bother to study his methods. He marches forward expecting to fight his way.

Zarkov

You're a pretty funny guy!

You can't make heads or tails of pseudo forces yet come up with a so-called gravity theory.

Are you sure centrifugal forces are not spun electrodynamically?

:p

Hey you guys have all this theory, hanging by a fictitious thread, what you going to do when your brain snaps, and the reality of the theory of physics becomes a mass illusion.....

It seems there is no REAL understanding, useful molecules, waves, electrons.... all conjecture, now forces...

In a rotating frame a centripetal force would have a signed vector, an outward force would have an oppositely signed vector... ball bearings have to have an outward force, yes due to inertia, but expressed between the balls and the race, and drawn from the spin. What sign will you give ?? to the force between the balls and the race ??

:)

Pysics IMO, is under an LSD trip..

What sign will you give ??

How about:

:rolleyes:

This real force is away from the centre, ... we have a term for forces towards the centre..centripetal force..
Away from the centre is centrifugal force, and in the above case it becomes a real force resulting from an inertial action..


No, centrifugal force is not real (in both inertial and non-inertial reference frames!). I think you (Zarkov) are getting your reference frames mixed up.

For inertial reference frames obviously there is not such a thing (I hope we all agree on that! Plenty of simple experiments will show this ie. spin a weight at the end of a rope above your head, let it go and the weight will fly off at a tangent.)

For non-inertial reference frames ie, sitting on a merry-go-round watching a ball roll off, this is VERY different to people standing on the ground wacthing you. They are in INERTIAL reference frames and Newton's law can be aptly applied. For you on the merry-go-round, it is a NON-INERTIAL reference frame and Newton's laws do not hold!

If you are in an inertial reference frame watching a ball on a merry-go-round, there is no net force acting on the ball from Newton's second law. Yet for the person on the merry-go-round (NON-INERTIAL) there is an acceleration respect to you!

So how do you account for this? Well physicists perform a neat trick. We write down F = ma is if there was some rotational force acting on it equal to mv^2/r. This extra force can be designated as centrifugal force since this SEEMS to act outward.

To people standing on the ground they do not see this effect at all! All we have done is make up this extra force (pseudoforce) so that we can make calculations in NON-INERTIAL reference frames. Therefore, people on the merry-go-round in NON-INERTIAL reference frames use F = mv^2/r instead of F = ma.

There is no centrifugal force to INERTIAL reference frames (ie. everyday life). The ball simply rolls off at a tangent. There appears to be a force directed radially outward in a NON-INERTIAL reference frame. The ball flies off directly outward (if you are sitting at there axis of rotation).

So if someone was in a position to say that centrifugal force was a real force they would have to convince me that they could accelerate an object without applying a force to it. Simple as that!

Is it not true that:
F = ma
?

Every "learned" person I know will say that this is specific to inertial systems.

OK, define "inertial system."

They come back with the definition something along the lines:
inertial system = a system in which F = ma is valid.

:bugeye:

Sure, whatever you say.

Now, let's introduce the conception of Einstein, that the geometrical construct of space-time is a thing (as opposes to nothing), and more importantly (for purposes of calculation) that is is not necessarily Euclidean. In such a case, how would you define forces, or even vectors themselves?

I hold that the centrifugal force is just as valid as the Lorentz force or the force of gravity.

We say that the centrifugal force is just an artifact of a rotating system. Rotating with respect to what? Some say the fixed stars. Well, how do we know that they're fixed? Probably something along the lines of the validity of F = ma again. I say horseshit! Let's just say that our system has a geometrical warping (just like we would say about the force of gravity) manifest in its rotation. We say similarly that the magnetic force is caused by the geometrical distinction between coordinate systems moving with respect to each other. Are the forces of gravity and magnetism considered fictitious? I don't think that so. So why call the centrifugal force fictitious, whereas the Lorentz and gravitational forces are "actual?"

Originally posted by errandir

Now, let's introduce the conception of Einstein, that the geometrical construct of space-time is a thing (as opposes to nothing), and more importantly (for purposes of calculation) that is is not necessarily Euclidean.

not even possibly euclidean! einstein s space is minkowski at best.

Lethe,
I'm glad to know that you've read my post. Is that all you have to say about it?:)

Originally posted by errandir
Lethe,
I'm glad to know that you've read my post. Is that all you have to say about it?:)
well, ok, you asked for it.

Originally posted by errandir
Are the forces of gravity and magnetism considered fictitious? I don't think that so. So why call the centrifugal force fictitious, whereas the Lorentz and gravitational forces are "actual?"
first of all, the gravitational force *is* fictitious. this is einstein s equivalence principle.

secondly, the difference between the lorentz force (a real force) and gravtity and centrifugal force (fictitious forces), is that you can make the fictitious forces disappear by a change of coordinates. you cannot do this with the lorentz force. therefore it is a real physical force.

remember, things that depend on your coordinate system are not physical. things that do, are.

Originally posted by errandir

Now, let's introduce the conception of Einstein, that the geometrical construct of space-time is a thing (as opposes to nothing), and more importantly (for purposes of calculation) that is is not necessarily Euclidean. In such a case, how would you define forces, or even vectors themselves?


vectors and forces are perfectly well defined on a curved spacetime, even if it is a dynamical object. why wouldn t they be?

If the surface is curved (i.e. a sphere) then I understand that vectors can be defined in a tangent plane, but I thought that they could not be defined in general on the sphere (which I mean to exemplify a 2-D curved space).

Originally posted by errandir
Is it not true that:
F = ma
?

Every "learned" person I know will say that this is specific to inertial systems.

OK, define "inertial system."

They come back with the definition something along the lines:
inertial system = a system in which F = ma is valid.

:bugeye:

Sure, whatever you say.


the existence of inertial frames is implied by the homogeneity of space. that all inertial frames are related by a uniform rectilinear transformation, and that F=ma in such frames, are the starting postulates of classical mechanics. they cannot be derived, nor justified, except by experiment. i can assure you that they have been verified by experiment, and remember, physics is a science, which means it starts with observations, and infers laws from them, which then form the axioms of theories.

do you have a problem with this approach to science?

I have a problem with circular reasoning.

Originally posted by errandir
If the surface is curved (i.e. a sphere) then I understand that vectors can be defined in a tangent plane, but I thought that they could not be defined in general on the sphere (which I mean to exemplify a 2-D curved space).

the tangent plane is how we define vectors on a sphere. that is the definition of a vector on a sphere. got it? perfectly well defined. your claim that these things are hard to define is unfounded.

Originally posted by errandir
I have a problem with circular reasoning.

there is no circular reasoning here. you start with axioms, based on observation, and you prove theorems about them. what s the problem?

Originally posted by lethe
remember, things that depend on your coordinate system are not physical. things that do, are. Classically speaking, if you had an electron orbiting a proton (Bohr model of hydrogen without discretization), then, in the coordinate system of the electron, there is no electric force attracting it to the proton. Thus, by the reasoning mentioned, the electric force can be changed (eliminated) by chosing a frame that moves with the electron. Since the electric force is part of the Lorentz force, then it seems that the Lorentz force must change under this change in coordinate system. I'm sure that there would be some electric tidal force, but the same can be said of gravity.

Originally posted by lethe
there is no circular reasoning here. you start with axioms, based on observation, and you prove theorems about them. what s the problem? The definition (at least the one that I was given just this morning) of an inertial system is one in which Newton's laws (specifically F = ma was given) are valid. F = ma is said to be the rule in Newtonian systems.

This came up in a discussion about curvilinear coordinate systems. Specifically, it was mentioned that "extra" terms appear in the expression for acceleration. My arguement was that these terms show favoritism for the cartesian system. On and on we went. I asked how we know that the cartesian basis vectors are not the ones actually changing (in time) along the trajectory of the particle. That is when we got into the F = ma <-> Newtonian system discussion.

Maybe circular was the wrong word. I will refrase.
I don't like using a term in its own definition.

Originally posted by errandir
Classically speaking, if you had an electron orbiting a proton (Bohr model of hydrogen without discretization), then, in the coordinate system of the electron, there is no electric force attracting it to the proton. Thus, by the reasoning mentioned, the electric force can be changed (eliminated) by chosing a frame that moves with the electron. Since the electric force is part of the Lorentz force, then it seems that the Lorentz force must change under this change in coordinate system. I'm sure that there would be some electric tidal force, but the same can be said of gravity.

of course there would be a force.

Originally posted by errandir
The definition (at least the one that I was given just this morning) of an inertial system is one in which Newton's laws (specifically F = ma was given) are valid. F = ma is said to be the rule in Newtonian systems.

This came up in a discussion about curvilinear coordinate systems. Specifically, it was mentioned that "extra" terms appear in the expression for acceleration. My arguement was that these terms show favoritism for the cartesian system. On and on we went. I asked how we know that the cartesian basis vectors are not the ones actually changing (in time) along the trajectory of the particle. That is when we got into the F = ma <-> Newtonian system discussion.

Maybe circular was the wrong word. I will refrase.
I don't like using a term in its own definition.

circular is not the wrong term. from your description, it sounds like you are dealing with circular reasoning. however, i do not believe *my* description above contains any circular reasoning. i cannot attest for whatever you heard this morning.

Originally posted by lethe
the existence of inertial frames is implied by the homogeneity of space.What is this homogeneity of space? I don't see how space can be homogenious with all the warping due to massive bodies. Hypothetically, if there were no massive bodies, then perhaps there would be some homogeneity, but then this is not testable.




Originally posted by lethe
that all inertial frames are related by a uniform rectilinear transformation, and that F=ma in such frames, are the starting postulates of classical mechanics.But "inertial frame" is not defined here.




Originally posted by lethe
they cannot be derived, nor justified, except by experiment. i can assure you that they have been verified by experiment, and remember, physics is a science, which means it starts with observations, and infers laws from them, which then form the axioms of theories.I never heard of any experiments to show F = ma. Where were they done? Where did the experimenter find an inertial frame?

Originally posted by lethe
you start with axioms, based on observation, and you prove theorems about them. what s the problem? If the axiom is:
F = ma
then the problem is, in some systems there is an F that most people like to call fictitious (the centripetal force). Why do they call it so? If you have an m in this frame, it will experience an a according to the vector sum that is F. If F is decomposed to yield some F_centripetal, where in the set of axioms is this fictitious?

Are you saying that there is an electric force in a frame moving with an orbiting electron. I see two points in space defined: the electron and the proton (classically speaking). In a frame in which neither the electron nor proton are moving, there is no acceleration of the electron.

Two obvious possibilities:
1) There is an electric force towards the proton and an equal centrifugal force away from the electron
2) There is no such thing as centrifugal force, therefore, since F = ma = 0, there is no force acting on the electron.

Originally posted by errandir
What is this homogeneity of space? I don't see how space can be homogenious with all the warping due to massive bodies. Hypothetically, if there were no massive bodies, then perhaps there would be some homogeneity, but then this is not testable.

check this out: drop a quarter and a dime. see which one lands first. then walk down the street and try it again. you should get the same result. why? because space is homogeneous. the laws of physics are independent of your location in space.




But "inertial frame" is not defined here.

let me try this again.

postulate: there exist reference frames which satisfy the following 2 conditions:

the laws of physics are the same in all such coordinate systems. (homogeneity)

any two such coordinate systems are related by a uniform rectilinear transformation. (galileo s principle of relativity)

definition: such reference frames are called inertial

I never heard of any experiments to show F = ma. Where were they done? Where did the experimenter find an inertial frame?
are you kidding? i think i did F=ma experiments in kiddie science lab in the 8th grade. you can find an inertial reference frame anywhere on the surface of the earth, if you, say, work on the surface of a table, to cancel the force of gravity.

more accurately, go up to a satellite in orbit. they do experiments up there all the time.

Originally posted by errandir
Are you saying that there is an electric force in a frame moving with an orbiting electron. I see two points in space defined: the electron and the proton (classically speaking). In a frame in which neither the electron nor proton are moving, there is no acceleration of the electron.

Two obvious possibilities:
1) There is an electric force towards the proton and an equal centrifugal force away from the electron
2) There is no such thing as centrifugal force, therefore, since F = ma = 0, there is no force acting on the electron.

i think i see the problem. a force is fictitious if you can make it zero by tranforming into an inertial reference frame (as defined above). you cannot do this with the lorentz force. in the inertial rest frame of the electron, the electron is accelerating.

Originally posted by errandir
If the axiom is:
F = ma
then the problem is, in some systems there is an F that most people like to call fictitious (the centripetal force). Why do they call it so?

because you are in a noninertial reference frame.

(be careful here. it is the centrafugal, not the centripetal force which is fictitious)

Originally posted by lethe
check this out: drop a quarter and a dime. see which one lands first. then walk down the street and try it again. you should get the same result. why? because space is homogeneous. the laws of physics are independent of your location in space.What do you mean by result? Do you mean specifically that homogeneity is defined on the race between the quarter and the dime to hit the ground? Of course you don't, because physics was considered before there were quarters and dimes (as they are today). I don't see the deeper meaning in your example. Can you give another one? Perhaps you can give the definition of homogeneity of space, and then I can ask questions about that.




Originally posted by lethe
postulate: there exist reference frames which satisfy the following 2 conditions:

the laws of physics in all such coordinate systems. (homogeneity)

any two such coordinate systems are related by a uniform rectilinear transformation. (galileo s principle of relativity)

definition: such reference frames are called inertialOK, how does a rotating frame violate this. BEFORE YOU ANSWER, take a look at my proposal, and then tell me what is wrong with it.

I make observations in a frame which in which there is some force in the r-direction, F_r. I change my observation to a different point. At this location, I observe a different force still directed away from the origin, F_r^'. I do this enough times that I realize a direct relationship with the distance from the origin and the force directed away from it. The relationship turns out to be:

F_x = Kx

Admittedly, I concede this to be approximately valid only in the region that I have tested.

I am secretly considering a rotating coordinate system, but why should I be able to figure it out just on these observations? How do I rule out the possibility that there is some natural law to cause this force. What does physics say to prevent the existence of some potential energy:

U = -(K/2)x²

in some region which contains the origin?




Originally posted by lethe
are you kidding? i think i did F=ma experiments in kiddie science lab in the 8th grade. you can find an inertial reference frame anywhere on the surface of the earth, if you, say, work on the surface of a table, to cancel the force of gravity.So, would this be a 2-D inertial frame?




Originally posted by lethe
more accurately, go up to a satellite in orbit. they do experiments up there all the time. But even up there aren't there tidal forces? I mean. Put two marbles on meter apart, and then see if they stay there. I thought that they would either move towards or away from each other under the infulence of the tidal field.

Originally posted by lethe
(be careful here. it is the centrafugal, not the centripetal force which is fictitious) Ya, that's what I meant, but you probably figured it out any way.

man. i don t get it. are you shitting me? this is basic classical mechanics shit. this is supposed to be easy.

Originally posted by errandir
What do you mean by result? Do you mean specifically that homogeneity is defined on the race between the quarter and the dime to hit the ground? Of course you don't, because physics was considered before there were quarters and dimes (as they are today). I don't see the deeper meaning in your example. Can you give another one? Perhaps you can give the definition of homogeneity of space, and then I can ask questions about that.

the laws of physics are the same here as they are there. dimes and quarters fall at the same rate. hydrogen has the same spectrum. maxwell s equations. newtons equations (in the classical limit), the speed of light, hooke s law, blah blah blah

i really did mean the example i used. i thought pennies and quarters would be a nice simple example.


OK, how does a rotating frame violate this. BEFORE YOU ANSWER, take a look at my proposal, and then tell me what is wrong with it.

I make observations in a frame which in which there is some force in the r-direction, F_r. I change my observation to a different point. At this location, I observe a different force still directed away from the origin, F_r^'. I do this enough times that I realize a direct relationship with the distance from the origin and the force directed away from it. The relationship turns out to be:

F_x = Kx

Admittedly, I concede this to be approximately valid only in the region that I have tested.

great. looks like hooke s law. a spring at the origin. looks good to me

I am secretly considering a rotating coordinate system, but why should I be able to figure it out just on these observations? How do I rule out the possibility that there is some natural law to cause this force. What does physics say to prevent the existence of some potential energy:

U = -(K/2)x²

in some region which contains the origin? physics does not rule out this possibility at all. in fact, it follows directly from hookes law. i can set this up for you in the lab if you need.

so what about it? what was the point of this? i am going to conveniently ignore what you are secretly planning or whatever.


So, would this be a 2-D inertial frame?
i don t really care about the dimensionality. it doesn t affect newton s laws.


But even up there aren't there tidal forces? I mean. Put two marbles on meter apart, and then see if they stay there. I thought that they would either move towards or away from each other under the infulence of the tidal field.

so what? if i am doing an experiment where i am measuring Newtons, i don t care about micronewtons. this frame is only approximately inertial. you want a frame that is exactly inertial? well you had better get rid of all energy in the universe. then we will talk.

i tire of talking classical mechanics. maybe someone else can pick up where i left off.

Originally posted by lethe
man. i don t get it. are you shitting me? this is basic classical mechanics shit. this is supposed to be easy.OK, so I'm a dumb shit. We've already established that months ago. I thought that you liked teaching. I guess you just thought that that sounded good to say, or maybe you just like the part that makes you feel smarter than everyone else. Well shit on me. I have never taken a course on classical mechanics (at least, not after high school). I do know Newton's Laws. I have done experiments in labs to demonstrate them. But excuse me for not wanting to take things for granted for the rest of my life.




Originally posted by lethe
the laws of physics are the same here as they are there. dimes and quarters fall at the same rate. hydrogen has the same spectrum. maxwell s equations. newtons equations (in the classical limit), the speed of light, hooke s law, blah blah blahDo the laws of physics include a uniform gravitational acceleration of ~9.81 m/s² directed toward the ground as one of the postulates?




Originally posted by lethe
so what about it? what was the point of this?Is this any different than the centrifugal force that would result from a reference frame rotating about the origin at &omega; = sqrt(K)
?

BTW, I meant for all dependence to be on r, rather than x:
F_r = Kr
U = -(K/2)r²
Sorry for the typo, I make a lot of them.




Originally posted by lethe
i don t really care about the dimensionality. it doesn t affect newton s laws.Neither do I, and I didn't think that it would. I don't understand why you have to be so defensive. If you don't want to answer my question, fine.




Originally posted by lethe
this frame is only approximately inertial. you want a frame that is exactly inertial?Interesting. It approximately obeys the laws of physics? Two frames are approximately related by a rectilinear translation? Or both?




Originally posted by lethe
maybe someone else can pick up where i left off. I think that is a wonderful suggestion. If there is someone else out there who is willing to help me dismast my ignorance, PLEASE take over.

Originally posted by errandir
Do the laws of physics include a uniform gravitational acceleration of ~9.81 m/s² directed toward the ground as one of the postulates?

no, that is not usually taken as a postulate.


Is this any different than the centrifugal force that would result from a reference frame rotating about the origin at &omega; = sqrt(K)
?
the centrifugal force is proportional to the mass, whereas the force in hooke s law is not. this means that you cannot change to an inertial frame to get rid of hooke s law, and you can to get rid of the centrifugal force.

BTW, I meant for all dependence to be on r, rather than x:
F_r = Kr
U = -(K/2)r²
Sorry for the typo, I make a lot of them.
doesn t matter.


Neither do I, and I didn't think that it would. I don't understand why you have to be so defensive. If you don't want to answer my question, fine.

i didn t mean that to sound defensive. i really meant what i wrote. Newton s laws do not care about the dimensionality of your reference frame. they work fine in any dimension. so it is totally irrelevent what dimenion i choose to work in.

Interesting. It approximately obeys the laws of physics? Two frames are approximately related by a rectilinear translation? Or both?
the latter. these frames are only approximately inertial. if you account for tidal forces, you will see that the laws of physics are exactly obeyed. if you neglect the tidal forces, then you have an inertial frame.


I thought that you liked teaching. I guess you just thought that that sounded good to say, or maybe you just like the part that makes you feel smarter than everyone else. Well shit on me. I have never taken a course on classical mechanics (at least, not after high school). I do know Newton's Laws. I have done experiments in labs to demonstrate them. But excuse me for not wanting to take things for granted for the rest of my life.
.....
I think that is a wonderful suggestion. If there is someone else out there who is willing to help me dismast my ignorance, PLEASE take over.

i respond to threads that are interersting to me, and where i think i would have something valuable to add. i as a rule, do not like to reply to crackpot threads , and as far as i can tell, Zarkov is a crackpot and this is his thread. i had no intention of replying to this thread, until you explicitly asked me to. i only wanted to point out the difference between euclidean and minkowski spaces. i did get involved, but mostly only because you asked me to.

but when you ask things like "what experiments have there been to verify F=ma?" it gets very hard for me to take you seriously.

Is a rotating cylinder a 2-D intertial frame?

no, a frame on the cilinder would be rotating, and hence continously accelerating.

I don't see how this is fundamentally different than a frame on the surface of the earth that is continually accelerating (due to gravity).

Strictly speaking, very very very strictly speaking, I suppose that you cannot call a frame of reference on the earth an inertial frame. It is a nearly perfect inertial frame, as the earth's rotation is quite slow in comparison to the typical experiment you would perform on the surface of the earth.

Are you hinting that the coriolis force is negligible since the earth is spinning at a rate much slower than would affect "normal" velocities in an experiment?

"A frame in which unaccelerated objects move in straight lines at constant velocity is called an inertial frame."

This is from a web site:

http://casa.colorado.edu/~ajsh/sr/postulate.html



Does anyone disagree with this?



I'm just having a problem seeing how this says anything. How is any arbitrary frame excluded from an inertial frame? Can a particle move in a not straight line without being accelerated?

>> Can a particle move in a not straight line without being accelerated?

no, any curving requires acceleration.

I hold that planets in a circular orbit are in an inertial state. These have a constant acceleration.

PS, is there ever a REAL straight line in the Universe ??

:)

Originally posted by errandir
Are you hinting that the coriolis force is negligible since the earth is spinning at a rate much slower than would affect "normal" velocities in an experiment?

That is why I said "typical" experiment. You don't shoot mortar shells over a range of 50 km everyday as an experiment I suppose ?

I don't like that definition much. It's a bit circular.

EXCELLENT, James, that's exactly what I'm saying (about all the definitions that I've heard)! Can you give me the definition for inertial frame with which you are satisfied?

An inertial reference frame is a volume of space with no detectable gravity and no detectable acceleration.

It is an ideal situation, since it can only be approximated. A rocket ship with its thrusters turned off in the middle of a huge void between galactic clusters is a very precise approximation to an inertial reference frame.

Oddly enough, an orbiting space station or a falling box seem to be inertial reference frames. From inside, there are only negligible forces acting.

Neither the orbiting space station nor the falling box are true inertial reference frames. The gravitational effects of the sun are measurable for inside. If the space station or the box are transparent or have windows, an observer inside cannot use the Lorentz transformation to deal with a space station or box rotation around or falling toward Jupiter.

The surface of the Earth is not an inertial reference frame. The gravitational effects are quite obvious, although we are so used to the force that we often think there are no forces acting on us.

Originally posted by Dinosaur
The surface of the Earth is not an inertial reference frame. The gravitational effects are quite obvious, although we are so used to the force that we often think there are no forces acting on us. I thought that it was agreed that approximating the surface of the earth as an inertial reference frame (2-D) only breaks down when one considers the ability to observe the coriolos force. Why would gravity have any effect (considering that the curvature is perpendicular to the surface)?

errandir:

<i>Can you give me the definition for inertial frame with which you are satisfied?</i>

How about:

"An inertial frame is a point of view in which objects experiencing no net force do not accelerate."

But this just returns to my question of why we can't call the centrifugal force a force and then say that the frame is inertial with this force in it.

Inertial (or fictitious) forces don't count, errandir, and centrifugal force is one of those.

Errandir: From my previous post: A rocket ship with its thrusters turned off in the middle of a huge void between galactic clusters is a very precise approximation to an inertial reference frame. The surface of the Earth is not an inertial reference frame due to the obvious gravitational force.

Various thought experiments in books on relativity can be misleading. For example, there is often mention of playing table tennis or throwing and catching a ball on a train moving smoothly at constant velocity. If you do not look out the window, you cannot tell that the train is moving. This (or a similar) situation is often given as an example of what is meant by an inertial reference frame. Such examples merely indicate that standing still and moving at constant velocity cannot be distinguished when enclosed in a room with no windows. The gravitational force can be felt inside such a room, preventing the room from being a true inertial reference frame.

As posted previously, centrifugal force does not exist. Centrifugal means center-fleeing. When on certain amusement park rides, one has the illusion of a force directed away from the center of rotation. What is felt is an inertial effect due to being forced to move in a circular path, which requires a force acting toward the center of rotation.

If you are in a high performance car that accelerates rapidly, you feel the seat pressing against your back. There is no backward fleeing force. What you feel is an inertial effect due to not moving at constant speed. This is analogous to what is referred to as centrifugal force.

The orbiting satellite is analogous to a weight at the end of a string being swung in a circular path. The person swinging the weight is applying a force toward the center of rotation. If the string breaks, the weight does not flee from the center of rotation. It moves in a plane tangent to the circular path at the point where it was when the string broke. In the absence of gravity, it would move along a tangent to the circular path (Perpendicular to a center-fleeing path), and it would be an unaccelerated object with no forces acting on it.

Before the string breaks, there is no center fleeing force. The person swinging the weight feels an inertial effect due to the force he is applying to cause the weight to follow a curved path. The force he is applying is analogous to the gravitational force acting on an orbiting satellite, and is directed toward the center of rotation.

>> Centrifugal means center-fleeing. When on certain amusement park rides, one has the illusion of a force directed away from the center of rotation

True, BUT

what of the ball bearings in a ball race, as the balls spin they push outwards on the bearing race ?

:)

Zarkov: Flash Gordon says that centrifugl force does not occur for ball bearings either.

As mentioned in previous posts, centrifugal force is an inertial effect.

Dinosaur..

Yes I agree but you really will have a trouble signing all these inertial forces. This inertial force can split ball races in half, and just to confuse there is no centripetal force in this case. IMO

:)

I am getting so frustrated!:mad: Don't get me wrong, I very much appreciate your attempts to answer my question, but I am getting the feeling that no one has yet understood my question, or they are just not taking me seriously (allusion to mr. know-it-all).

I need to understand the terms in the discussion. I grasp the concepts quite well (all except for one, but I will defer mention of that until I pin down the termonolgy). I request that, if you are attempting to repsond to <i>me</i>, that you do so with definitions, so that I may finally comprehend a consistent and nontrivial set thereof. Until then, I don't think that I can meaningfully carry on in this discussion.

The terms for which I need a consistent, nontrivial set of definitions:
-inertial frame
-force (true/actual/not fictitious, i.e. the force in Newton's law)
-acceleration
-straight line
-fictitious force

Please understand that I know a few definitions for each of these terms induvidually, but each definition makes reference to the other terms (i.e. it poses conditions regarding the other terms). Thusly, my problem has been that this set of terms seems closed and unmeaningful (i.e. self consistent/trivial/impossible to violate)

Thanks again for being so patient with me.

Hi errandir,

An attempt to answer your question. I have rearranged the terms so their definitions follow eachother as nicely as possible. I am pretty sure that everybody has a slightly different view on it, so except tons of different rephrasings ;).

"-inertial frame"

A frame of reference at rest or moving with constant velocity.

Note that this definition requires another frame of reference (to determine whether the original frame of reference was at rest or moving with constant velocity). To solve this dilemma, one postulates that there is at least one inertial frame of reference.

Then you get the two "definitions" or "postulates" that constitute the Galilean principle of relativity:
- There exists at least one inertial frame of reference.
- All frames that are at rest or move with a constant velocity with respect to the original inertial frame, are also inertial frames.

(note: there are tons of different formulations of this principle going around, I prefer this one as IMHO it is not ambiguous - the other definitions usually have the problem I sketched above, that they require another inertial frame)

"-acceleration"

In a frame of reference (this is defined now), the second derivative of the position with respect to time.

"-straight line"

The motion followed when acceleration is zero.

"-force (true/actual/not fictitious, i.e. the force in Newton's law)"

There is no real definition of a force I've ever seen, except for the one encoded in Newton's second law: a force is the cause for acceleration.

"-fictitious force"

Forces that arise due to working in a non-inertial frame of reference.


Hope these short definitions (without too much blabla) help you understand this.

Bye!

Crisp

Originally posted by Crisp

"-force (true/actual/not fictitious, i.e. the force in Newton's law)"

There is no real definition of a force I've ever seen, except for the one encoded in Newton's second law: a force is the cause for acceleration.


hmmm... perhaps this is the source of errandir s claims of circular reasoning: if you can t define what is a real force, and what is a ficticious force, than how can you distinguish an inertial frame from a noninertial frame?

newton envisioned a force as some kind of push. all forces that one can point to can be broken down into the 4 fundamental forces. let s forget the nuclear forces for now, and just look at classical forces. then a force is any dynamical influence that is due to either electromagnetism or gravity. in einsteinian gravity, electromagnetism (the lorentz force) is the only force left.

this is very natural. suppose for a second that i wanted to call the lorentz force a fictitious force, as errandir wants to do. let s look at the reference frame of the electron orbiting the nucleus (or some other system of charged particles, large enough to permit a classical treatment). it would seem that the electron is not accelerating, so this is an inertial frame. but in this frame, the nucleus is moving. and in a way that is not governed by the lorentz force law. if the electron passes by a neutron, the neutron will be accelerating, even though the lorentz force law tells us that neutral particles in an inertial frame to not accelerate.

if you lived in this frame, with enough experiment you could discern the proper force law, but it would not be same in other reference frames. if you changed to a frame at rest relative to the first one, in a different location, the force law would change. if you changed to a frame in uniform motion with respect to the first, same story, your force law would change.

this is a violation of galileo s principle of relativity, which means the reference frame of the orbiting electron is unambiguously not an inertial frame. the only valid types of force laws are those which are invariant under galilean transformations. this is galileo s postulate. the laws of physics do not depend on your reference frame.

the proper noncircular way to formulate mechanics is this: postulate galileo s principle, postulate newton s laws, and then postulate the dynamical laws, like maxwell s equations and the lorentz force law. you can add more force laws if you like. for example, you could postulate some harmonic force (like hooke s law). this is allowable. but if it is not the same in every reference frame, then you have violated galileo s postulates.

Originally posted by Crisp
"-inertial frame"
...
...this definition requires another frame of reference. To solve this dilemma, one postulates that there is at least one inertial frame of reference.

Then you get the two "definitions" or "postulates" that constitute the Galilean principle of relativity:
- There exists at least one inertial frame of reference.
- All frames that are at rest or move with a constant velocity with respect to the original inertial frame, are also inertial frames.I find this ambiguous. Are you suggesting that this principle is a way of defining an inertial frame?




Originally posted by Crisp
"-acceleration"

In a frame of reference (this is defined now), the second derivative of the position with respect to time.I suppose I should add the term "point" to my list. I imagine that most people think of this as a set of numbers (in Newtonian mechanics, a set of three numbers). In that case, the second derivative of the point would mean another set of numbers, each of which corresponding to the numbers in the set that represents the point (assuming an orthogonal coordinate system). In other words:

P = {x_i}
a = d²X/dt² = d²{x_i}/dt² = {d²x_i/dt²}

The problem I see with this is when I want to use "cylindrical" coordinates, for instance. Now, I know that in the calculation of the acceleration in this system, one must take into account the derivatives of the basis vectors themselves (and in this light, the above def. for a is a poor one). My problem with this is that we have to presuppose that the basis vectors are changing in time (with respect to the "cartesian" system). Who's to say that the basis vectors in the "cyl" system are not the ones that are in fact constant, and that it is the "cart" basis vectors that are not changing in time?

You would probably point me back to the establishment of the inertial frame, right? I'll assume so, since that's what everyone else seems to be doing. "The cartesian coordinate system represents an inertial frame." Well, as dull as it might make me sound, I still don't understand our definition of "inertial frame."




Originally posted by Crisp
"-fictitious force"

Forces that arise due to working in a non-inertial frame of reference.Again, without going into similar detail as above, this relies on the definition of an inertial frame, which I do not yet understand.




I have no beef with the other definitions that you proposed. Perhaps you could focus on revising the definition of "inertial frame," and that might make the a and F_fictitious become clear to me.

Thanks for trying to help me out with this.

Originally posted by errandir
You would probably point me back to the establishment of the inertial frame, right? I'll assume so, since that's what everyone else seems to be doing. "The cartesian coordinate system represents an inertial frame." Well, as dull as it might make me sound, I still don't understand our definition of "inertial frame."


cartesian coordinates have nothing to do with whether the frame is inertial or not. an inertial frame can be described by cylindrical coordinates, while a noninertial frame can be described by cartesian coordinates.

I think that I have muddled my own initiative a bit. I was giving the example of "cyl" vs. "cart" coordinates to show the ambiguity of "the second derivative of a point with respect to time." I think that this was intended to mean "the second order change in the spatial position of a particle with respect to time." I don't really know what "the second derivative of a point with respect to time" would mean.




Anyway, so the position has a representation, i.e. three numbers.

The second order change relates to these three numbers.

Considering two coordinate systems, "cart" and "cyl."
- First and foremost, let us define them each with an orthonormal basis (for simplicity).
- Let us say that they share a basis vector, e_z.
- Let us say that "cart" has two more basis vectors, e_x and e_y.
- Let us say that "cyl" has two more basis vectors, e_&rho; = e_xcos(&omega;t) + e_ysin(&omega;t) and e_&phi; = -e_xsin(&omega;t) + e_ycos(&omega;t).

Now, consider a particle with a trajectory such that its position vector is e_xcos(&omega;t) + e_ysin(&omega;t) = e_&rho;.

I appeal to the reasoning with which I'm a bit uncomfortable. According to said reasoning, one would conclude (the conclusion intended to demonstrate said reasoning) that, since the particle is moving in a circular path, it has some acceleration directed towards the center (that acceleration being a = &omega;²). This is concluded with respect to the "cart" system (more specifically, it is based on the assumption that "cart" is fixed). But, in the "cyl" system, the position vector is static. What I mean by that is that if I turn things around and say that "cyl" is fixed, maintaining the relationship between the two systems, then there is no acceleration, and it is "cart" that is rotating. But how can this be? What has changed? I don't see any fundamental difference between saying that "cyl" rotates with respect to "cart" and saying that "cart" rotates with respect to "cyl." Even if we say that one of the two coordinate systems is fixed, to <i>what</i> is it fixed?

I have mentioned earlier this notion of the "fixed stars," but that doesn't put me quite at ease. What is so special about the stars that "inertial" systems should follow them (in terms of orientation)?

I find this ambiguous. Are you suggesting that this principle is a way of defining an inertial frame?

Yes, that is indeed what I wanted to suggest. You start with one inertial frame, which is postulated to exist, and then you define other inertial frames using that.

You would probably point me back to the establishment of the inertial frame, right? I'll assume so, since that's what everyone else seems to be doing. "The cartesian coordinate system represents an inertial frame." Well, as dull as it might make me sound, I still don't understand our definition of "inertial frame."

As lethe pointed out, there is no connection between the type of coordinates and whether a frame is inertial or not.

To get the idea of an inertial frame, try the following: you should think of the frame you start with, of which the existance is assumed, as a frame that is not subject to any form of acceleration - you assume hence, by common sense, that it is an inertial frame. You can check this studying a particle from this frame of reference, then you transform to a frame that moves with a constant velocity with respect to our first frame. This second frame - by the Galilean principle of relativity - should also be an inertial frame. If the laws of physics remain the same in both frames of reference (i.e. there are no forces that disappear or appear, these are the fictuous forces), then they are both inertial frames.

But I see your point with the definition of inertial frames, it is indeed a bit problematic, as it requires two frames of reference to define properly (I had not thought it over thoroughly before actually). The principle of relativity relies a bit on "common sense" to chose the initial frame of reference I guess.

"I appeal to the reasoning with which I'm a bit uncomfortable. According to said reasoning, one would conclude (the conclusion intended to demonstrate said reasoning) that, since the particle is moving in a circular path, it has some acceleration directed towards the center = ù2. This is concluded with respect to the "cart" system (more specifically, it is based on the assumption that "cart" is fixed). But, in the "cyl" system, the position vector is static. What I mean by that is that if I turn things around and say that "cyl" is fixed, maintaining the relationship between the two systems, then there is no acceleration, and it is "cart" that is rotating. But how can this be? What has changed?"

If you compare between the two frames of reference, you see there is a problem: in the cilindric, rotating, frame of reference, there are no forces working on the particle. When you go to another frame of reference, let's say a cartesian frame of reference, you see that a new fictuous force appears: the particle is forced to move on a the cilinder (this is called a "binding" force). Hence, "the laws of physics" are not the same in both frames of reference, a new force has appeared.

"I don't see any fundamental difference between saying that "cyl" rotates with respect to "cart" and saying that "cart" rotates with respect to "cyl." Even if we say that one of the two coordinate systems is fixed, to what is it fixed?

No coordinate system is prefered above the other, or better: no inertial frame is prefered above the other. The choice of frame of reference and coordinates is completely arbitrary, and I hope I pointed out that it is not necessary to "fix" a frame of reference. Any inertial frame would do.

Feel free to ask any questions of course, that is why we are here :)

Bye!

Crisp

I purposefully didn't mention any forces (or even use the word "force") in my previous post for a reason. Is it possible to define an inertial system without appealing to some force condition?

I want to emphasise the acceleration (kinematics) without getting hung up on force (dynamics), if that's possible (I know that Newton said F = ma, but I hope that it is not important in the definition of "inertial frame"). If this cannot be done, then I don't think that I will ever resolve this issue to my own personal satisfaction.

I am not sure if there is a way to define an inertial frame without mentioning forces (or equivalently, potentials). The idea of inertial frames is that physics remains unchanged for all inertial frames, hence it is sufficient to pick one frame and calculate.

But in order to do that, you need to verify that this is indeed correct. So there are two ways to it:

1. You postulate the existance of an inertial frame, and define all other frames at rest or that move with constant velocity w.r.t. the first as inertial frames. You can easily verify that this does not change Newton's law of motion, or the Hamilton/Lagrange equations, ... Hence the postulate predicts the uniformity of physics.

2. Or you assume the uniformity of physics, which allows you to define two inertial frames as two frames (related in a linear way) for which the laws of physics are the same.

Bye!

Crisp

Are you suggesting that the "laws of physics" don't work in a "rotating" frame.

Also, I find the term "constant velocity" ambiguous. How is that defined? If acceleration = 0, then velocity = constant? How is acceleration defined?



Can someone evaluate this suggestion:
"inertial frame" - a frame of reference in which, for every geodesic, s, and event, P, not on that geodesic, there exists exactly one geodesic, s^', through the event, P, that does not intersect the other geodesic, s

Originally posted by errandir

Can someone evaluate this suggestion:
"inertial frame" - a frame of reference in which, for every geodesic, s, and event, P, not on that geodesic, there exists exactly one geodesic, s^', through the event, P, that does not intersect the other geodesic, s

this also applies to noninertial frames

Originally posted by errandir
Are you suggesting that the "laws of physics" don't work in a "rotating" frame.

No, not at all. I am saying that there is a difference in "the laws of physics" in inertial and non-inertial frames. If you have an object that is described by F = m*a in the following way

F(push) + F(gravity) = m*a

in some inertial frame of reference, then in all other inertial frames of reference, you will have something similar. No new forces appear. If you go from inertial to non-inertial, you have to add coreolis forces, or electromagnetic fields or ...

"Also, I find the term "constant velocity" ambiguous. How is that defined? If acceleration = 0, then velocity = constant? How is acceleration defined?"

You have the position as a function of time, it is called x(t). This is what you measure in your (inertial or non-inertial) frame of reference. By definition, velocity is the change of this variable x(t) with respect to time. By definition, acceleration is the second derivative with respect to time.

Constant velocity means that the change of x(t) in time is constant. This implies that the acceleration is zero. If the acceleration is zero, this implies that the velocity is constant. I don't see any ambiguity here, these statements are simply equivalent.

Bye!

Crisp

Originally posted by Crisp
I am saying that there is a difference in "the laws of physics" in inertial and non-inertial frames. If you have an object that is described by F = m*a in the following way

F(push) + F(gravity) = m*a

in some inertial frame of reference, then in all other inertial frames of reference, you will have something similar.Can you clarify similar?

What about F(push) + F(centrifugal) = m*a. I'm sure that there are other frames in which this could be valid in such a form. Why can't I say that an inertial frame must satisfy F(push) + F(centrifugal) = m*a?




Originally posted by Crisp
You have the position as a function of time, it is called x(t).Can you clarify x(t)? Could it be defined as "x(t) is the number of meter sticks that can be put between the particle and the origin at time = t?"

I follow your definitions for velocity and acceleration <i>assuming there exists a suitable definition for position</i>. However, I still need some clarification for the definition of position.

Originally posted by errandir
Can you clarify similar? What about F(push) + F(centrifugal) = m*a. I'm sure that there are other frames in which this could be valid in such a form. Why can't I say that an inertial frame must satisfy F(push) + F(centrifugal) = m*a?

Similar = the same form, but perhaps using different coordinates. If you have gravity and some pushing force in an inertial frame, then transforming to another inertial frame you will see there is still gravity working, and that same pushing force.

If you use F(push) + F(centrifugal) and you transform from that frame (which will be non-inertial) to an inertial frame, you will see that F(centrifugal) disappears.

Can you clarify x(t)? Could it be defined as "x(t) is the number of meter sticks that can be put between the particle and the origin at time = t?"

I follow your definitions for velocity and acceleration <i>assuming there exists a suitable definition for position</i>. However, I still need some clarification for the definition of position.

Yes, I would define "x(t) as the number of meter sticks that can be put between the particle and the origin along the x-axis at time t". Ok, it doesn't get any more fundamental than this :)

Bye!

Crisp

Originally posted by Crisp
If you use F(push) + F(centrifugal) and you transform from that frame (which will be non-inertial) to an inertial frame, you will see that F(centrifugal) disappears.We are trying to define "inertial frame." I thought that, if we found some frame in which we observed some laws of physics (some sum of forces = ma), and then we found another frame in which the laws of physics had the same form (some sum of the same forces = ma), then we could say that these were "inertial frames." The definition should not contain the term (IMHO, and in my feable ability to comprehend).




Originally posted by Crisp
Yes, I would define "x(t) as the number of meter sticks that can be put between the particle and the origin along the x-axis at time t". Ok, it doesn't get any more fundamental than thisWell, now we need to define the x-axis. Also, I really hope that you don't find this definition fundamental or sufficient after you consider that we have not specified an arrange/mechanism for putting the meter sticks between said points.

If I put them end to end, and then specify that there should be no obliquity between meter sticks, then I think that this might be a good way to define it. Then, velocity would be the rate at which the number of meter sticks would have to increase, and acceleration would be the rate at which the velocity would have to increase. In order to use any of this, though, an appropriate definition needs to be found for the x-axis.

When you say <i>along the x-axis</i>, this seems inappropriate. If we start putting meter sticks along the x-axis between two points that are not both on the x-axis, then, we would obtain an infinite result for the number of required meter sticks. No amount of meter sticks will ever reach the other point if we keep putting them along the x-axis.

Originally posted by errandir
We are trying to define "inertial frame." I thought that, if we found some frame in which we observed some laws of physics (some sum of forces = ma), and then we found another frame in which the laws of physics had the same form (some sum of the same forces = ma), then we could say that these were "inertial frames." The definition should not contain the term (IMHO, and in my feable ability to comprehend).

Okay, but now you are going around in circles, the sum of forces and how this changes was merely an example to illustrate what I meant by "the laws of physics" change in a previous post (the one with two possible definitions of an inertial frame). It is an illustration to the definition, not the definition itself.

Well, now we need to define the x-axis. Also, I really hope that you don't find this definition fundamental or sufficient after you consider that we have not specified an arrange/mechanism for putting the meter sticks between said points.

I was already assuming that you would put them "end to end" as you said. That is how common sense tells you to measure a distance, no ?

When you say <i>along the x-axis</i>, this seems inappropriate. If we start putting meter sticks along the x-axis between two points that are not both on the x-axis, then, we would obtain an infinite result for the number of required meter sticks. No amount of meter sticks will ever reach the other point if we keep putting them along the x-axis.

Ehhhh... that was not what I meant. I meant to say that you need meter sticks for every axis you consider. Since I was only talking about the x-axis, the point you are measuring must also be on that axis, otherwise you need to drag in y/z axes aswel, with their own set of rulers.

Bye!

Crisp

To what do we measure along the axes?

If we start at the particle, we measure along the x-axis until <i>what</i>, and it can't be the origin, because the particle and the origin don't necessarily lie on a common x-axis?

If we start at the origin, we measure along the x-axis until <i>what</i>, and it can't be the particle, because the particle and the origin don't necessarily lie on a common x-axis?

Not always no, then you also include the other directions... y and z. First you keep putting rulers along the x-axis until you think you've reached the x-coordinate of the particle, then you start in the y, putting rulers, until you think you have reached the y position, then you start in the z direction, put rulers until you reach the particle. If you're not right on the particle, adjust x/y such that in the end you do. Then read the number of rulers required in every direction, these give you the coordinates.

What would make someone "think" that they had reached the x-coordinate? Can you give some example indications?

If we use this as our working definition of position, then do we require that we make a right turn when we start measuring along a different axis?

"What would make someone "think" that they had reached the x-coordinate? Can you give some example indications?"

More and more, I am getting the feeling you are going to keep asking questions until I finally say "because it is so" ;) ...

Concerning how you can know the x-coordinate: well, a priori you cannot. You have to make a lucky guess and try. If you get to the particle, then you guessed right, if you didn't, then though luck.

Ofcourse in practice when measuring something, you either don't use this silly method but advanced methods (lasers or something), or you change your basis such that your x-axis points directly at the particle. Then when you start putting rulers along the x-axis, you will surely hit the particle.

"If we use this as our working definition of position, then do we require that we make a right turn when we start measuring along a different axis?"

Well, depends how your axis are aligned and what way the particle is.


Look, we're pushing the (classical) "measurement" proces here to a point where it is becoming ridiculous. Nobody will use rulers and put them next to eachother; nobody will put a huge metal frame of reference with three arrows pointing in the x/y/z direction and start measuring with respect to this metal thing. In principle you can ofcourse.

Bye!

Crisp

Originally posted by Crisp
More and more, I am getting the feeling you are going to keep asking questions until I finally say "because it is so" ;)I promise that this is not my intent.




Originally posted by Crisp
Ofcourse in practice when measuring something, you either don't use this silly method but advanced methods (lasers or something)...I'm up for it. I just don't see how you would use lasers to define position.




Originally posted by Crisp
...or you change your basis such that your x-axis points directly at the particle. Then when you start putting rulers along the x-axis, you will surely hit the particle.What's the difference between this and using spherical polar coordinates. In this case, position is just the distance from the origin. Surely we don't want this to be what we use for position.




Originally posted by Crisp
Look, we're pushing the (classical) "measurement" proces here to a point where it is becoming ridiculous. Nobody will use rulers and put them next to eachother; nobody will put a huge metal frame of reference with three arrows pointing in the x/y/z direction and start measuring with respect to this metal thing. In principle you can ofcourse.Well, I'm not suggesting practicability. I don't care about actually making the measurement. What I'm trying to get a fix on is the definition of the more abstract object called position. I suspect that light is going to have to come into play, though I don't really want to get off into relativity.

Hi errandir,

When working with cartesian coordinates, the position of an object with respect to a given reference point (the origin) is defined as the number of units in each direction. A unit physically corresponds with a ruler of length "one meter". Hence to determine the position, you would have to put rulers along each direction until you finally reach the object in question.

For spherical or circular coordinates, you measure the distance to the object (hence you put rulers straight towards the object and count those) and the angle(s) the object makes with two axes of choice. To measure this angle, you can use geometrical instruments, or the goniometric rules such as sin(a) = b/c where b and c are for example the radius and projection on an x-axis (hard to put in words).

Concerning the measurement with lasers: there exist small devices that emit a laser pulse and measure the time required for the laser light to be reflected by the object you want to measure the distance to. This is not very useful for measuring cartesian coordinates (ok, hadn't thought of that, I was just trying to say that using rulers does not always happen ;)).

To grap the concept of position, I think it is best to think of it in the mathematical way as a set of coordinates. The question is then how to measure the coordinates. As pointed out, this depends on what type of coordinate system you use.

I think I have reached the point where I cannot say any more about this (or be more precise). Feel free to ask questions though.

Bye!

Crisp

With due appreciation, I am getting the feeling that I have exhausted the insight from this thread. Thanks again, Crisp, for all of your genuine tolerance of my plausably inane inquiry. I am now going to unsubscribe to this thread.

Hi all, I'm just a stupid 15 year old kid here. But from my understanding centrifugal and centripetal forces are the resultants of "real" forces right?

Well Woot, we may never know.

I am sure the ball bearing example is pure centrifugal force, it is certainly not inertial motion as the forces induced can shatter a ball race.

:)

Has anyone considered how centrifugal and centripetal forces can co exist. They are exertions of matter on matter in opposite directions and should produce a zero force within the matter of a turning wheel which they don’t. The answer is probably going to be both are intermittent forces dependent on an alignment of electrons from hub to perimeter to be alive.

Regards

leeaus

What do you mean they don't produce zero net force?

The circumference of the wheel isn't moving in or out!

There is exactly 0 net force.

You may be confusing "net force" with tension inside the spokes.

THAT is precisely what produces the "centripetal" force.

It is surely not necessary to talk about "intermittant" forces or alignment of electrons!

When something rotates, there is a center-seeking force and there is a tangential velocity. The force pulls the object into a circular path. There is an inertial reaction to the force. If the force ceases, the object follows a tangential path, not a radial or center-fleeing path.

The above is high school physics, and is usually associated with the orbital motion of Earth satellites, moons, planets, et cetera.

The ball bearing race and amusement park rides are more complicated. There are frictional forces, bearing forces, and other forces involved in these devices, but no center-fleeing or centrifugal force involved.

What seems like centrifugal force is an inertial reaction, analogous to the inertial reaction you feel against your back when a car accelerates, or to the force you feel sending you toward the windshield if the car stops suddenly.

If you do not believe me or others posting similar remarks, try searching the internet for more authoritative articles.

How oppositely directed forces co-exist within the one particle of a rotating body was the suggested consideration. Centrifugal force is an outward exertion, centripetal inward. Fairly straight forward that each must be intermittent to exist without annulling the other.
regards

leeaus

They are macroscopic manifestations of the electromagnetic force which bonds atoms together, resulting from the principle of inertia.

How many of you know it alls can come up with a satisfactory explanation of the van der Waal's force between electrically neutral planes. No wonder I haven't been here for three months, still arguing about 19 th century physics.

Ryans which side of the bed did you get out of three months ago. The same one again today no doubt. Would be useful if it was known what your “they” referred to. (“They” are macroscopic manifestations ……..)

Regards

leeaus