Do Centrifugal and Centripetal Forces Exist in Outer Space?

[Hootenanny]

Now let us suspend the ball by a spring in the rotating frame. Now there is an action-reaction force. The centrifugal force acts outwards while the centripetal force exerted by the spring in the oppsite (i.e. inward) direction.

Forgive me for intruding and allow me to point out that I have not followed the discussion in it's entirety. However in the quoted example, surely the centrifugal force arises purely as a consequence of the rotating reference frame. In other words, both the centrifugal and centripetal forces are not observable in the same reference frame and hence one cannot consider them an action-reaction pair. Perhaps I have misinterpreted your example and I apologise if that is the case.

 

[Doc Al]

Okay. This is for both of you - Let us consider the electric force as an example; A styrofoam ball as an excess of charge on it and is initially at rest in an inertial frame of reference. Now let there be an electrical field applied. The charged ball will then start to accelerate. There is only an action force and no reaction force acting on the ball. But the force is quite real. Newton's 3rd law does not apply to this case since it applies to situations when there are a pair of forces acting on a body.

Newton's 3rd law (in its simplest version) applies whenever two bodies interact. The "pair" of 3rd law forces acts on different bodies. The "reaction" force in this case will be an electrostatic force on whatever is producing the field.

Now let us suppose that the charged ball is in contact with a wall whose unit normal is parallel to the electric field and the direction of the electric force is towards the wall. When the ball comes in contact with the wall we can then apply Newton's 3rd law. The force exerted by the ball on the wall is equal and opposite to the force the wall exerts on the ball. This is the action-reaction pair.

That's true. Now there are two forces identified as acting on the ball and those forces are not 3rd law pairs.

Similarly if there is an uncharged ball but now the force of gravity is acting on a ball on a table. The table exerts and equal and opposite force on the ball. This too is an action-reaction pair.

The force of gravity and the normal force from the table are not 3rd law pairs.

If the ball is in free-fall then there is no reaction force and Newton's 3rd law does not apply.

Sure there is and sure it does. (Using the non-GR view of gravity for the moment.)

Now let us consider a (non-charged) ball in free-fall as observed in the rotating frame. Since the object is not in contact with anything and there is no counter force acting then there is no reaction force. Now let us suspend the ball by a spring in the rotating frame. Now there is an action-reaction force. The centrifugal force acts outwards while the centripetal force exerted by the spring in the oppsite (i.e. inward) direction.

The "centrifugal force" acts outward on the ball and is an artifact of viewing things from a rotating frame. It is obviously not the 3rd law "reaction" to centripetal force for two reasons: (1) Both centripetal and "centrifugal" forces act on the same body!; (2) "Centrifugal" force is not a "real" force--there is no actor!

As I mentioned in an earlier post, there is an old-fashioned usage of the term "centrifugal force" to refer to the reaction force to centripetal force, but used in that sense it has no relationship to the modern usage. (I haven't seen that usage since I was a kid reading ancient physics books in the library--I seriously doubt you'll find that usage in any standard books anymore.) Sounds to me like you're equivocating between that old-fashioned usage and the modern meaning of the term "centrifugal force".

 

[Cryptonic]

If a circular space station is rotating, deep in some intergalactic void with no visible stars or galaxies, how do you know it is rotating? Wouldn't it seem as if something "outside" is pulling you towards the floor (ie outer hull of stationary space station)?

I guess what I'm asking is, how can we say convincingly that the space station is rotating, and not the rest of the universe rotating around the stationary space station?

Isn't all motion relative?...or something?

 

[Mentz114]

If a circular space station is rotating, deep in some intergalactic void with no visible stars or galaxies, how do you know it is rotating? Wouldn't it seem as if something "outside" is pulling you towards the floor (ie outer hull of stationary space station)?

I guess what I'm asking is, how can we say convincingly that the space station is rotating, and not the rest of the universe rotating around the stationary space station?

Isn't all motion relative?...or something?

There was a discussion about this here recently -

https://www.physicsforums.com/showthread.php?t=223264

Rotation can only be defined for extended objects like the space station, and so has an absolute nature. If one calculates the influence of the universe rotating around the space station, the effect is much smaller than when the station is rotating.

Rotation is not relative.

 

[Cryptonic]

great, thanks Mentz114.

 

[nuby]

If Centripetal and Centrifugal 'forces' exist in space (say on a space shuttle). What is the magnitude of these forces, compared to on earth. And, where is the proof of this? Is there any experiment data available on the web? I haven't been able to find any.

If you spin a ball attached to a string on earth, it spins around in circles. Would the same happen in space? Or would you get wrapped up in string?

Pete, I believe you said something along the line that .. these forces originate from the matter itself? How would this work?

 

[Dale]

Okay. This is for both of you - Let us consider the electric force as an example; A styrofoam ball as an excess of charge on it and is initially at rest in an inertial frame of reference. Now let there be an electrical field applied. The charged ball will then start to accelerate. There is only an action force and no reaction force acting on the ball. But the force is quite real. Newton's 3rd law does not apply to this case since it applies to situations when there are a pair of forces acting on a body.

No, Newton's 3rd law does not apply here because it applies to situations where there are two bodies interacting. You can have as many forces as you wish acting on a single body, you don't invoke the 3rd law until you are considering another object interacting with the first.

Now let us suppose that the charged ball is in contact with a wall whose unit normal is parallel to the electric field and the direction of the electric force is towards the wall. When the ball comes in contact with the wall we can then apply Newton's 3rd law. The force exerted by the ball on the wall is equal and opposite to the force the wall exerts on the ball. This is the action-reaction pair.

Yes, and neither is centrifugal.

Similarly if there is an uncharged ball but now the force of gravity is acting on a ball on a table. The table exerts and equal and opposite force on the ball. This too is an action-reaction pair. If the ball is in free-fall then there is no reaction force and Newton's 3rd law does not apply.

If you are considering only the ball then you are right. But there is an equal-and-opposite (Newtonian) gravity reaction force on the Earth if you are considering the Earth too.

Now let us consider a (non-charged) ball in free-fall as observed in the rotating frame. Since the object is not in contact with anything and there is no counter force acting then there is no reaction force.

Yes, there is no reaction force, but there is a centrifugal force. The centrifugal force is therefore not a reaction force.

Now let us suspend the ball by a spring in the rotating frame. Now there is an action-reaction force. The centrifugal force acts outwards while the centripetal force exerted by the spring in the oppsite (i.e. inward) direction.

Sure, but the action-reaction pair is the compression force of the spring on the ball and the compression force of the ball on the spring. The centrifugal force is not part of any action-reaction pair.

Would you like me to discuss the four properties that you mentioned or can you obtain what I meant from the explanation I gave in this post?

Yes, I would like you to discuss them.

You spoke of distinguishing characteristics of force. No two forces of a different nature are identical for the sole fact that they are different. E.g. the magnetic force may exist in one frame but may not exist in another frame. In such cases the magnetic force becomes the electric force.

This is a valid and interesting piont. I will have to modify or remove my property number 1. To be honest, I was not considering boosts, particularly not relativistic boosts. I was considering linearly accelerating and rotating reference frames. I need to think about that a bit.

But when you're making an observation in a single frame then there is nothing about the motion that will tell you whether the force is inertial or not.

Yes there is! Strap an accelerometer on it.

 

[goeat]

There's a somewhat archaic usage of "centrifugal force" to mean the reaction force to the centripetal force. I guess that's what you mean. But that's totally different from the standard usage of the term centrifugal force.
It even acts on a different body! (And if that's all you mean, there's not much to argue about!)

That's all I mean. I prefer the archaic usage of centrifugal force because it had a meaning. The "standard" usage of the term defines it to be non-existing. As posted previously, I am not alone in preferring the archaic usage of centrifugal force:

http://www.encyclopedia.com/doc/1E1-centripe.html

See section on "reactive force":
http://peswiki.com/index.php/PowerPedia:Centrifugal_force

Examples of action-reaction pairs

How would you describe the forces of a motorcycle is in a stable, non-accelerating (and no drag), coordinated turn?

At the contact patches, the tires push downwards and outwards on the pavement. The pavement pushes upwards and inwards on the tires. Then to describe the vertical and horizontal components of these forces. Gravity pulls downwards on the motorcycle, which in turn pushes downwards on the tires at the contact patch, where the pavement pushes upwards with equal and opposite force. The tires push outwards on the pavement at the contact patch, and the pavement pushes inwards on the motorcycle. The forces result in an inwards acceleration of the motorcycle and an outwards acceleration of the earth. The roll axis torques cancel out, so the motorcycle maintains it's current lean angle.

The tough call here is which forces are the action forces and which are the reaction forces. Forward speed and slip angle of the tires cause the tires to generate "outwards" forces on the pavement at the contact patch, which reacts with an equal and opposite "inwards" force at the contact patch.

In my case, I have no problem calling the inwards force from the pavement to the tires a centripetal force, and the outwards from the tires to the pavement a centrifugal force, regardless of which is the action or which is the reaction force.

Maybe a better example would be a ball looping inside a larger sphere in outer space, free from any friction or significant gravity effects. At the contact patch, the ball exerts an outwards force on the sphere, and the sphere exerts an inwards force on the ball. The center of mass of the ball and sphere will each follow the path of a circle, with the relative radius of the paths depending on the relative mass and size of the ball and sphere. Again, I would be OK to call the inwards force from the sphere to the ball inside a centripetal force, and the outwards force from the ball to the sphere a centrifugal force, since to me, these terms just help describe the direction of forces perpendicular to the path of an object.

 

[Dale]

I prefer the archaic usage of centrifugal force because it had a meaning. The "standard" usage of the term defines it to be non-existing.

The centrifugal force (modern meaning) does exist in the rotating frame. I think you are just getting hung up on the word "ficticious". It doesn't mean that it doesn't exist.

Your archaic usage does nothing but add a lot of confusion to this thread, which has been dealing with the modern usage.

The tough call here is which forces are the action forces and which are the reaction forces.

That is always a completely arbitrary distinction. Together they are an action-reaction pair, beyond that it does not matter.

 

[goeat]

Your archaic usage does nothing but add a lot of confusion to this thread, which has been dealing with the modern usage.

I'm not sure that the OP was concerned about modern versus classis usage.

It's difficult to see this as "archaic" usage when there are current reference sources, such as the links I provided in previous posts, that use the "classic" definition, or include both definitions.

Then again, if some group decides to change the meaning of a "classic" term, why not create a new term instead of changing the meaning of the old one? This is how to prevent confusion.

 

[Doc Al]

That's all I mean. I prefer the archaic usage of centrifugal force because it had a meaning. The "standard" usage of the term defines it to be non-existing.

Centrifugal force, in the modern sense, most certainly has a precise meaning and an existence.

So it seems like your digression about "centripetal force" and its "reaction" has nothing to do with the topic of this thread, which was clearly about its modern usage as an inertial force.

The tough call here is which forces are the action forces and which are the reaction forces.

As Dale already pointed out, the distinction between "action" and "reaction" is arbitrary. Better to call them "3rd law pairs".

 

[pmb_phy]

No, Newton's 3rd law does not apply here because it applies to situations where there are two bodies interacting. You can have as many forces as you wish acting on a single body, you don't invoke the 3rd law until you are considering another object interacting with the first.

Hmmm .. you're right. I now see that I had something else in mind. In any case, in reality there actually is a reaction force present. The charged particle exerts a force on the source of the field. I now see that this was a poor example.

Let me take a different tack on this. Consider instead two charged particles moving along two straight lines which intersect at a right angle. Each particle has a force exerted on it which is caused by the other particle. However, at least in this case, Newton's 3rd law does not apply. Contrary to your claim that "real" forces must obey Newton's 3rd law the forces acting here, i.e. the Lorentz forces, are quite real.

Were you aware that Newton's 3rd law does not always apply? When Newton stated this law the Lorentz force was totally unknown. Since this is a counter example of Newton's 3rd law Newton never would have stated his law as such since it doesn't always hold. This notion is covered in good physics texts such as Classical Mechanics - 3rd Ed., by Goldstein, Safko and Poole.

Yes there is! Strap an accelerometer on it.
[/qoute]
So what? An accelerometer in free-fall in a gravitational field will read zero, contrary to what you are attempting to imply. We are discussing Newtonian physics here right??

Pete

 

[Doc Al]

Consider this - An observer is at rest in a uniformly rotating frame of reference. In that frame there is a ball suspended by a spring. The centrifugal force exerts a force on the ball and the ball exerts a force on the spring. The force exerted on the ball is the centrifugal force. The force the string exerts on the ball is the centripetal force. This is an action-reaction pair.

"Action-reaction" pairs do not act on the same object!

 

[Dale]

Consider instead two charged particles moving along two straight lines which intersect at a right angle. Each particle has a force exerted on it which is caused by the other particle. However, at least in this case, Newton's 3rd law does not apply. Contrary to your claim that "real" forces must obey Newton's 3rd law the forces acting here, i.e. the Lorentz forces, are quite real.

Don't forget that the EM fields carry momentum. In fact, in this case each particle interacts directly with the local field, rather than the other particle. Taking that into account momentum is conserved and therefore Newton's 3rd law is satisfied at all times.

An accelerometer in free-fall in a gravitational field will read zero, contrary to what you are attempting to imply. We are discussing Newtonian physics here right??

I already mentioned exactly this point in post 24. I also have not introduced an example using gravity and when someone else has I made it clear if I thought they were using the Newtonian approach. I like this way of classifying inertial forces, and I am perfectly content to consider gravity an inertial force. I have no qualms about using ficticious/inertial forces.

 

[Dale]

Then again, if some group decides to change the meaning of a "classic" term, why not create a new term instead of changing the meaning of the old one? This is how to prevent confusion.

I agree with you 100% on this point. It is one of my personal "pet peeves". In poetry it is great for words to have multiple meanings and all sorts of ambiguity. In physics it only causes problems.

 

[Dale]

Hi Pete,

One quick thought. Newton's 3rd law essentially defines the conservation of momentum in terms of forces. One way to see that the centrifugal force does not follow the 3rd law is simply to consider the momentum of an isolated system in a rotating reference frame. In the rotating reference frame the isolated system will accelerate, so momentum is not conserved in the rotating frame. Therefore the 3rd law is violated.

 

[YellowTaxi]

Another point is that any object rigidly attatched to a rotating reference frame (spacestaion) appears stationary but it actually has spin angular momentum as well as orbital, and spins precisely once for every complete orbit of the station.

Any object that falls off the station, it will certainly be spinning as it flies off into outer space.
And likewise an object that's dropped inside the spacestation will spin as it falls to the floor.

Circular forces..!

[edit.. on second thoughts I think that what I said wasn't quite right: The spin of the falling object is invisible to anyone standing in the spacestation, because they're basically spinning too, so in effect it remains 'invisible' in that frame. Curious.]

And
Sure Newton's 3rd law is just the conservation of momentum re-written in terms of forces. All he did was turn it into 'The consevation of Forces law'.

 

[dipstik]

dale: i think coriolis forces are the result of the conservation of angular momentum in rotating frames, but i could be wrong.

 

[YellowTaxi]

dale: i think coriolis forces are the result of the conservation of angular momentum in rotating frames, but i could be wrong.

no its actually the conservation of linear momentum that requires the coriolis to explain motions seen from the rotating frame: From the rotating spacestation straight line trajectories in free-space look curved. that's why it's fictitious and is required to model the curvature mathematically. Objects that fall off the frame appear to follow curved paths. A ball thrown around on a roudabout appears to be curved by an invisible field. We all know you don' t actually need a force to move in straight lines.

The curvature seen from the frame is only an apparent curvature, it's kind of an illusion really.
http://ie.youtube.com/watch?v=_36MiCUS1ro

The bottom line is that when something moves around on a rotating frame things get wierd.
I think you can only use Newtons laws within the frame if everything stays put [statics].
As soon as things start moving around you get the coriolis and centrifugal effects messing with everything.

 

[pmb_phy]

Hi Pete,

One quick thought. Newton's 3rd law essentially defines the conservation of momentum in terms of forces. One way to see that the centrifugal force does not follow the 3rd law is simply to consider the momentum of an isolated system in a rotating reference frame. In the rotating reference frame the isolated system will accelerate, so momentum is not conserved in the rotating frame. Therefore the 3rd law is violated.

Conservation of momentum states that when the total force acting on a system is zero then the momentum of that system is conserved. If a centrifugal force is acting on a particle then there is no reason to assume the momentum should be conserved. In GR this is well defined because the source of the centrifugal field (which Einstein viewed as a gravitational field) is the mass of the universe, i.e. the "fixed stars." This isn't all that hard to demonstrate since you merely have to consider a rotating spherical shell. The spacetime inside the shell is flat but the rotating shell causes the spacetime to rotate.

Likewise, the momentum of a particle in free-fall in a gravitational field is not conserved either.

Pete

 

[pmb_phy]

no its actually the conservation of linear momentum that requires the coriolis to explain motions seen from the rotating frame: ..

Actuallu it is the effect of transforming to a rotating frame of reference. Conservation of momentum, whether linear or angular, is not utilized in the transformation.

From the rotating spacestation straight line trajectories in free-space look curved. that's why it's fictitious and is required to model the curvature mathematically.

What do you mean by "curvature"? If you're speaking about spacetime curvature or spatial curvature then it has nothing to do with this subject. In the case of rotating frames in otherwise flat spacetime there is no spacetime curvature and also no spatial curvature and curvature cannot be introduced by the introduction of another coordinate system. Therefore curvature has nothing to do with inertial forces.

Objects that fall off the frame appear to follow curved paths. A ball thrown around on a roudabout appears to be curved by an invisible field. We all know you don' t actually need a force to move in straight lines.

It is the trajectory which is curved and nothing else. When physicists use the term "curvature" in GR they are referring to the non-vanishing of the curvature tensor.

Pete

 

[YellowTaxi]

Actually it is the effect of transforming to a rotating frame of reference. Conservation of momentum, whether linear or angular, is not utilized in the transformation.

Maybe you misunderstood me, but I seriously wouldn't expect anybody to try to analyse free-body straight-line trajectories at constant speeds in terms of coriolis forces unless they had the misfortune to be trying to make sense of them from a rotating reference frame; like from a playground roundabout or a rotating space-station.

What do you mean by "curvature"? I...

I simply mean the apparent (ficticious) curvature of an object falling away in a straight line (with no accelerative forces acting on it whatsoever) but viewed from a rotating reference frame where the motion does seem very curved indeed.
One like in this video here for example:
http://ie.youtube.com/watch?v=49JwbrXcPjc

I don't fully understand gen rel , and I don't talk about it - not to anybody ;-)
At the present moment in time I don't even fully understand circular motion. (And I'm not convinced that anybody does fully understand that either.)

 

[pmb_phy]

"Action-reaction" pairs do not act on the same object!

I guess I was unclear. Thanks for correcting me. I had meant to say that the ball exerts a force on the string and the string exerts a force on the ball.

Regarding Newton's third law; before I can reasonably comment on that further I need to know two things (1) what does it have to do with this subject since the existence of an action reaction force has, in my opinion, very little to do with the OP's original question which I am attempting to address and (2) please provide a reference for Newton's 3rd law because different texts will define this in slightly different ways and answering a question regarding it will depend on the exact wording. For example: Feynman defines Newton's 3rd law as follows action equals reaction. This is how it was taught to me in my first college physics course. In that course the following example was used - One can say that when you push on a wall with a force F then the wall pushes back with a force -F. This forms an action-reaction pair. Other authors use it only to refer to pairs of particles and even then there are two forms of it. The weak form requires only that the forces are equal and opposite. The strong force adds the additional requirement that the forces act along a line connecting the two particles. In other circumstances one is merely given a field with the source not specifically stated, e.g. a uniform gravitational field, or a uniform electric field, or an EM wave. As such one cannot specify another body. The Lorentz force is a good example. To say that the Lorentz force is not real because it doesn't obey Newton's third law is contrary to the modern view of what a force is. The example of an EM wave is illustrative in this case. First off, one need not be concerned with the source since the field is decoupled from the source. As such, while there is an action (the force on the charge due to the EM wave) there is no reaction force. Since Newton's third law came up as an attempt to define what is "real" then I'd rather focus on that rather than side track into whether two particles are implied in Newton's third law. Can we agree to do this Doc? Thanks for your response Doc Al.

Best wishes

Pete

 

[pmb_phy]

Hi Pete,

One quick thought. Newton's 3rd law essentially defines the conservation of momentum in terms of forces.

To be precise it, at most, is a theorem which is derived from the Newton's 3rd law, not a definition. Regarding the Lorentz force, momentum is conserved because the EM field has momentum and this is where the momentum comes in regarding the conservation of momentum between interacting charges.

One way to see that the centrifugal force does not follow the 3rd law is simply to consider the momentum of an isolated system in a rotating reference frame. In the rotating reference frame the isolated system will accelerate, so momentum is not conserved in the rotating frame. Therefore the 3rd law is violated.

Are you familiar with Mach's Principle?

Pete

 

[nuby]

Was my question answered somewhere? lol... Sorry about being a total layman, here.

 

[YellowTaxi]

I've heard recently that centrifugal "force" doesn't exist. If this is true what is the actual force that creates the centrifugal effect?

Also, do centrifugal and centripetal effects/forces exist in outer space, i.e. on space shuttle.

Thanks

I'd say centrifugal force only exists for someone who's rotating or spinning but is pretending they weren't aware of their rotation.

 

[Dale]

Conservation of momentum states that when the total force acting on a system is zero then the momentum of that system is conserved. If a centrifugal force is acting on a particle then there is no reason to assume the momentum should be conserved.

Don't you see that you are making my point for me here? If a system is isolated then, by definition, there is no external body generating any force on it. Therefore if there is a force acting on an isolated system that force must be in violation of the 3rd law since there is no other body for the "reaction" force.

Pete, I must admit that you have me quite surprised. The fact that inertial forces violate the 3rd law should be obvious to you by your own words in this and previous posts. It is fine by me if you want to claim that the 3rd law is or that a classification of forces as "real" or "ficticious" on that basis alone is wrong. But you cannot seriously still believe that the centrifugal foce satisfies the 3rd law.

 

[YellowTaxi]

If a system is isolated then, by definition, there is no external body generating any force on it. Therefore if there is a force acting on an isolated system that force must be in violation of the 3rd law since there is no other body for the "reaction" force.

I think that's the same as saying an object can't accelerate unless a force acts on it.
Obvious really. ie It's the 2nd law too.

.'. the forces are fake/ficticious

It's the same as what I said. The coriolis and centrifugal forces are ficticious because they explain an apparent curvature which isn't really there in reality. The trajectory of free moving objects (actually perfect straight lines) only looks curved when viewed from the dodgy rotating frame. I'm pretty sure they look like Archimedes spirals.

 

[Dale]

Was my question answered somewhere? lol... Sorry about being a total layman, here.

I would say that the centrifugal force does exist in the rotating reference frame. It does not exist in an inertial (non-rotating) reference frame.

Such forces are called "ficticious forces" or "inertial forces", they exist in non-inertial frames, and not in inertial frames.

 

[YellowTaxi]

I would say that the centrifugal force does exist in the rotating reference frame. It does not exist in an inertial (non-rotating) reference frame.

- but it does not exist on objects that are moving with an angular rotation about the centre axis with an equal and opposite rotation to that of the frame. (because such objects are in fact not rotating and not moving at all).

well you could say they DO have a centrifugal force acting on them, but it's of magnitude zero.