Do Centrifugal and Centripetal Forces Exist in Outer Space?

[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.

 

[Dale]

- 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.

The centrifugal also acts on such objects. Its value is mω²r, so it is only equal to 0 for r=0.

 

[YellowTaxi]

I would say that the centrifugal force does exist in the rotating reference frame.

The centrifugal also acts on such objects. Its value is mω²r, so it is only equal to 0 for r=0.

No,
If you move with an ω numerically equal to the ω of the rotating frame but in the opposite direction, you aren't moving at all. And then mrω² for all values of r is zero. Even though from the rotating frame you seem to have an angular speed ω, you in reality are standing still.

That's why I think the rules for statics will work fine on a rotating frame , but anything moves at all [in any direction] and it all gets bizarre, and Newton flies out the window.

 

[Dale]

No,
If you move with an ω numerically equal to the ω of the rotating frame but in the opposite direction, you aren't moving at all. And then mrω² for all values of r is zero. Even though from the rotating frame you seem to have an angular speed ω, you in reality are standing still.

That's why I think the rules for statics will work fine on a rotating frame , but anything moves at all [in any direction] and it all gets bizarre, and Newton flies out the window.

I think you are thinking about the Coriolis force, which is -2m(ωxv). So it does depend on the velocity in the rotating reference frame.

If an object is stationary in the inertial reference frame then it obviously travels in a circular motion in the rotating reference frame. To travel in a circular motion it must have a centripetal force, which is provided by the Coriolis force. The Centrifugal force, of course, is in the opposite direction of any centripetal force, but the factor of 2 in front of the Coriolis force makes it twice the magnitude of the Centrifugal force resulting in a net centripetal acceleration in the rotating reference frame.

PS sorry, that is confusing to read but I am too sleepy to write more clearly

 

[YellowTaxi]

I think you are thinking about the Coriolis force, which is -2m(ωxv). So it does depend on the velocity in the rotating reference frame.

no I'm not talking about coriolis at all here. We were both discussing mω²r which is not the coriolis force ;-)

If an object is stationary in the inertial reference frame then it obviously travels in a circular motion in the rotating reference frame.

yes , that was exactly my point in my previous post. This object which appears to have circular motion when viewed from the rotating frame will in fact be standing perfectly still in space. The tension in the string (say), or the force from a retaining wall required to hold it there will be zero.

 

[Doc Al]

No,
If you move with an ω numerically equal to the ω of the rotating frame but in the opposite direction, you aren't moving at all.

You aren't moving with respect to the inertial frame.

And then mrω² for all values of r is zero.

No. The ω in the formula for centrifugal force is due to the rotation of the frame; it's not the ω with respect to the rotating frame.

Even though from the rotating frame you seem to have an angular speed ω, you in reality are standing still.

Again, you are at rest with respect to the inertial frame, not the rotating frame.

That's why I think the rules for statics will work fine on a rotating frame , but anything moves at all [in any direction] and it all gets bizarre, and Newton flies out the window.

To be used from a rotating frame, Newton's laws, including their application to statics, must be modified to include all relevant "fictitious" forces.

no I'm not talking about coriolis at all here. We were both discussing mω²r which is not the coriolis force ;-)

You're not talking about coriolis, but you should be. If you are moving with respect to the rotating frame, coriolis force must be considered.

yes , that was exactly my point in my previous post. This object which appears to have circular motion when viewed from the rotating frame will in fact be standing perfectly still in space. The tension in the string (say), or the force from a retaining wall required to hold it there will be zero.

It's certainly true that an object at rest in an inertial frame requires no net force to remain at rest. But if you choose to analyze the situation from the view of the rotating frame, it is certainly moving. Both centrifugal and coriolis forces are at work.

Let's say the frame is moving counterclockwise at angular speed ω with respect to the inertial frame. Thus there will be a centrifugal force = mω²r acting outward. If the object also moves clockwise with an angular speed ω with respect to the rotating frame, there will be a coriolis force = 2mω²r acting inward. Thus, from the rotating frame, there will be a net inward force equal to mω²r. Which makes sense, since from the rotating frame the object is accelerating inward. (No "real" force is required.)