# The meaty part of centripetal acceleration

Discussion in 'Physics & Math' started by nicholas1M7, Jul 29, 2011.

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Kid, you're disappointing everyone here. Understand that I'm NOT trying to be mean and that what I'm about to explain to you is very simple - it's just that you haven't caught on to it yet.

You're trying to show how smart you are, yet what you say - and your awkward way of saying it - gives the impression that you really aren't all that bright. But I think the REAL problem is that you've just begun your scientific education and haven't learned many of the basics yet.

So, instead of trying to tell the MUCH MORE educated people here about basic physics, you actually should be asking questions to help you understand what you're studying.

3. ### nicholas1M7BannedBanned

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Okay. I think I got you.

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Good - and thanks for the positive response.

I feel certain that the more you learn, the more you will enjoy it.

7. ### OnlyMeValued Senior Member

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James could you elaborate? I am probably just crossing wires here, as I am deep into a couple of papers, kinda sorta on a different track.

Wouldn't the context involved in centrifugal force itself be non-inertial? in that it involves constant acceleration.

8. ### James RJust this guy, you know?Staff Member

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nicholas1M7:

That's what I wrote, didn't I? You can probably take it as given that I mean what I write, most of the time at least.

Speed is the magnitude of velocity. If velocity changes then you have an acceleration. Therefore, if speed changes you have an acceleration.

Clear?

9. ### James RJust this guy, you know?Staff Member

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OnlyMe:

One problem is that you can see "centrifugal force" used in two different senses. The sense that physicists normally use is that centrifugal force is a pseudoforce (fictious force) that appears when you work in a rotating (and hence accelerating) frame of reference. It points outwards from the centre of rotation. If you work in an inertial frame instead, there is never any centrifugal force in this sense.

The other way that the term "centripetal force" is sometimes used, mostly by amateurs, is to refer to ANY force (real or fictitious) that acts away from the centre of rotation.

To take an example, if you tie a ball to a string and swing it in a circle around your head, then your hand exerts a centripetal force on the ball. By Newton's third law of motion, the ball exerts an equal and opposite force on your hand, which points away from your hand (towards the outside of the circle). You could call that outwards force a "centrifugal force" if you like, but in analysing that situation you're usually interested in the force on the ball and not the reaction force on the hand that is swinging it, so that kind of centrifugal force almost never comes up in elementary physics discussions.

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11. ### OnlyMeValued Senior Member

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James R

That's all consistent with my understanding. I just forgot you usually come at things from the perspective of particle physics, where I begin with GR and work the other way.

Centripetal forces is toward the center... The force I exert on the ball through the string holding it in. Centrifugal the force the ball exerts on the string and me away pulling out. In that sort of example it is a clear distinction because both can be felt and measured.

I started by seeing the non-inertial system as a satellite in a stable orbit. Gravity in that case is the centripetal force and the satellites linear velocity contributes to the centrifugal force. Neither wind up being felt by an observer on the satellite and yet contribute to the stable orbit.

And yes both terms are to differing degrees archaic.

Thanks for the clarification.

12. ### James RJust this guy, you know?Staff Member

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OnlyMe:

Centripetal/centrifugal force has nothing to do with particle physics or GR, in particular.

That's one sense in which "centrifugal force" is used, as I said, and not the most common one used by physicists.

The centrifugal force in the non-inertial frame has nothing to do with velocity. In its own frame the satellite doesn't move, so its velocity is zero. The centrifugal force is purely an effect of the reference frame itself.

Actually, in the Newtonian picture both must be felt by the observer, so they can cancel out to give zero net force in the accelerating frame.

Neither term is archaic.

13. ### OnlyMeValued Senior Member

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This is true. However, how you see things is often influenced by your knowledge base.

So what you are claiming is that satellite needs no velocity to remain in orbit. The above quote is a semantic debate. Even the satellite knows that it[/] is in motion relative to the planet beneath. And yes it could be the planet above or out the side window.

Archaic, adjective, First among other definitions is, very old.

Both terms are very old and in the case of centripetal force, it was commonly used to refer to what Newton later called gravity. That does not mean gravity is the only centripetal force.

14. ### James RJust this guy, you know?Staff Member

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OnlyMe:

In the inertial frame of the Earth, the only force on the satellite is gravity. If the satellite had no velocity, it would be pulled to Earth by the gravitational force. So in that frame it certainly needs velocity to orbit.

In the non-inertial frame of the satellite, the satellite is subject to two forces: (a) the force of gravity pulling it towards the Earth and (b) the centrifugal force pushing it away from Earth. These two forces are equal and opposite, so there is no net force on the satellite. The satellite is at rest in its own frame.

There's no experiment you can do inside the satellite to determine that the satellite is in motion. You need to look out of the window to see motion, and then you only see relative motion.

I read it in the sense of "outdated".

Gravity is a centipetal force. Any force that always points towards the centre of rotation/revolution is a centripetal force.

15. ### OnlyMeValued Senior Member

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Here is where the perspective one acquires from their experience base affects how, one "see" the world.

When we separte frames of reference in relativistic terms, inertial and non-inertial and the many versions of each, we gain a perspective on the world in which we live that is useful and at times enlightening. But the world and the universe exist as a whole with all of the different frames of reference subject to the same rules and the mechanics of how things interact do not change simply as a result of from which frame of reference we choose to view them.

I really do believe there is an underlying unity to.., everything. We are not there yet. But simply because we cannot see the whole of the truth around us, does not mean that that truth changes depending upon our frame of reference.

The most important role that understanding frames of reference and relativity plays in our understanding of the universe is in providing us with the tools to understand how things appear from a frame of reference other than the one we are currently in.

Just because an observer in a satellite cannot know without looking out a window that the satellite is moving does not mean that it is not moving. Additionally, any observer in a satellite that has any understanding of how gravity affects objects has to know that the satellite is in motion.

The forces at work between the satellite and the planet do not change depending upon the frame of reference. Only how an observer perceives events unfolding changes.

The satellite's non-inertial frame of reference is the product of both its linear velocity and counter balancing constant acceleration toward the planet. That is true in both frames of reference. The only difference is how observes on the satellite and the planet measure those two forces.

Keep in mind that centrifugal force is not a real force it is an effect. The result of the counter balance, in this case between the satellites linear velocity and the gravitational force acting between the planet and the satellite.

16. ### James RJust this guy, you know?Staff Member

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OnlyMe:

The rules don't change, but the physical quantities certainly do. And in fact the rules are only the same in a special subset of reference frames, namely inertial frames. In other frames, the laws of physics have to be modified to make things accord with observation.

That's the whole point of relativity.

Galileo made the point that absolute motion is undetectable by any experiment, back in the 17th century.

Only if they know the satellite is orbiting something. And that's an intellectual understanding, not one based on direct observation.

Yes they do. In any non-inertial frame, pseudoforces will exist that do not exist in inertial frames.

A satellite's acceleration in no way "counter balances" its velocity. The acceleration and the velocity are at always at right-angles to each other (for a circular orbit).

No. It's very real, with very real effects. Centifugal forces can kill you if they are large enough. Centifugal force is the same kind of force that throws you through the windscreen when you have a head-on car crash and you're not wearing a seatbelt.

17. ### Motor DaddyValued Senior Member

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He was wrong. A light source can travel away from the point in space it emitted light during the time of light travel of the light it emitted.

Last edited: Aug 3, 2011
18. ### AlexGLike nailing Jello to a treeValued Senior Member

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According to MD, every physicist in the world is wrong, and he's right.

19. ### OnlyMeValued Senior Member

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James, this discussion seems to be degrading into a semantics debate, where comments are directed toward statements out of context rather than the overall intended context.

The way it appears to me there are two basic issues involved in the overall context. One involves how centrifugal and centripetal forces are defined and described and the other involves the context involved in frames of reference.

Centripetal force is a center seeking force. A force acting toward the center. That can be the force one exerts on a string attached to a ball being swung around in a circle. In the case of a satellite in a stable orbit around a planet, the centripetal force is the result of the gravitational interaction between the satellite and the planet. In a very crude example of the relationship between an electron and a proton in an atom, the opposite charge attraction, could be viewed as a centripetal force. Centripetal force is a generic term for any force acting toward the center of an involved system.

Centrifugal force... One can look at centrifugal force from both inertial and non-inertial frames of reference. How it, (centrifugal force), is defined in each case can be misleading and the various definitions, to some extent, originate from different perspectives.

Generally as applied to non-inertial frames of reference, centrifugal force is considered to be a fictitious or pseudo force.

Just one of the many definitions a Google search returns, when searching "pseudo force", http://www.google.com/search?q=pseudo forces&ie=UTF-8&oe=UTF-8&hl=en&client=safari
In the case of inertial frames of reference, most descriptions fall back to a Newtonian model and label centrifugal force as reactive.., consistent with Newton's Third Law of Motion.

Both, at the risk of using the term again, are archaic, though still used. And neither actually describes the mechanism involved.

All centrifugal force is an artifact of an objects inertia, whether the frame of reference is inertial or non-inertial. And in both inertial and non-inertial frames of reference, centrifugal force acts away from a central point determined by a curved path.

Being a little redundant; In all cases centrifugal force is an artifact of a change in an objects motion, in the form of an acceleration, which changes the objects motion from a straight line of motion to a curved motion.

In most cases, both in inertial and non-inertial frames of reference, the centrifugal force can be measured and is directed away from the center of the curved path. In this a stable orbit is an exception...

In the case of a stable orbit, the centripetal force is the force of gravitation acting between the two masses. In our case a satellite and a planet. Here the centrifugal force, an artifact of the objects inertial resistance to its change in motion, cannot be felt as the gravitational force acts equally on all parts of the satellite and all objects and individuals in the satellite. Note: The satellites inertial resistance to the "pull" of gravity is a direct result of its linear velocity, which is a 90 degrees to the gravitational force. The centripetal force of gravity and the centrifugal force resulting from the satellite's inertial resistance to a change in motion, are equal and opposite. The result is that an observer in a stable orbit "feels" as though she/he is in free fall, feeling the effects of neither. In this situation both the inertial resistance to a change in motion and the gravitational force act equally on all of the involved elements.

The satellite here represents a special case involving the Equivalence Principle, where the satellite's constant and uniform acceleration toward the planet, cannot be felt, as it would be, should the satellite's curved path be the result of a centripetal force other than gravity.

To the issue of frames of reference... when ever we begin to compare observations made from differing frames of reference, whether they are inertial or non-inertial, we are discussing how things "look" to the observers in the various frames of reference involved in the comparison. That is the subject of relativity. And yes different tool sets are required when we attempt to reconcile observations made involving inertial and non-inertial frames of reference. When adequate tools are available and properly applied, no individual frame of reference can be assumed to be special or preferred. They must all be treated as equivalent. That is not to say that any observation from any inertial or non-inertial frame of reference represents a accurate macroscopic description of the involved events and interactions.

In the case of the satellite/planet example we have been using, we are dealing with a satellite in a non-inertial frame of reference orbiting a planet. How an observer with no knowledge outside of the satellite capsule, experiences the inertial frame of reference within the capsule's limited environment, has no significance to the satellite's orbit of the planet, nor any of the involved forces (or pseudo forces).

One last thought, as a question of sorts. Given the satellite in a stable orbit and an observer with no windows or instruments to know of anything outside of his/her inertial frame of reference (confined to the satellite).., should the observer have the ability to perform an experiment equivalent to the GP-B experiment, obviously without the guide star or another outside reference..., would the measured results, still influenced by geodetic and frame dragging effects be completely consistent with his/her closed inertial frame of reference? It is my guess without all the work that would be required, that the results would not be equivalent to those expected from a completely inertial frame of reference.

20. ### James RJust this guy, you know?Staff Member

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Please keep your nonsense to its own threads, rather than trying to peddle it to new ones.

21. ### James RJust this guy, you know?Staff Member

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OnlyMe:

I have no issues with your previous post; I think we're on the same page here.

General-relativistic orbital reference frames are only locally inertial. That is, at any point on the orbit you can't really distinguish being in orbit from moving in a straight line through "flat" space. However, obviously if you make measurements on a large enough scale it is possible to detect the curvature of the overall spacetime caused by the planet or other body being orbited.

So, I'm sure you can measure frame-dragging and other effects without looking outside the satellite. In fact, GP-B shows that this must be the case.

22. ### PeteIt's not rocket surgeryRegistered Senior Member

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Didn't gravity probe B use a star as a direction reference?

23. ### James RJust this guy, you know?Staff Member

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I'm no expert on GP-B, I must admit.