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# Centrifugal g force testing2

## Centrifugal g force testing

(OP)
I have trouble grasping direction of a g force.

I'm doing g force testing in a centrifugal apparatus. My question is- in which direction is g-force acting?

Working with rotating equipment for a long time I know that centrifugal force is acting to the outside. That force is counteracted by stiffness of the rotating arm. One of the engineers said it's acting to the inside and I called up the testing lab and they said it's acting to the inside (center).

Is that the case and if it is what is the reasoning behind it?

William

### RE: Centrifugal g force testing

FeldmanWill:
Centrifugal force acts outward , away from the center of a circular motion. The reacting force, Centripetal force acts inward, toward the center of a circular motion. It is the force which holds the mass in place on your machine.

You would do well to find a copy of a freshman Uni. Physics textbook and study it a bit, this is pretty basic stuff.

### RE: Centrifugal g force testing

From a non-mechanical engineer's perspective:

If a body is moving in a circle there is a radial force, consisting of a force in one direction and an equal reaction in the opposite direction.

Which direction is the action, and which the reaction is pretty arbitrary, depending on circumstances and convention, but the outward force is always called centrifugal, and the inward force is always called centripetal.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

Careful. Most of the time people use the phrase "outward force", they are mistaken. In the sense they are using, there is no such force. I don't mean to imply that you don't know this, Doug. With the original question being what it is, the phrase "outward force" is definitely a landmine.

### RE: Centrifugal g force testing

#### Quote:

Careful. Most of the time people use the phrase "outward force", they are mistaken. In the sense they are using, there is no such force. I don't mean to imply that you don't know this, Doug. With the original question being what it is, the phrase "outward force" is definitely a landmine.

I'm not sure what circumstances you have in mind, but in any case where a body is moving in a circle (relative to an inertial frame of reference) there is an inward force and an equal and opposite outward force.

I think the Wikipedia article on "fictitious forces" is highly misleading in this respect.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

There is no outward force acting on an object moving in a circle.

### RE: Centrifugal g force testing

"There is no outward force acting on an object moving in a circle."

Why is an inward force necessary to keep the object moving in a circle?

Walt

### RE: Centrifugal g force testing

Fictitious forces are required to explain effects in accelerating frames of reference.

When you stomp on the accelerator in a car, you become part of an accelerating frame of reference that is the car. There is an apparent(g)force that pushes you into your seat, but that's what Wikipedia refers to as the fictitious force, because in the external, inertial reference frame, the seat is actually pushing on you to accelerate you to the same velocity as the car.

Likewise, in a tilt-a-whirl, there is an apparent (fictitious) force pushing you against the wall of the cylinder, but that force is not real. The real force is the centripetal force pushing you to move in the circle that follows the motion of the cylinder. There are not two canceling forces; there's only one force that's apparent in the inertial frame, and the fictitious force in the accelerating frame. There actually cannot be equal and opposite forces in this case because the centripetal force has to be unbalanced to provide the centripetal acceleration that moves you in the circle. If it were "balanced", you would, by definition, have to move in a straight line.

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### RE: Centrifugal g force testing

Because the object wants to go in a straight line. Not away from the center (radially) but always in a straight line (tangentially). The inward force toward the center changes the direction of motion.

-handleman, CSWP (The new, easy test)

### RE: Centrifugal g force testing

This is because an object moving in a circle is accelerating, even if the rotational speed remains constant. The acceleration is inward,toward the center of the circle.

### RE: Centrifugal g force testing

#### Quote:

There is no outward force acting on an object moving in a circle

Newton would disagree:

Law III: To every action there is always opposed an equal reaction: or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

#### Quote:

Fictitious forces are required to explain effects in accelerating frames of reference.

When you stomp on the accelerator in a car, you become part of an accelerating frame of reference that is the car. There is an apparent(g)force that pushes you into your seat, but that's what Wikipedia refers to as the fictitious force, because in the external, inertial reference frame, the seat is actually pushing on you to accelerate you to the same velocity as the car.

Yes, fictitious forces are required to explain effects in accelerating frames of reference, but no, the inertial force felt when you are actually accelerated is not an example of a fictitious force.

A common example of a fictitious force is the force that appears to accelerate a parcel sideways across the back seat when you drive around a sharp bend. The parcel is actually travelling in a straight line (ignoring friction forces on the base), but appears to be accelerating from your accelerating frame of reference, so the force is entirely fictitious.

For a body accelerated in a straight line there is a real inertial reaction force, whether the body is observed from an inertial frame of reference, the accelerated frame of reference, or anything in between.

To call inertial reaction forces imaginary is both unnecessary and confusing.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

#### Quote (IDS)

For a body accelerated in a straight line there is a real inertial reaction force, whether the body is observed from an inertial frame of reference, the accelerated frame of reference, or anything in between.

In the example of the car, you mash the gas:

1. The seat exerts a force on you, you accelerate. F=ma
2. You exert a force on the seat. Exactly the same F=ma...the force exerted by the seat, your mass, your acceleration.

We could trace this chain all the way back to the tires on the pavement, or even further. Nowhere in this chain is a force pushing you back into the seat. That pseudo force is only apparent to you in the car. Nobody in this thread is claiming that reaction forces are not real or significant...you could accelerate fast enough to break the seat mounting bolts, for example, but not because there is a force pushing you back into the seat.

### RE: Centrifugal g force testing

Nescius - it comes down to terminology, but the inertial reaction force is the force that pushes you back in the seat. It is felt as a compressive force by the person sitting in the car, and it can be observed as a compressive strain by an external observer, in any frame of reference.

To call this real reaction force an "apparent" or "imaginary" force seems to me to be unnecessary and confusing, since there are apparent accelerations, with associated imaginary forces, which are a result of a non-inertial frame of reference, and do not cause any strain in the body experiencing these "accelerations".

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

"1. The seat exerts a force on you, you accelerate. F=ma
2. You exert a force on the seat. Exactly the same F=ma...the force exerted by the seat, your mass, your acceleration."

in 1, you get accelerated and move in inertial space
in 2, if the force is exactly the same and opposite, then you do not move.

You can't be moving and not moving in the same reference frame, so the reference frames have to be different.

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

Doug, that is not correct. You in the seat of the car are subject to a non-zero net force, and thus an acceleration, forward. That forward force is the only force. A compressive strain results simply because every "slice" of you is exerting force on the slice in front of it. In fact, the compressive strain would not be uniform. Your back slices would be under more strain than your chest slices. Imagine this and try to describe any force pushing you back into the seat...where does it act upon you...by what mechanism?

Another question to provoke thought: In the case of the twirling object at the end of a string, what is the path of the object when the string is suddenly cut?

Of course, the new path is straight and tangent to the former, circular path. The object does not fly radially outward; there is no force acting radially outward. The only force exerted on the object is directed radially inward. When this force is removed, the object immediately ceases accelerating.

### RE: Centrifugal g force testing

IRstuff...wha? Number 2 is the driver exerting an equal and opposite reaction on the seat as he or she is accelerated, NOT a pseudo force pushing on the driver.

### RE: Centrifugal g force testing

I'm going to drop out of this discussion. In my opinion the terminology is illogical and confusing, as evidenced by the OP, but people get the right answer once they have sorted out the illogicalities, so I'll leave it at that.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

If you are the piece of string used to whirl a stone around there is an inward force exerted by your outer hand on the stone, and an outward force exerted by your inner hand on the axis of rotation. They are both real forces. Note that in this frame of reference the stone is stationary, and physics is complex.

If you change your frame of reference to an inertial one then you can have a smug look on your face as the centrifugal force is no longer required to explain anything.

Cheers

Greg Locock

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### RE: Centrifugal g force testing

isn't this "simply" load and reaction ?

of course there's an outward acting force on a body in circular motion. for a rock on a string it is presented as tension in the string, for a planet it balances the inward gravity force. Where does it come from ? I'll accept accelerating frames of reference; I'm a "simple" engineer, I'm content to design to the load that I know will happen and I'm content to let philosophers and physicists argue about why the force happens.

### RE: Centrifugal g force testing

Greg, no.

1. The force your hand exerts on the stone is inward, real, and exists in the inertial reference frame. The stone accelerates.
2. The force the stone exerts on your hand is outward, real, and exists in the interial reference frame. Equal and opposite. The string is in tension.
3. There is no real force acting on the STONE in an outward direction

#### Quote (rb1957)

of course there's an outward acting force on a body in circular motion. for a rock on a string it is presented as tension in the string

Please explain how you can push with a string.

### RE: Centrifugal g force testing

"tension" is not "pushing" ... there is an outward force on the rock as evidenced by the tension in the string.

if the string is in tension, then surely the rock is pulling on the string, no?

If your inertial frame is anchored to the rock, then yes, there probably is no force, but my inertial frame is anchored to the outside world.

### RE: Centrifugal g force testing

It is a frame of reference issue, as IRStuff has correctly explained.

But nobody is being sufficiently detailed to state the frames of reference in their discussions.

In the linear accelerating car example:
Seat applies force to driver. Driver accelerates with respect to road.

The driver does not accelerate with respect to the car.
Because "inertia force" opposite and equal to the seat pushing against the drivers back.

### RE: Centrifugal g force testing

XKCD explains it all.

~ Sze Kwan (Jason) Cheah

### RE: Centrifugal g force testing

Nexius, if the frame of reference is the string then the centrifugal force is the one exerted on the axis by my inward hand. Since in this frame of reference there is no acceleration of the stone, then there must be a centrifugal force on the stone, to balance the centripetal force.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Centrifugal g force testing

Greg,

I don't know what you mean by inward, inner, or outer hand. In any case, I am aware that certain forces are apparent in a non-inertial reference frame. We've come full circle. See the 3rd post in this thread where I provide a few links to set the original poster on his/her journey.

The disagreements here, mostly, concern precision of language. There are examples of incorrect wording in this thread. That does nothing to help the original poster, where his/her issue is largely one of terminology.

### RE: Centrifugal g force testing

Shrugs. It would appear you haven't thought through the implications of a rotating frame of reference. If your FOR is the string holding the stone to the axis then there is a centrifugal force on the stone, since it is in equilibrium and it has an inward (in our terms) force acting on it.

Cheers

Greg Locock

New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?

### RE: Centrifugal g force testing

#### Quote:

Shrugs. It would appear you haven't thought through the implications of a rotating frame of reference

Is it not my post, third in this thread, that directs the OP to materials describing precisely what you claim I haven't "thought through?

#### Quote:

...an inward (in our terms) force acting on it.

Yes, the key issue! As you imply, there is no "inward" because there is no rotation...no acceleration, no net force.

My effort has been to prevent future readers of this thread from drawing a force vector on a rotating object to oppose the tension in the string. The object isn't flying outward, so people assume it's in equilibrium and assign a force the duty of opposing the string to maintain harmony. Most times people do this, they have no notion of what a reference frame is.

Have a good weekend, all.

### RE: Centrifugal g force testing

let's see if i understand ...

consider a rock on a string being spun around a center ...

surely we cannot deny the tension in the string ?

if our frame of reference is the outside world, then we see the rock rotating, accelerating (w^2/r) and the tension force that balances this inertial force.

if our frame of reference is the rock, then i guess we're doing linear, constant velocity motion (w*r, in the tangential direction). I assume ('cause I haven't thought enough about it) that the rock would also have the tension force applied to it. So to create equilibrium we "create" a special force due to the rotating frame of reference, ie everything with the rotating FOR has this force applied to it.

if our FOR was a truly (whatever that means !?) stationary body in space, then everything on the earth would be within a rotating FOR ?

### RE: Centrifugal g force testing

"if our frame of reference is the rock," then the rock is stationary, which is why there must be a force to compensate for the string pulling on it.

The Earth is rotating, orbiting, and linearly traveling toward the edge of the universe.

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

ok, i'll buy that (since the origin of the FOR is moving, the rock isn't moving within the FOR)

agreed, the earth is rotating, about itself and about the sun (and etc). But whilst our FOR is the earth then we don't have to account for these motions, yes?

### RE: Centrifugal g force testing

"But whilst our FOR is the earth then we don't have to account for these motions, yes?

And we do. Coriolis is one such example of a force that we have to account for due to using a rotating frame of reference. The Foucault pendulum https://en.wikipedia.org/wiki/Foucault_pendulum is a manifestation of that force.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: Centrifugal g force testing

Yes, I've been gently noodling away at how you would do schoolboy newtonian physics if the frame of reference is the string (or the rock). A stationary object at the axis of the FoR just sits there, no forces required. One further out needs an inward force to stay 'still', so there must be some mysterious equal and opposite outward force.

Now this is all horribly complex. But saying 'in an inertial FoR centrifugal force is fictitious' is begging the question. That is you have just defined a theoretical construct that eliminates the other construct (analagous to what happens when an irresistible force meets an unmovable object, the simplest answer is that they both can't exist in the same universe). It may be a more useful construct in day to day use for us on Earth, but it comes with some assumptions of its own. A child that grew up on a 2001 type space station would think otherwise.

Cheers

Greg Locock

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### RE: Centrifugal g force testing

"When analyzing earthbound motions, the Earth frame is not an inertial frame, but rotates about the local vertical ..."

so the statement that centrifugal force is fictitious in an inertial FOR may be true, and not inconsistent with what we experience in the real world.

### RE: Centrifugal g force testing

Sure, when an object breaks free from the centripetal force, such as when an object on a turntable overcomes the friction force, the inertial frame sees a tangential motion of the object, relative to the point at which it left the turntable. But from a vantage point at the circumference of the rotating turntable, the observer initially sees a radial object motion away from the center, since the observer is moving at the same tangential velocity. Eventually, though, the motion of the object will deviate from the perceived initial trajectory, and it will become evident that either the FoR is not inertial or there are even more oddities in the force.

A good example of this type of anomaly is the geocentric solar system that resulted in a perceived retrograde motion of outer planets like Mars and Jupiter in their celestial motions. These anomalous motions were instantly removed once the FoR shifted to being heliocentric. Clearly, in the geocentric solar system, fictitious forces were needed to explain the oscillatory retrograde motions of Mars and Jupiter. No such forces are needed in the heliocentric FoR, although it's not an inertial FoR either.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: Centrifugal g force testing

"According to Ganse, “Centripetal force and centrifugal force are really the exact same force, just in opposite directions because they're experienced from different frames of reference.” This brings us to Newton’s Third Law, which states, “For every action, there is an equal and opposite reaction.” Just as gravity causes you to exert a force on the ground, the ground appears to exert an equal and opposite force on your feet. When you are in an accelerating car, the seat exerts a forward force on you just as you appear to exert a backward force on the seat. In the case of a rotating system, the centripetal force pulls the mass inward to follow a curved path, while the mass appears to push outward due to its inertia. In each of these cases, though, there is only one real force being applied, while the other is only an apparent force."

isn't that just being, umm, "forcist" ? the two forces co-exist, they cannot exist in isolation; they are (as stated) a load/reaction pair. The string is pulling on the rock just as much as the rock is pulling on the string.

How can you call one a real force and the other a mock force ? is the ground's reaction to my weight a mock "reaction" or a real force ??

With different FOR you'll perceive the same forces, the reason why the force is there may well be different.

### RE: Centrifugal g force testing

If they're in opposite directions, they are, by definition, not the same force.

Moreover, the two forces do not exist in the same FoR. There's one in one FoR, and the other in a different FoR.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: Centrifugal g force testing

FoR is a rotating record turntable (RT), with an icy surfece on it. In RT FoR a stone sat on the ice does not move if there is an inward force on it.But if it isn't accelerating then there must be no net force on it. So there must be an equal and opposite force pulling it 'outward'. Call it centrifugal force.

But the equivalent to Newton's laws in a RT FoR are complex.

Cheers

Greg Locock

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### RE: Centrifugal g force testing

"If they're in opposite directions, they are, by definition, not the same force." ... ok, but if they are a load and reaction pair they have to co-exist. there's no chicken and no egg, ie it's not which comes first, they both happen at the same time.

in the example of someone on a round-about, both the person on the round-about and an observer outside would see the same inertial force acting away from the center of rotation. I thought forces would be independent of FOR, so long as you were careful in describing the line of action.

### RE: Centrifugal g force testing

"in the example of someone on a round-about, both the person on the round-about and an observer outside would see the same inertial force acting away from the center of rotation"

Take a similar example of astronaut in spacecraft orbiting the Earth. In the inertial frame, the only force is gravity, which is clearly inward, and there is zero additional force imposed on the astronaut. Within the spacecraft, the astronaut does not even perceive any centrifugal force, because they and spacecraft are "falling" at exactly the same velocity.

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

"In the inertial frame, the only force is gravity" ...
if the only force is gravity, then there is an inertial acceleration (1st law), no?
isn't gravity balanced by the acceleration due to the circular motion, w*r^2, the outward acting centrifugal force ?

I agree within the capsule the astronaut probably doesn't perceive the forces acting on him. He would feel weightless, but only because the two forces acting on him balance.

### RE: Centrifugal g force testing

"isn't gravity balanced by the acceleration due to the circular motion, w*r^2, the outward acting centrifugal force ?"

No, because, if gravity were balanced out, there would be zero circular motion in the inertial frame, so that is physically impossible. Gravity is what causes the constant amplitude velocity vector to change direction. If gravity were canceled out, the astronaut would go in a straight line. Circular motion must have an unbalanced inward acting force.

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

I think we're just repeating our positions. I don't understand an "unbalanced force".

If I'm on a round-about I can feel the outward force, the inertial acceleration that is reacting the inward acting force due to the circular motion. That is, the circular motion produces an inward acting acceleration which is balanced by an outward acting inertial acceleration (or force for a rock on a string). Now someone who is unaware that they are on the round-about would describe the force differently to someone who knows that they are rotating about a point. In the first case they may describe a force acting in a constant direction (x-axis, aligned radially) and motion in the y-direction; in the second case they'd describe a force towards the center and motion tangentially around the center.

If I'm observing someone on the round-about I can see how they are affected by the motion in exactly the same way.

### RE: Centrifugal g force testing

In Earth orbit, in the inertial frame, there is only a single force, gravity, which is what make the linear velocity change direction. If there were a centrifugal force, the object would not orbit, and continue on in a straight line. This is the simplest case of circular motion. In the FoR of the object, there isn't even a perceived centrifugal force, hence, it doesn't really exist

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

the capsule in orbit is not an inertial FOR. The astronaut doesn't feel gravity acting, he feels weightless; which would probably confuse him if he didn't understand the motion of his FOR. An observer outside the capsule, seeing it orbit, would be able to explain to him why he feels weightless.

### RE: Centrifugal g force testing

rb1957,

At home, a couple of years ago, I got curious about how some vibration analysis. I wanted to know how a system would vibrate under a series of impact loads. I also question that damping is some constant multiplied by velocity.

ma + Cv + kx = 0 = mx'' + Cx' + kx

Obviously, I had to solve the thing numerically. My results were weird. They drifted strangely. In free, unforced vibrations, my amplitude increased. Eventually, I realized that the ma term had to be left out of the program and/or the spreadsheet. When you draw a free-body diagram, the assumption is that the forces are balanced and the mass is not accelerating. When we do dynamic systems, we throw in the inertial force (Fi) which is imaginary.

--
JHG

### RE: Centrifugal g force testing

If you whirl a stone attached to a string, that person exerts a centripetal force via the string to keep the stone moving in a circular motion, the stone attempts to move in a straight line i.e. Tangential to the circular arc of motion and it is this tendency of the stone to move in a straight line that keeps the string taught, if you release the string then the stone travels in a straight line and so the only force acting is that due to the centripetal acceleration, once you let the string go there is no centripetal acceleration and hence no force to keep the stone following a circular path.

As Newton said a body will continue to move in a straight line unless acted upon by an external force.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Centrifugal g force testing

"the capsule in orbit is not an inertial FOR. "

I didn't say that anywhere, and therefore, I can invoke gravity in the inertial frame. The astronaut's FoR is an accelerating frame, hence no gravity perception. In either case, there are no outward forces to be found in either frame.

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

that's what I took "In Earth orbit, in the inertial frame," to mean.

how can forces appear or disappear depending on your viewing point ?

"The astronaut's FoR is an accelerating frame, hence no gravity perception." do you mean gravity can disappear ??

### RE: Centrifugal g force testing

"do you mean gravity can disappear"

I think that's where we came into this movie. In the FoR of the astronaut, if they're aligned to the gravity vector, there's a difference of 0.27 milli-g along their height. If the astronaut were completely enclosed in a light-tight box, there would be no perception of the Earth gravity, nor even the orbital motion itself, so it essentially disappeared from their FoR, as it should.

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

"there would be no perception of the Earth gravity, ... so it essentially disappeared from their FoR" ... because the whole FOR is accelerating at g ?

### RE: Centrifugal g force testing

I said I was going to drop out of this argument, but I can't help myself.

It seems to me that we should get some agreement on how this works for forces accelerating a body in a straight line before we worry about circular motion or gravity.

The Wikipedia article on reactive centrifugal force (linked in post 3 of this thread) says:

"In accordance with Newton's first law of motion, an object moves in a straight line in the absence of any external forces acting on the object. A curved path may however ensue when a physical [force] acts on it; this force is often called a centripetal force, as it is directed toward the center of curvature of the path. Then in accordance with Newton's third law of motion, there will also be an equal and opposite force exerted by the object on some other object,[1][2] such as a constraint that forces the path to be curved, and this reaction force, the subject of this article, is sometimes called a reactive centrifugal force, as it is directed in the opposite direction of the centripetal force.

Unlike the inertial force or fictitious force known as centrifugal force, which always exists in addition to the reactive force in the rotating frame of reference, the reactive force is a real Newtonian force that is observed in any reference frame. The two forces will only have the same magnitude in the special cases where circular motion arises and where the axis of rotation is the origin of the rotating frame of reference. It is the reactive force that is the subject of this article."

Addition in [] and bolding are my edits.

The same applies to acceleration in a straight line. For any accelerating body there is a real nett unbalanced external force, and an equal and opposite real internal inertial reaction force. These forces exist and are measurable from any frame of reference, because frames of reference only affect apparent velocity and acceleration, they don't affect the real forces.

For any non-inertial frame of reference there is also an imaginary external force, which is imagined in order to make the laws of motion work. For a frame of reference accelerating at the same rate as the body, the imaginary external force has the same magnitude and direction as the real inertial reaction force, but they are not the same thing. One is a real force and can be felt and measured and the other is an imaginary force that has no physical effect on anything.

In my opinion the quoted Wikipedia article states this all reasonably clearly and consistently, other than referring to the fictitious force as an "inertial" force. Unfortunately the other Wikipedia articles on the subject are inconsistent, and written as though the imaginary force and the inertial reactive force were the same thing, which they are not.

It seems to me that this inconsistency in terminology is the source of widespread confusion on this issue.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

sorry, I don't like imaginary forces. For circular motion there is a force applied to the body (tension on the string, gravity on the planet) acting towards the center that is balanced by the inertial force due to the rotational acceleration (w^2*r) that is outward. Nothing imaginary about either of these.

now for an accelerating FOR, ok there is a force acting on everything within the FOR that would be undetectable (imperceptible) within the FOR, but still a real force. As I understand it if my FOR is my car, I can detect the acceleration of the FOR as I accelerate and brake, so these are not imaginary.

### RE: Centrifugal g force testing

"For circular motion there is a force applied to the body (tension on the string, gravity on the planet) acting towards the center that is balanced by the inertial force due to the rotational acceleration (w^2*r) that is outward."

Again, there is no balanced outward force in this case. If there were, the object would move in a STRAIGHT line, because the net force would be ZERO.

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### RE: Centrifugal g force testing

next time I'm on a round about I must remember this discussion ...

### RE: Centrifugal g force testing

#### Quote (rb1957)

For circular motion there is a force applied to the body (tension on the string, gravity on the planet) acting towards the center that is balanced by the inertial force due to the rotational acceleration (w^2*r) that is outward. Nothing imaginary about either of these.

That is pretty much what I said, for every external force there is an equal and opposite internal reaction force, neither of which are imaginary. That doesn't mean there are no imaginary forces though, you can imagine whatever you want. If you want the standard equations of motion to work in any non-inertial frame of reference then you need to add an imaginary force to get the right acceleration. In the particular case where the chosen frame of reference reduces the apparent acceleration to zero the imaginary force is exactly equal to the inertial reaction force, but in any other case it is different.

#### Quote (IRstuff)

Again, there is no balanced outward force in this case. If there were, the object would move in a STRAIGHT line, because the net force would be ZERO

No, if the total external force vector is not zero, the object will accelerate. That doesn't mean there is no inertial reaction force. The inertial reaction force is a real force generated by the acceleration.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

IRstuff - I'm not sure what your point is:

#### Quote:

If sum of forces zero, then there is no acceleration

and

#### Quote:

if the total external force vector is not zero, the object will accelerate.

seem to me to say exactly the same thing, other than that I mention that it is the external forces that must be summed.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

My point was that if something is moving in a circle, it must have a net force applied. You keep stating there's an equal and opposite force; this means that the net force is zero, hence, the object cannot move in a circle.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: Centrifugal g force testing

#### Quote:

My point was that if something is moving in a circle, it must have a net force applied. You keep stating there's an equal and opposite force; this means that the net force is zero, hence, the object cannot move in a circle.

Try reading what I did say, rather than what you assume I said, to make it wrong.

A body accelerates if there is a net external force. That doesn't mean that the inertial reaction force that results from the acceleration is imaginary, because quite clearly it isn't.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

Nescious....perhaps you should check Physics 101 again.

Doug...you were right to begin.

Greg...do you think he got that?

### RE: Centrifugal g force testing

Let's look at horrible record turntable (kids ask your parents) world. Let's make it easy and assume that the turntable is covered in frictionless material. In RT FoR for a mass to remain stationary a force inwards is required, proportional to m*r. If we assume that Newton 1 still holds true then there must be an outwards force m*r on the mass. We can't tell where that comes from in RT FoR but either it exists or Newton 1 is broken. So, yet again, we can make assumptions that exclude certain laws, or we can have the laws and have to invent some other laws. In many cases it is easiest to assume that you have an inertial FoR, but that is an abstraction in itself. It leads to simple laws, which is nice, and may even be true, but somewhere in the universe somebody is busy deriving the laws of dynamics in a rotating FoR and for them centrifugal force is as real as mass. It is not always easiest to work in an inertial FoR, for example battleship gunnery tables didn't, as in the heat of battle working out the absolute 3D vectors was probably a bit beyond the average gunnery officer, who could more easily account for it by using Coriolis and the known position of his ship and the target.

Cheers

Greg Locock

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### RE: Centrifugal g force testing

"You'll need to show what the free body diagram looks like. If sum of forces zero, then there is no acceleration."

not if one of the forces is a body force (like ma); a FBD doesn't have to be static.

and the acceleration we're talking about is due to the motion (w^2*r), much like if the body was accelerating in a linear direction.

### RE: Centrifugal g force testing

The external force provides the acceleration, which causes the deviation from a straight line. The motion is due to the acceleration, not the other way around.

TTFN (ta ta for now)
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### RE: Centrifugal g force testing

"any" reference frame would include rotating ones, no?
perhaps inertia FoR and non-inertia (ie accelerating) FoR.

If I'm sitting on a round about (which sounds like a rotating FoR), I know the inertial force acting on me (ie it's not fictitious). If I stop reacting this inertial force, I know what'll happen ...

### RE: Centrifugal g force testing

Yep. "Any" means any frame of reference you can think of.

I'll hold on commenting further, for now.

### RE: Centrifugal g force testing

#### Quote:

Open trap question for anybody...do you agree or disagree with the attached table?

I have the following disagreements:

- The right hand column should be headed Imaginary "Inertial" Centrifugal Force, since it is nothing to do with inertia, but people sometimes use that term.
- Exerted upon should say "nothing" in the right hand column, since imaginary forces are not "exerted".
- Direction should say "opposite the imaginary centripetal force" on the right, or "away from the imaginary axis of rotation". That is the definition of "centrifugal"

Other than that, I agree. The last row is really all that needs to be said.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

"If I'm sitting on a round about (which sounds like a rotating FoR), I know the inertial force acting on me (ie it's not fictitious). If I stop reacting this inertial force, I know what'll happen "

For the case of you on the string, you are NOT acted upon by an inertial force, you are acted upon by the centripetal force manifested by the tension in the string. Only the string is acted upon by the inertial force that you exert on it as a reaction to the centripetal force, which is why there is tension in the string.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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### RE: Centrifugal g force testing

#### Quote (IDS)

- The right hand column should be headed Imaginary "Inertial" Centrifugal Force, since it is nothing to do with inertia, but people sometimes use that term.
- Exerted upon should say "nothing" in the right hand column, since imaginary forces are not "exerted".
- Direction should say "opposite the imaginary centripetal force" on the right, or "away from the imaginary axis of rotation". That is the definition of "centrifugal"

1. I don't have a dog in that particular nomenclature fight.
2. The imaginary force certainly apparently is "exerted" on something. If you're going to use the imaginary force in a calculation, you must know what it is exerted on.
3. The centripetal force is never imaginary. It exists in all reference frames, equal and opposite to the reactive centrifugal force which, as the table states, exists in all reference frames.

### RE: Centrifugal g force testing

When a body travels in a straight line without acceleration it's considered to be in equilibruim and the sum of all the forces are zero, so a car travelling in a straight line has zero net force acting on it but the force to overcome air resistance, friction at the wheels resisting motion still have to be present albeit they sum to zero.

So in the case of constant velocity in a circle of a body, it generates a centripetal acceleration due to a change in direction and not due to a velocity, this acceleration generates a radial inward force centripetal force. Now in order that the body continues to rotate in a circle then the centripetal force needs to be opposed, which it is by the centrifugal force, otherwise the body will not continue to rotate in a circular motion.

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

### RE: Centrifugal g force testing

#### Quote:

1. I don't have a dog in that particular nomenclature fight.
2. The imaginary force certainly apparently is "exerted" on something. If you're going to use the imaginary force in a calculation, you must know what it is exerted on.
3. The centripetal force is never imaginary. It exists in all reference frames, equal and opposite to the reactive centrifugal force which, as the table states, exists in all reference frames.

1. OK, so let's call it the imaginary centrifugal force, rather than inertial.

2. I don't like the word "exerted" in relation to an imaginary force, but I agree it must have a point of application. I'd say the point of application was the end of the string, which is also the point of application of the reactive centrifugal force. I don't understand the "moving or not" bit. I thought the table was specifically looking at a body following a circular path, with the right hand column viewed from a rotating FoR at the centre of rotation.

3. OK, I'd agree that the imaginary centrifugal force, being imaginary, does not have an imaginary centripetal force. I should have said that the direction is opposite the real centripetal force in both cases. For the imaginary case we only introduce the imaginary force because without it the real centripetal force would appear to be an unbalanced force not associated with an acceleration, when viewed from the rotating FoR.

Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/

### RE: Centrifugal g force testing

"For the case of you on the string, you are NOT acted upon by an inertial force, you are acted upon by the centripetal force manifested by the tension in the string. Only the string is acted upon by the inertial force that you exert on it as a reaction to the centripetal force, which is why there is tension in the string."

I talk about sitting on a round about, and you talk about the rock on a string; ok.

yes, the rock is in force balance in the radial direction, there are two very real forces acting on the rock. One inward due to the circular motion (w^2*r), and it's outward inertial reaction/companion. And yes, the string only feels the tension due to the outward inertial force. If the string breaks (or if I let go of the round about), so that the loadpath for resisting the outward force disappears, I know what'll happen next ...

"When a body travels in a straight line without acceleration it's considered to be in equilibruim and the sum of all the forces are zero, so a car travelling in a straight line has zero net force acting on it but the force to overcome air resistance, friction at the wheels resisting motion still have to be present albeit they sum to zero." True enough, you can also draw a FBD for a body with linear acceleration (ie add the inertial force, m*a).

### RE: Centrifugal g force testing

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