Centrifugal g force testing 2

[FeldmanWill]

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

 

[IDS]

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.

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/

 

[IRstuff]

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

TTFN (ta ta for now)
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[IDS]

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

If sum of forces zero, then there is no acceleration

and

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
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[IRstuff]

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.

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[IDS]

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

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[Ron]

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

Doug...you were right to begin.

Greg...do you think he got that?

 

[GregLocock]

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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[desertfox]

http://www.diffen.com/difference/Centrifugal_Force_vs_Centripetal_Force

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

 

[rb1957]

"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.

another day in paradise, or is paradise one day closer ?

 

[IRstuff]

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.

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[Nescius]

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

 

[rb1957]

"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 ...

another day in paradise, or is paradise one day closer ?

 

[Nescius]

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

I'll hold on commenting further, for now.

 

[IDS]

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
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[IRstuff]

"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.

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[Nescius]

- 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.

 

[desertfox]

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

 

[IDS]

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/

 

[rb1957]

"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).

another day in paradise, or is paradise one day closer ?

 

[cowski]

I had the weirdest dream the other night. I found myself on a flat, smooth surface with a kind of checker board pattern on it. The segments were not quite square, they were more like segments of a ring. It reminded me of an oversize dart board. There was a tall pole located some distance away at the "bulls-eye" and I was tethered to it. My stomach rumbled; I was a bit hungry, but my curiosity was stronger... for now, at least. I was expecting a flag to be flying from the pole, but there wasn't one; nor could I make out any type of statue on top. If there was a plaque commemorating the large pole, I was too far away to see it. Looking off to my right revealed... not much. More checker board floor pattern and beyond that - just blackness. Not a McDonald's or Starbucks in sight - what an odd place this is. I swiveled to look behind me and promptly fell down when my feet unexpectedly slipped out from under me. Fortunately, I had a tight grip on my clipboard and my pencils were firmly clipped to my pocket protector. My knee was bruised and it took a few moments to recover my footing; the floor was slicker than snot on a doorknob. The struggle wasn't really worth it; the view behind me was much the same: checkerboard pattern floor as far as I could see and beyond - only blackness. I was wondering just how fast Jimmy John's could deliver me a sandwich when some noise caught my attention. I looked back to my left; there appeared to be three people waving their arms and motioning to me. Happy to see other people in this otherwise barren place I took a step toward them and once again found myself sprawled out on the floor. Cursing softly to myself and rubbing my sore knee, I used what traction I could find to make it over to them. After introductions and some small talk, I learned that Zack, Al, and Steve had all been here longer than I had and they had started theorizing about this "universe" that we found ourselves in. They were friendly and intelligent; pooling our resources, we found that Zack had a force gauge, Steve had a scientific calculator, and Al had some extra rope and an empty chinese takeout box. My stomach rumbled again. We were all tethered to the pole with roughly the same length of rope (give or take a few feet) Zack explained how they had been using the force gauge to measure the tension in each of the ropes; he had found that the tension was proportional to the mass of each person. Al mumbled something about an "anti-pole" force that was pushing us away from the pole. This seemed to agitage Steve and he launched a monologue about the "void", and something about dark forces and singularities. Al countered with his own argument - something about fields and tensors. I was trying to make sense of it when Zack pulled me aside. We slid some distance away (without falling this time) to leave the other two to their debate. Zack explained that he wanted to use the extra rope and the force gauge to take more measurements. One of them postulated that the force is proportional not only to the person's mass, but also to the length of their rope. This sounded crazy to me - a force proportional to the length of a rope? - but there wasn't much else to do and perhaps by taking more measurements we could bring some clarity to the currently competing theories. I agreed to be the test subject; we rigged up the test gauge and the rope. Zack slowly let out more line and I noted the force readouts. I was nearly at the end of my rope when a frayed portion snapped. Surprise gave way to pain as I fell yet again (why did it have to be on the same knee?); pain gave way to bewilderment as I realized that the checkerboard pattern was passing under me faster and faster. The others were yelling and frantically waving their arms as I slid further away from them; they also seemed to be moving to my right? Bewilderment gave way to sheer terror as the blackness of the void approached; I could see the edge, my end was near. My heart was racing as I fell over the edge and landed with a thud on a grainy, slightly squishy surface. Dirt? the void isn't a void, but is made up of dirt? I got up and brushed myself off; my clipboard was nowhere to be seen, at least most of my pencils were still accounted for - that was some small comfort. I stood for a while trying to make sense of my new situation when I saw my friends come into view somewhere off to the left - wait... weren't they moving to the right after my rope broke? They still appear to be moving to the right. Have I been launched into orbit? It was about this time that I had an epiphany... I could walk around without being tethered and without sliding like Bambi on ice! Thank God for friction! No, hold on, that was nice, but it wasn't my epiphany. Ok, here goes: what if I'm not in orbit, but rather the "dartboard" is rotating and I'm standing still? While standing on the dartboard we were trying to solve a statics problem (the sum of the forces equals zero); we were not moving in our frame of reference, so the rope had to be balancing some other unknown force. However, viewed from my new vantage point, it is a dynamics problem! (the sum of the forces equals mass times acceleration). The rope was exerting a force on us, causing us to accelerate around the pole. Then I woke up; my knee hurt from being in the same position too long and my stomach rumbled. I made my way to the fridge and assembled possibly the best ham sandwich ever.