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

 

[rb1957]

from the above link ...
"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.

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

 

[IRstuff]

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

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[GregLocock]

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


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[rb1957]

"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.

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

 

[IRstuff]

"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)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

"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.

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

 

[IRstuff]

"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)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

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.

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

 

[IRstuff]

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)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

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.

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

 

[drawoh]

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 (F[sub]i[/sub]) which is imaginary.

--
JHG

 

[desertfox]

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

 

[IRstuff]

"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)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

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 ??

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

 

[IRstuff]

"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)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

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

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

 

[IDS]

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/

 

[rb1957]

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.

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

 

[IRstuff]

"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.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

https://www.youtube.com/watch?v=BKorP55Aqvg

faq731-376 forum1529 Entire Forum list

http://www.eng-tips.com/forumlist.cfm

 

[rb1957]

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

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