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]

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.



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

 

[Nescius]

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

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.

 

[rb1957]

"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.

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

 

[MintJulep]

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:
Tires apply force to road. Car accelerates with respect to road.
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.

 

[scheah]

XKCD explains it all.

~ Sze Kwan (Jason) Cheah

 

[IRstuff]

LOL ;-)

TTFN (ta ta for now)
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[MacGyverS2000]

LPS for sk

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[GregLocock]

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


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[Nescius]

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.

 

[Compositepro]

https://en.wikipedia.org/wiki/Equivalence_principle

 

[GregLocock]

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


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[Nescius]

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?

...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.

 

[rb1957]

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 ?

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

 

[IRstuff]

"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.

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[rb1957]

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?

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

 

[IRstuff]

"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.

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[GregLocock]

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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[rb1957]

"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.

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

 

[IRstuff]

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.

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[desertfox]

http://www.livescience.com/52488-centrifugal-centripetal-forces.html

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