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

 

[dhengr]

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.

 

[Nescius]

Forget terminology, temporarily, and think in terms of a free body diagram. Check some of these out:

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

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

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

 

[IDS]

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/

 

[Nescius]

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.

 

[IDS]

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/

 

[Nescius]

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

 

[Strong]

"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

 

[IRstuff]

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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[handleman]

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)

 

[Nescius]

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.

 

[IDS]

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/

 

[IDS]

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/

 

[Nescius]

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.

 

[IDS]

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/

 

[IRstuff]

"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.

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[Nescius]

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.

 

[Nescius]

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.

 

[IDS]

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/

 

[GregLocock]

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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