# Centrifugal forces don't exist in reality?

• Aeronautic Freek
In summary: Centripetal force... Causes compression in the blade. What centrifugal force does is it causes the blade to rotate in a circle. As the blade rotates, the centrifugal force straightens out the blade, and because the tension is in the blade, the centrifugal force also causes the blade to stay in that position.

#### Aeronautic Freek

We can often hear that centrifugal force don't exist in reality...Helicopter mi-26 can lift 56tons ,it has 8 blades,so each blade hold 7 tons of force!
Do you know if you put 7tons at blade when blade is not rotating(static) ,bending moment will be way too much and blade will broke at root.

But when blade is rotating centrifugal force "straighten" blade so centifugal force basicaly "reducing" bending moment and this is reason why so tiny blades can hold so much load on it..
So how you can say,that centrifugal force don't exist?

centifugal force has outward direction so it cause tension in blade..

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Aeronautic Freek said:
So how you can say,that centrifugal force don't exist?
Because it doesn't. The force on the blades is centripetal force, an inwards force applied by the fastenings at the hub. This force is pulling the blades into a circular path; withou that force they'd go flying off in a straight line at a constant speed just like Newton's first law says they should.

But you are right that the tension in the blades is what straightens them out - it's just that the tension comes from the inwards-directed centripetal force and not the fictitious centripetal force.

Klystron, dextercioby, Delta2 and 2 others
This has come up before. There's a seventeen page argument about it here if you want to read. Its link is broken, but appears to reference this Wikipedia page.

I think the OP is referring to the "reactive centrifugal force". In the case of a ball on the end of a string being swung around, the string exerts a centripetal force on the ball and the ball exerts a "reactive centrifugal force" on the string. That's a real force (if you accept the analysis - I haven't finished reading the links myself).

The centrifugal force that emerges from transforming to a rotating frame is an inertial (also called fictitious) force and is sometimes said not to exist (although I don't think that's helpful). Either way, it's a different thing.

vanhees71
Nugatory said:
Because it doesn't. The force on the blades is centripetal force, an inwards force applied by the fastenings at the hub. This force is pulling the blades into a circular path; withou that force they'd go flying off in a straight line at a constant speed just like Newton's first law says they should.

But you are right that the tension in the blades is what straightens them out - it's just that the tension comes from the inwards-directed centripetal force and not the fictitious centripetal force.
force with inward direction (so called centripetal)will cause compression of blade,in reality blade is in tension so force has outward direction..
if you put load-cell between hub and blade it will show tension not compression..

Aeronautic Freek said:
force with inward direction (so called centripetal)will cause compression of blade
This is only true if the blade is not accelerating. A rotating blade is accelerating

Aeronautic Freek said:
force with inward direction (so called centripetal)will cause compression of blade,in reality blade is in tension so force has outward direction..
if you put load-cell between hub and blade it will show tension not compression..
That's not measuring the centrifugal force, at least not in the sense of the words "centrifugal force" people mean when they say it doesn't exist. That would be measuring either the centripetal force or the reactive centrifugal force, depending on how you wish to interpret it.

Dale
Dale said:
This is only true if the blade is not accelerating. A rotating blade is accelerating
when blade is rotating it is little bit longer than when is not rotating,because centrifugal force cause tension in blade structure..

Aeronautic Freek said:
when blade is rotating it is little bit longer than when is not rotating,because centrifugal force cause tension in blade structure..
Nobody is disagreeing with this, except for your assertion that this is due to the centrifugal force. There are two things you could mean by that. One meaning is the "reactive centrifugal force", and you are correct that this is a real force. This is not the force that people say doesn't exist. The other meaning of centrifugal force is a coordinate dependent effect that arguably "isn't real", and certainly doesn't appear in the inertial frame you appear to be using. You would be wrong to attribute anything to this.

You seem to be arguing against a straw man. Your example of "centrifugal force" is not an example of the "centrifugal force" that is said to be not real.

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Dale
Aeronautic Freek said:
when blade is rotating it is little bit longer than when is not rotating,because centrifugal force cause tension in blade structure..
Agreed, but that fact is not incompatible with what @Nugatory and @Ibix said. We all agree that the blade is in tension.

What you said that is incorrect is that an inward force implies compression. That is not correct.

If you imagine cutting the blade at any point along its length you wind up with two surfaces. One surface is the outward facing surface of the part of the blade attached to the center, and the other surface is the inward facing surface of the part of the blade that is “free”. In tension the second surface has a force pointing inward, this is the real centripetal force and it is an inward force that is present in tension.

If you had an inward pointing force on the first surface then that would be compression.

nasu, etotheipi and Ibix
Aeronautic Freek said:
force with inward direction (so called centripetal)will cause compression of blade,in reality blade is in tension so force has outward direction.
Every part of the blade (except the last bit) has a inwards force (from the next inner part) and a smaller outwards force (from the next outer part), The net force from these two real forces is inwards, to allow circular motion in the inertial frame.

The outwards force, that people say doesn't exist, comes into play in the co-rotating frame where the blade is at rest. It cancels the net real force inwards, to make Newton's 2nd Law work in that frame. It's only used to explain the acceleration (or the lack of it) in the non-inertial frame. It has nothing to do with the measurable tensions, which are frame invariant.

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etotheipi, Delta2, Ibix and 1 other person
Aeronautic Freek said:
centifugal force has outward direction so it cause tension in blade..
View attachment 264167
How and why are you distinguishing between centrifugal force and linear inertia? Why aren't you just calling this linear inertia? The blade mass wants to go straight due to linear inertia, but the connection to the rotor forces it to curve. As @Nugatory said, the only true force is the centripital force.

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Centrifugal force is real in a rotating reference frame, causing stationary objects to accelerate without external force being applied, or to have weight where they are prevented from accelerating. Coriolis force is a similar effect where angular acceleration can be achieved without application of torque. The discussion above is all using inertial reference frame in which yes, centrifugal force is a psuedo-force.

weirdoguy
Halc said:
Centrifugal force is real in a rotating reference frame...
It's not exactly wrong to say this (that depends on what you mean by "real") but if you're going to take this position you are also accepting that fictitious forces are exactly those that are "real" in a non-inertal frame... so a force can be fictitious and real at the same time and we're left wondering what the value of the word "real" is.

It is much more helpful and consistent with mainstream practice to consider "real" things to be those that cannot be made to go away by the mathematical trick of a coordinate transform.

weirdoguy
Or just avoid the word “real” altogether.

Dale said:
Or just avoid the word “real” altogether.
Which is why we have the word "invariant", of course.

Dale
Well what adjective do we use to label a non-pseudo force?
Centrifugal is only meaningful in a rotating frame? I'm not trying to make any ontological claim here.

Nugatory said:
so a force can be fictitious and real at the same time
But not in the same way, so it isn't a contradiction.

Aeronautic Freek said:
if you put load-cell between hub and blade it will show tension not compression..
Of course it will. That's why the blade is fastened to the hub.

But that is not relevant. The hub exerts a centripetal force on the blade and the blade exerts a centrifugal force on the hub. Newton's Third Law!

Note, though, that the net force on the hub is centripetal. It's made of a material strong enough to withstand the centrifugal exerted on the hub by the blade yet still remain intact.

Halc said:
The discussion above is all using inertial reference frame in which yes, centrifugal force is a psuedo-force.

In inertial frame centrifugal force is non-existent, so I don't see any point in calling it a "pseudo-force in inertial frame".

Delta2, Dale and A.T.
Halc said:
Well what adjective do we use to label a non-pseudo force?
Using less overloaded terms helps to avoid that whole philosophical mess about what is real/fictitious/pseudo:

Interaction force : frame invariant, obeys Newtons 3rd Law
Inertial force : exist only in non-inertial frames, frame dependent, doesn't obey Newtons 3rd Law

etotheipi, Dale and jbriggs444
A.T. said:
Every part of the blade (except the last bit) has a inwards force (from the next inner part) and a smaller outwards force (from the next outer part), The net force from these two real forces is inwards, to allow circular motion in the inertial frame.

The outwards force, that people say doesn't exist, comes into play in the co-rotating frame where the blade is at rest. It cancels the net real force inwards, to make Newton's 2nd Law work in that frame. It's only used to explain the acceleration (or the lack of it) in the non-inertial frame. It has nothing to do with the measurable tensions, which are frame invariant.

But if I called centrifugal force "reactive centrifugal force" than everything is OK?

Imagine ball connect to the blade in the way that can slide in and out .When blade start rotate ball will moves outward.
What is force which push ball outward in inertial frame of reference?
Or how centrifugal clutch works if centrifugal force "dont exist"(whatever that mean)?

Aeronautic Freek said:
But if I called centrifugal force "reactive centrifugal force" than everything is OK?
There are two different outward forces mentioned in my post. They are not the same thing and must not be conflated, so you need two different names for them. I don't care which you use, as long you have two different ones.

Aeronautic Freek said:
Imagine ball connect to the blade in the way that can slide in and out .When blade start rotate ball will moves outward.
What is force which push ball outward in inertial frame of reference?
Moving in a straight line also brings you away from the center (outward), but you don't need a force to move in a straight line in an inertial frame.

Aeronautic Freek said:
But if I called centrufigal force "reactive centrifugal force" than everything is OK?
The inertial centrifugal force is a fictitious force. The reactive centrifugal force is a real force. They are different things, but if you use each correctly then both are “OK”

Aeronautic Freek said:
Imagine ball connect to the blade in the way that can slide in and out .When blade start rotate ball will moves outward.
What is force which push ball outward in inertial frame of reference?
In an inertial frame the ball never accelerates outward. Don’t confuse “move” with “accelerate”. Forces are required to accelerate, not to move. There is nothing inconsistent with Newton’s laws in moving outward without an outward force as long as there is never any outward acceleration.

Dale said:
In an inertial frame the ball never accelerates outward. Don’t confuse “move” with “accelerate”. Forces are required to accelerate, not to move. There is nothing inconsistent with Newton’s laws in moving outward without an outward force as long as there is never any outward acceleration.

Do you know how centrifugal clutch works?

if i put ball close to blade root when blade is not rotating,than start rotate blade,ball will accelerate outward,so which force push ball outward?

Aeronautic Freek said:
...wich force push ball outward?
In which frame? What is the acceleration of the ball in that frame that needs to be explained by a force?

A.T. said:
In which frame? What is the acceleration of the ball in that frame that needs to be explained by a force?
i know that in non-inertial frame is centrifugal force..
so i ask for inertial frame..

Aeronautic Freek said:
so i ask for inertial frame..
What is the acceleration of the ball in that frame that needs to be explained by a force?

Aeronautic Freek said:
Do you know how centrifugal clutch works?

if i put ball close to blade root when blade is not rotating,than start rotate blade,ball will accelerate outward,so which force push ball outward?
The contact force with the blade pushes the ball outwards. If you put a ball at rest on a frictionless turntable, then the ball stays at rest (in an inertial frame) when the turntable rotates. If there is friction, the friction force pushes the ball as the turntable rotates.

Aeronautic Freek said:
if i put ball close to blade root when blade is not rotating,than start rotate blade,ball will accelerate outward,so which force push ball outward?
The ball never accelerates outward. It only accelerates tangentially.

And no, I don’t know how a centrifugal clutch works. The fact that it has the word “centrifugal” in the name doesn’t change anything I said above.

Dale said:
The ball never accelerates outward. It only accelerates tangentially.

... the ball is constrained to move in the radial direction (relative to the rotating blade). To remain at rest (relative to the blade) would require a centripetal acceleration. If there is insufficient friction for this, then the ball has no option other to accelerate outwards relative to the blade.

The real force is a tangential force in the rotating frame, but this tangential direction is constantly changing in an inertial frame, which results in constrained radially outward motion.

Delta2
Aeronautic Freek said:
Do you know how centrifugal clutch works?
I'm not @Dale whom you are asking, but I can say with confidence that the the answer to that question will be "Yes".
if i put ball close to blade root when blade is not rotating,than start rotate blade,ball will accelerate outward,so which force push ball outward?
If the ball is unconstrained (no friction, not in a trough that forces it to move along the length of the blade, no mechancal connection to the blade, ...) it won't accelerate outwards. It will stay put (Newton's first law) while the blade moves out from under it, and then it will fall straight to the ground.

In any realistic situation, there will be some frictional force on the ball as it wants to stay put while the blade moves under it (this is how we can use flatbed trucks to move things around). This force will have a radial outwards component and that's what pushes the ball outwards.

Dale said:
And no, I don’t know how a centrifugal clutch works.
A centrifugal clutch is found for instance on go-karts or small motorcycles. You have a rotating assembly that is attached to the motor, often directly to the motor shaft. It spins with the shaft. This assembly contains a pair of weights which are spring-loaded so that, at rest, both are together hugging the motor shaft.

Around this assembly is a steel drum which is attached to the drive mechanism. Often a chain ring is bolted on and a drive chain runs from here to the rear wheel.

When the motor is idling, the rotation rate of the clutch assembly is insufficient to drive the weights apart. The force of the spring is sufficient to keep them together. When the motor is revved up, the rotation rate increases, the weights move apart and contact the drum. The drum spins and the kart or bike takes off.

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

It is a convenient design since it engages and disengages automatically with no risk of stalling the engine.

As has been noted, when viewed from the inertial frame the weights move in a near-circular path. There is some degree of outward spiral as the clutch engages, but the acceleration of both weights is always centripetal.

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Dale and PeroK
jbriggs444 said:
A centrifugal clutch is found for instance on go-karts or small motorcycles. You have a rotating assembly that is attached to the motor, often directly to the motor shaft. It spins with the shaft. This assembly contains a pair of weights which are spring-loaded so that, at rest, both are together hugging the motor shaft.

Around this assembly is a steel drum which is attached to the drive mechanism. Often a chain ring is bolted on and a drive chain runs from here to the rear wheel.

When the motor is idling, the rotation rate of the clutch assembly is insufficient to drive the weights apart. The force of the spring is sufficient to keep them together. When the motor is revved up, the rotation rate increases, the weights move apart and contact the drum. The drum spins and the kart or bike takes off.

View attachment 264278
https://en.wikipedia.org/wiki/Centrifugal_clutch

It is a convenient design since it engages and disengages automatically with no risk of stalling the engine.

As has been noted, when viewed from the inertial frame the weights move in a near-circular path. There is some degree of outward spiral as the clutch engages, but the acceleration of both weights is always centripetal.
and how you will explain someone what push weights outward every time when engine increase RPM?

weirdoguy and PeroK
Aeronautic Freek said:
and how you will explain someone what push weights outward every time when engine increase RPM?
In an inertial frame the weights are never pushed outwards. Their acceleration is inwards at all times.

Dale said:
In an inertial frame the weights are never pushed outwards. Their acceleration is inwards at all times.
How do you mean accelerate inwards?if weights accelarte inward,like you said than spring will compressed and clucth will never transmit power from engine to wheel?

every time when you increase enigne RPM,weights moves outward,stretch spring and transmit power from engine to wheel
i am talking about real life situation what happened with weights in clucth..