Centripetal vs Centrifugal Force

Back when I was a kid I was taught about centrifugal force. I could swing a bucket of water up over my head and back down in a circle fast enough, and the water would stay in the bucket. I was told that centrifugal force held the water in the bucket.

Later, I think it was in college, I found out that explanation was completely wrong. There was no force acting on the water to push it away from me to keep it in the bucket. The bucket itself was pushing on the water, in the direction of my arm (towards the center of the circle), resulting in the water moving in a circle. Whoever told me this said it was called centripetal force and they explained there's no real force as centrifugal force. Centripetal force always acts towards the center of the circle an object is moving in.

Several times in my adult life I've gotten into conversations with people who refuse to accept this. They insist that centrifugal force is real and they defend it with some really silly reasoning. Some have suggested that it's a real force when you look at the forces in a moving frame of reference, but that only makes it seem like there's a force that isn't there. It's an apparent force used to make the math easier to deal with in that reference frame. Others just seem to *know* it's a force because they've believed it all their lives and refuse to consider otherwise.

This morning I had a debate online with someone and their best argument was an xkcd cartoon about the two forces.

Has anyone else had this conversation and attempted to change someone's mind about it?

Centripetal or centrifugal force is the force required to keep a body moving in a circle. The body has a constant acceleration magnitude pointed to the center of rotation. F=ma applies so the force must exist otherwise the body would move in a straight line.

Equal and opposite reactions of the same force. Tension in a line on the tetherball acts inwards (on the ball) and outwards (on your arm)

As long as you account for both in your design, I really don't care how long you want to argue about what to call it.

But since we're on the topic, I'll share for those that haven't seen

jderimig said:

Centripetal or centrifugal force is the force required...

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Not either, just Centripetal.

Centrifugal force doesn't exist.

Nytrunner said:

Equal and opposite reactions of the same force.

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Not true.

Nytrunner said:

Tension in a line on the tetherball acts inwards (on the ball) and outwards (on your arm)

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The tension between the two is a real force. The force pulling on the ball is indeed centripetal force. The force pulling on the pole, the opposite direction down the line, is inertia. That's not what centrifugal force is normally used to describe. There's nothing pushing on the ball away from the pole.

Love this thread! Got me thinking.
Gotta go with Mushtang on this one. Good explanation here:
https://www.diffen.com/difference/Centrifugal_Force_vs_Centripetal_Force
What people call centrifugal force is the inertia of motion.

Ah, but there is a reaction! Both objects, tether/ring and ball/Bond experience stress which must be quantified and accounted for!

Oh it is absolutely an Inertial Force, ie a Fictitious Force! But if you have to do rotational statics or dynamics, its handy too!

You can say "it's not real" and be pedantically correct, as long as you don't mind engineers and other folks writing it down and using the term casually if they so desire
But my lunch break is over now, so you'll have to argue with someone else for a bit

So I guess they should rename centrifuges to.....centripetes?

Convincing someone that there isn’t centrifugal force is about as tough as convincing a flat earther they’re wrong. I just pick my battles - some fights aren’t worth having.

GrouchoDuke said:

Convincing someone that there isn’t centrifugal force is about as tough as convincing a flat earther they’re wrong. I just pick my battles - some fights aren’t worth having.

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I wonder how many people that post on this forum regularly believes in a Flat Earth? Or even a Young Earth? Probably more than will admit it here, haha.

Mushtang said:

I wonder how many people that post on this forum regularly believes in a Flat Earth? Or even a Young Earth? Probably more than will admit it here, haha.

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Not sure. I just try not to waste time on it...I’d much rather wonder about how consciousness works or how to best design my next rocket.

I was taught that centrifugal force is the combination of centripetal force and inertia, but that there is no actual force called centrifugal force.

All the explanation you need.

Mushtang said:

Not either, just Centripetal.

Centrifugal force doesn't exist.

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Centrifugal is the "apparent" force keeping the water in the pail. The pail applies centripetal force to keep the water going in a circle. Its all very simple if you know basic newtonian physics.

When you are spinning the bucket in a vertical loop on the rope, and if you cut the rope at the instant when the bucket is at the top of the loop, which direction does it go? Not straight up, since there is not a force, or acceleration, acting in the straight up vertical direction at that point based on the rotational motion after the rope is cut. The rope can only support a tension force, you can’t put a compression load on it.

The bucket and water travel horizontally initially tangent to the circle and then starts falling due to the acceleration due to gravity.


Now, if you put a hole in the bottom of the bucket, how fast can you spin it so that it does not leak when at the top?

GlenP said:

When you are spinning the bucket in a vertical loop on the rope, and if you cut the rope at the instant when the bucket is at the top of the loop, which direction does it go? Not straight up, ....

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If at the top of the vertical loop you eliminate (somehow, thought experiment here) the bottom of the pail, which way does the water go?

If it's any help, here's another perspective. You can compare:

- the water to a baseball and
- the bucket to a catcher's mitt

It's the same situation except the water in a bucket can push against the bucket for a minute, while a catch lasts only a fraction of a second.

Would you say a baseball applies a force to a catcher's mitt? I would, and this can allow for useful calculations, which is the whole purpose behind the original concept of "force". If you stick with classical, Newtonian, engineering mechanics, then no, there's nothing wrong with defining a "centrifugal force" for water, which is opposed by the centripetal force from your arm and bucket.

If however, you open up modern physics (post-1900 theory), with its relativity and quantum physics, then "force" takes on another meaning: there are 4 forces in nature and Newton isn't relevant anymore. But this is a whole other realm and no water bucket really has to go there.

Mushtang said:

Has anyone else had this conversation and attempted to change someone's mind about it?

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Yes. In fact, I do it for money.

A student complained to the department head after I tried -- and failed -- to convince her that she was not being pushed back into her seat by the "g forces" in an accelerating automobile. Or, rather, when I failed to convince her that she was going to keep making the same sign error in her analyses unless she picked one frame of reference and stuck with it all the way through a solution.

I think she dropped the class the day after I worked through the man-walking-on-a-barge problem on the board.

The best conversations I've ever had about this were probably in a Science of Science Fiction class, where I presented the "artificial gravity" scene from Armageddon as a lab excercise. I'd have to dig out my notes, but I think we worked out that an effective 1g at the core of the station meant that Ben Affleck's character would have faced a centripetal acceleration of ~8g by the time he climbed "down" to the fuel pod (with a significant gradient in that acceleration from his head to his feet).

Centrifugal pumps?

You can really go in circles on this topic. I got the same thing in college physics and I understand why physicists prefer centripetal over centrifugal. If you are in the rotating reference frame with no windows to the outside, you see a fictious force, which most people call centrifugal force. Arguably, you are in a non-inertial reference frame, but you don't know that.

Funkworks said:

If it's any help, here's another perspective. You can compare:

- the water to a baseball and
- the bucket to a catcher's mitt

It's the same situation except the water in a bucket can push against the bucket for a minute, while a catch lasts only a fraction of a second.

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No, it's not the same situation at all.

Funkworks said:

Would you say a baseball applies a force to a catcher's mitt? I would, and this can allow for useful calculations, which is the whole purpose behind the original concept of "force".

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The pitcher has applied a force to the baseball, and the moving baseball is then stopped by a non-moving glove. Nothing is adding a force to the water to push it towards the bucket as it spins around. The bucket is the only thing pushing against the water as the bucket moves in a circle.

Funkworks said:

If you stick with classical, Newtonian, engineering mechanics, then no, there's nothing wrong with defining a "centrifugal force" for water, which is opposed by the centripetal force from your arm and bucket.

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Yes, there is something wrong with that. There's no force pushing the water into the bucket. None. So there's nothing to apply the name "centrifugal" to. The only force on the water from anywhere is from the bucket, pushing the water in the direction of the center of the circle.

I wrote a letter to the editor of Air and Space magazine referring to an article that mentioned centrifugal forces acting on a pilot and suggesting that an aerospace publication written for the general public should not perpetuate the myth of centrifugal force. He didn’t reply. [emoji851]

jderimig said:

Centrifugal is the "apparent" force keeping the water in the pail. The pail applies centripetal force to keep the water going in a circle. Its all very simple if you know basic newtonian physics.

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What he said ^ .

Centrifugal force isn't real, one isn't prime, and other arguments entirely irrelevant to the average person.

Einstein say gravitation forces are not real either. It just an apparent force due to space-time distortion by mass, like the water in the pail.

jderimig said:

If at the top of the vertical loop you eliminate (somehow, thought experiment here) the bottom of the pail, which way does the water go?

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ask Goliath when David throws a rock and lets go of the bottom of the sling which way it goes...



overhanded demo at 0:30

*In an out of control spinning aircraft*
"Hit the eject lever! I'm pinned against the wall by the force!"
Mushtang: "ACtually, it's more like the wall's pinned against you"

For the record, I know there's no 2nd law formal thing as the centrifugal force, but I also can't help poking folks that get heated about it

Don't be confused by the phrase "fictitious forces." Just because they are "fictitious," doesn't mean that it is wrong to refer to fictitious forces like centrifugal force (and, yes, gravity). From some frames of reference you definitely feel them and may have no way of telling that they are caused by a "real" force in another reference frame.

An issue arises, however, from us needing to know that there are multiple frames of reference and what's happening in another frame to understand that the force in the rotating frame is due to centrifugal force. From inside the rotating frame, you wouldn't know that the centrifugal force is centrifugal force without knowing that you are in a rotating frame. If you didn't know you were in a rotating frame, you might attribute the force to something like gravity (see: Equivalence Principle).

Inside a windowless car, we would say that we are pushed toward the door by centrifugal force due to the car turning. But without knowing for sure that the car is rounding a bend, the force you feel could be caused by some other acceleration like a truck hitting the side of the car or the car tilting on its side. So labelling a force as "centrifugal" is mixing frames of reference. That's not a good thing to do, but in most cases it wouldn't be confusing. We understand what is meant by "centrifugal force."

“This is the kind of pedantry up with which I will not put!”

-Winston Churchill on being told not to end a sentence with a preposition

The reason this question comes up is that centrifugal force is something that you feel. If you are on a merry go round, the feeling you get is that you are being pushed outward, not that you have to be pushed toward the center to move on a curved path.

It is very hard to convince people experiences they have had repeatedly are fictitious.

jadebox said:

Inside a windowless car, we would say that we are pushed toward the door by centrifugal force due to the car turning.

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That right there is exactly what I was talking about in my original post. You make it sound like you'd say it was centrifugal force even if you did know the car was turning, but there's no such thing, there's nothing pushing you against the door.

If you're standing in a room and I walk up and push you forwards, you'd never say that in some situations you may believe that you were pushed against me by some imaginary force.
If you're running on a track and I run up and push you to the left, and as we run I continue pushing you left, you'd never say that you were pushed against me by some imaginary force.