Hi
There see to be quite a few websites calling the Coriolis a pseudo-force or an illusion. I want to clear up any misinformation that is out there. I look forward to your feedback
It is a fictitious force. Akin to a centrifugal force.
It’s perfectly valid to call it a force, so long as you’re invoking a rotating frame of reference. Those are extremely useful in physics – and moreso in engineering. They didn’t get the Apollo spacecraft to the moon by calculating the minute-by-minute rotation of the earth!
The word “fictitious” does not mean “fictional.” It certainly isn’t an illusion.
It is certainly correct to call it a pseudo force as a pseudo force is a force that appears in a non-inertial frame of reference and is due to that frame of reference.
Whether it is an “illusion” depends on precisely what you mean by that word, but I would avoid describing it as such as it arises from the physics of the situation.
Isn’t it though?
Say you are twirling a bucket around your head. You will feel a force puling the bucket straight away from you. But, when you let go of the rope, the bucket does not fly straight away from you but rather on a tangent to the circle it was making as it was twirling.
I guess for an engineer they need to calculate the “straight-away force” (i.e. centrifugal force) when working out how strong the cable needs to be to hold it. Still, it is a “fake” force.
EDIT: This just has to be here: xkcd: Centrifugal Force
It’s perfectly fine to call it a pseudo-force, as long as you also call gravity a pseudo-force.
Remember … what we observe is the Coriolis acceleration … and we blame this on Coriolis force because only a force can cause an acceleration …
Also remember … the Coriolis acceleration can only be observed in a rotating frame-of-reference … in an inertial frame-of-reference it contributes nothing to what we observe …
It’s sometimes called a pseudo-force because when we observe this acceleration, we see that kinetic energy is being added to the object we’re observing, however we have no source for this energy and this violates the conservation of energy … some people are kinda nitpicky about that …
By no means is it an illusion … it’s very real and specifically in gunnery it must be considered … we do have to aim a little to the west to hit our target if we’re shooting north, for example …
The sciencey techno-babble is to be in a rotating frame-of-reference … we the observers must be experiencing an acceleration … of equal magnitude but opposite direction of the observed Coriolis acceleration … and an inertial frame-of-reference is where we the observers are not accelerating …
I’m not clear on how you would see k.e. being added to the mix. As the xkcd cartoon notes, so long as you translate coordinates, everything balances. You can do physics in a rotating room, a falling room, even a room that’s tumbling like the dice on a table in a casino. The books will always balance.
It’s called a fictitious force because it doesn’t have any actual origin in an inertial frame of reference. Centripetal force is real, necessary to counter inertia. If you spin the f.o.r., the inertia appears to vanish, and the object “feels” as if it’s tugging at the rope. That’s pretty hefty for a mere illusion!
Remember when we took our kids down to the Kentucky State Fair and they got into a rock fight on the merry-go-round? … All the witnesses on the ground testified Mikey threw the rock at Henry, but the merry-go-round carried Henry away and that lil’ girl right into the rock … the girl said the rock curved right into her, she didn’t think she had to duck … we had the acquittal in hand until Billy broke out laughing … [sigh]
A frame of reference that is rotating is undergoing an acceleration; travelling in a circle requires constant acceleration. Similarly, if you were in a rocket accelerating in a straight line, you would have the illusion of experiencing gravitational force - a force pulling you down to the floor.
Coriolis, centrifugal, and apparent gravity (due to acceleration) are all fictitious forces that are introduced to compensate for the fact that the frame of reference we think of as stationary (i.e. here on earth, or in a rocket, or on 2001’s space station) is in fact undergoing acceleration.
It’s not just apparent gravity that’s a pseudo-force. According to General Relativity, all gravity is a pseudo-force. Is the Coriolis force real? That’s a philosophical question, not a scientific one. But if it’s not real, then neither is gravity.
I’m sorry, but I’m not clear on what’s going on here. Are some people getting hot and bothered about recognizing that things moving in a straight line, don’t appear to be moving in a straight line, if the observer is in motion as well? And that some very complex calculations have to be one, in order to allow the observer who is in motion, to successfully predict the observed result of applying force to something?
Seems everyone is dancing around this. So, help us dimwits out.
The question at hand is about the Coriolis effect but let’s make it more simple and something we all “understand”: centrifugal force. Understanding that is more intuitive and we can get to the Coriolis effect.
So:
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Twirl a bucket around your head and you feel a force pulling on the rope.
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Let go of the rope and the bucket does not fly straight away but on a tangent.
If the centrifugal force is real why doesn’t the bucket fly straight away?
Because you’re switching frames of reference mid-description.
In the frame of reference rotating with the bucket, there’s an outward force, and when the rope is let go of, the bucket moves outward (from your still rotating arm or body) with the same acceleration you’d expect due to the outward force you felt (for short periods of time - which is all the Principle of Equivalence demands of the universe).
Meanwhile in the non-rotating frame of reference, the force you feel is the resistance of the bucket to your demand that it curve its path - and so in that frame of reference, when the rope is cut, the bucket gets its revenge by refusing to bend its path anymore - so it moves out on a tangent.
OK. I learned something today. One good part of my day.
I would not have concluded that gravity is a pseudo-force, but I don’t have any understanding of the Einstein field equations.
To help keep my feet on the ground here, can someone tell me whether newtonian gravity (no general relativity, just the approximation that Newton came up with) is a force or a pseudo-force?
I would call it Coriolis a force. Granted it is often small and not always applicable.
Gravity as Newton understood it was a force, yes, and gravity can be mathematically treated as a force in most circumstances, especially if you do all your math from the perspective of someone standing on a planet.
That last bit is the important piece of all of this: These pseudo-forces appear and disappear depending on where your observer is relative to other things. If you write all your equations from the perspective of an observer in a box which just got dropped out of a plane, there is no gravity*: Everything they observe moves in straight lines, balls hang in the air when they let go, and so on, just as if they were in a sealed box out beyond Pluto. From their perspective, the apparent force is the one dragging the ground towards them at an ever-increasing rate; they certainly don’t feel it, and it doesn’t impact them at all yet. In short, someone in freefall isn’t in an accelerating reference frame, and the only people who see them accelerating are the ones on the ground, who are in an accelerating reference frame.
*(This is human-scale objects immediately near Earth. Tidal effects are, like, a zillionth-order correction in this context.)
This gets into something very important in the study of General Relativity: The equivalence principle, which states that there’s no observable difference between accelerations due to things like rockets and accelerations due to being on the surface of a planet. The usual statement of this principle, however, is imprecise, and some introductions to the concept would have you believe that a scientist in a sealed room could under no circumstances tell whether they were in Hoboken or on a rocket to Jupiter and beyond the infinite accelerating at 9.8 m/s[sup]2[/sup]. This overly-naïve platitude is falsified by Foucault’s pendulum, which will precess at high and low latitudes on Earth and not on the rocket due to Coriolis forces. Full circle, complete with parallel transport. 
I think of it in this way, spin around twirling a bucket of water on a rope. Have a camera follow this rotation looking from above. You and the bucket look as though you are standing still. Let go of the rope and the bucket flies away in a straight line does it not?
Yes - the instantaneous motion of the bucket will be acceleration away from you (though as time goes on, the motion will deviate from that).
Exactly. “Pseudo” forces are fictional forces that compensate for the fact that the entire frame of reference itself is undergoing acceleration. Hence anything that is released or free to move in that frame will experience that pseudo-force due to its inertia because the force acting on the frame of reference does not act on it. It’s just a shorthand to explain the tendency of objects to apparently move in the (accelerating) frame of reference due to the object’s inertia.
Again remember that circular motion is constant acceleration toward the center, i.e. acceleration perpendicular to direction of motion. .