5X5 Episode 75

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5X5 Episode 75
The Coriolis Effect
12th August 2009

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5X5 74 5X5 76
Skeptical Rogues
S: Steven Novella
R: Rebecca Watson
B: Bob Novella
J: Jay Novella
E: Evan Bernstein
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The Coriolis Effect[edit]

You're listening to the Skeptics' Guide 5x5, five minutes with five skeptics, with Steve, Jay, Rebecca, Bob and Evan.


S: This is the SGU 5X5 and tonight we are talking about the Coriolis effect. The Coriolis effect or the Coriolis force is actually a pseudo-force or an illusionary force that appears to cause things to curve as they travel a long distance across a rotating or spinning frame of reference.

E: It's called the Coriolis effect because it was first discovered by Gaspard-Gustave Coriolis, a French mathematician, a mechanical engineer and scientist. He's best known for work on motions of moving parts in machines relative to their fixed parts. He had a talent for interpreting and adapting these theories to applied mathematics and mechanics and while he was an assistant professor and the director of studies at the École Polytechnique in Paris from 1816 to 1838, he gave the terms work and kinetic energy their scientific and practical meanings and they're still used today. But in 1835, he published a paper called "On the equations of relative motion of system bodies" which he described this force, which in the 20th century became known as the Coriolis effect.

R: The funny thing is I think that most people think of the Coriolis effect as something that it is not. A lot of people have heard of it when they're talking about the way that, say, water swirls down the drain or the way your toilet flushes. There's a really common thing that's done on tours around the equator where they show you a bowl of water with a hole at the bottom and they show water swirling down it one way on one side of the equator and then they step over to the other side and show it swirling the other way. This effect has even been—if you've been fooled by it, don't feel bad because I read a book by Douglas Adams where he talks about it, believing that it's totally true and at the time that he wrote the book he still thought it was true. So you're not the only one and it's a very good illusion and good kind of trick. But the trick is really just in the bowls; the drains that you use; the way that they are pock-marked or the subtle variations in a bowl can have a lot more to do with the way water drains than, for instance, what side of the equator you're on. You could also influence it by spinning the bowl a certain way; by tipping it a certain way. So unfortunately, that's not actually the Coriolis effect that you see.

S: That's right, because the Coriolis effect is proportional to the distance over which something is moving, like, say, a cannonball shooting a mile or two or, let's say, the size of a weather system like a tropical storm or a hurricane. However, the size of a bathtub or a sink is far, far, far too small for the Coriolis effect to be significant. This is something called the Rossby number, which describes the magnitude of the Coriolis Effect and for things the size of a tub it is literally negligible. Even the slightest momentum in the water in the tub will overwhelm any Coriolis force and that will determine the direction that the water spins when it goes down the drain.

B: In fact the Coriolis effect is two or three orders of magnitude smaller than any of the random influences that—that of much greater influence on the way that the water drains. But it can—you can pull it off; you can show the Coriolis force in a small body of water; something maybe tub-sized or maybe smaller but it's an extreme effort to make it work. First off, you need an incredibly smooth and uniform bowl. You will need a small drain hole; not a big one because you're going to need time for this force to build up. And you're also going to need about a week and a lot of patience for the water to become completely still so that all motion that might have been induced by you, by the water going in it, will be completely washed away and then only then would you be able to potentially see the Coriolis effect in action but it's very difficult experiment to pull off on something that small-scale.

J: You know I have to admit I had a hard time really understanding what this meant until I had a discussion with Steve about it and then I actually went to Wikipedia and looked at the graphic that they had on there so I highly recommend that you do this. Now, I did come up with a way to help explain the idea of what this is visually. So let's say that an ant is traveling around a hula hoop and the hula hoop is rotating. And then a cannonball is shot from the centre of that circle. It would appear to the ant that the cannonball's path is curving when it actually isn't. Because the ant is actually moving on the outside of a sphere, just like we are on the Earth and that's the basic explanation of why it looks like the path of that cannonball is actually curving when it isn't. And the other thing is in regards to ballistics—let's say someone is shooting something above a half a mile. They may have to take into account the Coriolis effect because the Earth is actually spinning underneath that missile after it's shot and the position isn't going to be accurate as if it was being shot on something that isn't spinning. A ballistics expert has to take into account the spin of the earth in order to calculate where that shot is actually going to hit.

B: And the faster you're moving the more pronounced the Coriolis effect will be and the further north or further south you are below the equator will also make it more and more pronounced. Another great way to visualise this is to picture four kids on a playground carousel that's spinning around and with each kid at the 12, 3, 6 and 9 position. If the carousel is spinning and the kid at the 12-o'clock position shot the ball straight out, just pushed it straight out, the ball will curve to the left, depending on rotation of course, it will curve to the left to the person at the 3-o'clock position and then if he did it the same thing would happen it would go straight out briefly and then gently curve to the person at the 6-o'clock position. So if you were outside of that carousel and you looked at it, the ball would seem to be moving in a straight line but if you're on that rotating frame of reference, only then will it appear to have a curved aspect to it.

E: It's a good point Bob; it's a good lesson for the kids so if there are four kids on that carousel and you're ninety degrees off-centre to the left then if that other kid vomits then, you know, be careful. It's not going to go straight ahead to the other kid; it's going to go the left.

B: (laughs)

S: That's right. So, to summarise, the Coriolis force is an artefact of making observations within a rotating or spinning frame of reference. But if you are within that frame of reference you can actually treat it as if it is a real force and everything will work out as if that were true—all the calculations and observations that you make. But really, it's just an artefact of making observations from within a—what we call "non-inertial frame of reference".

B: There are those that will argue that although it appears the force seems to just appear within the rotating frame of reference, even though it's not a fundamental force of nature, it is a bona fide force and should be treated as thus.

S: SGU 5x5 is a companion podcast to the Skeptics' Guide to the Universe, a weekly science podcast brought to you by the New England Skeptical Society in association with skepchick.org. For more information on this and other episodes, visit our website at www.theskepticsguide.org. Music is provided by Jake Wilson.


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