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What is the Coriolis Effect and does it really affect my toilet water?

by Dani Johnson

August 17, 2013

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Donate By now, everyone's heard that the water that goes down the drain in their sinks and toilets spins clockwise in the northern hemisphere and counterclockwise in the southern hemisphere because of The Coriolis Effect. Most of us have been using little factoids, like this, about the Earth to impress our friends or would-be boyfriends/girlfriends since the days when recess was the highlight of our afternoon. Is there any truth to the claim, and what the heck is the Coriolis Effect, anyway?

By definition, the Coriolis Effect (TCE) is an apparent deflection of the path of an object that moves within a rotating coordinate system. For us, the rotating coordinating system is the Earth, and the objects are various things like hurricanes or airplanes. When something, like an airplane, is set into motion the rotation of the Earth literally moves the ground from underneath the plane. Because of this, pilots need to adjust their course so they can navigate to the correct destination, otherwise the Earth would have moved so far underneath them that they would not arrive at the correct destination. Now, imagine the pilot disables the adjustments and lets the plane fly from point A to point B as if the Earth weren't moving underneath it. If we were watching the plane from the ground it would look to us as if the pilot were steering the plane to purposely miss the target, but that's just an illusion because the Earth is what "veered off course", not the plane.

Another way to observe TCE is on a merry-go-round with a few friends and an inflated ball. Sit across from a friend on the merry-go-round and have another friend keep the merry-go-round going at a slow and constant speed. Now, try to throw the ball to your friend that's sitting in front of you (as if you didn't already know about TCE), and observe how it seems to veer off course and go to one side instead of to your friend. When something (your ball) is set into motion it will travel in a straight line unless otherwise disturbed. In this case, the disruption is the merry-go-round moving underneath the ball so that the ball misses the target (your friend). It's almost as if the merry-go-round thinks your playing dodge ball, so it's jerking your friend out of the way before the ball can get to them.

If you're a visual learner like me, you'll enjoy this short YouTube video. It describes the basics of TCE with helpful graphics.

TCE also affects long-range shooting weapons because the Earth will move the target away from the bullet's path. Check out this video to see what kind of changes TCE can have on long-range weapons:

TCE affects the wind and ocean currents, as well, affecting the weather systems we experience here on the ground. Because of TCE, hurricanes in the northern hemisphere spin to the right and their path is in a spiral that curves to the right. In the southern hemisphere, the hurricanes spiral to the left and their path is in a spiral that curves to the left. TCE changes its effect on things the closer or farther away the object is from either pole. This change in effect is caused by something physicists refer to as the Conservation of Angular Momentum. For instance, if you are spinning around in circles in your office chair, tucking in your arms and legs will cause you to spin faster while extending your arms and legs will cause you to spin slower. In this scenario, imagine that the crown of your head is the North Pole of the earth, and the center of the bottom of the chair is the South Pole of the Earth. The "line" that connects these two points is the center of the spin, or the axis. Now, imagine the Earth spinning and see how the equator is farther from the center of the spin than the poles. When something is set into motion at the equator its speed will increase as it gets closer to the North Pole because it is getting closer to the center of the spin, like when you pulled in your arms and legs to gain speed in the chair.

Here is a YouTube video that explains how TCE affects the weather with helpful graphics:

Now that you are more familiar with the Coriolis Effect and how it affects us here on Earth, let's revisit the original question. Does the Coriolis Effect really affect the direction my toilet water spins when I flush, or the direction that my bath water spins when I pull the plug? The short answer is no, not in any way that you can measure with your eyes. The scale is just too small, but remember that episode of Spongebob Squarepants where he finds a drain in the bottom of the ocean and pulls the plug? If that scenario were real, if there were actually a drain pipe in the ocean and someone pulled it, the direction that the water would flow down that drain would absolutely be affected by TCE because the scale is much larger.

Because of the copious use of the words "affect" and "effect", please forgive me if I got them in the wrong spot a couple of times. Also, my understanding of TCE is very general, no matter how many times I've watched the included videos or read the webpages listed in the links below. If anyone knows of something I missed, or just has something to add, your comments are welcome!

Further Reading



by Dani Johnson

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