The Coriolis effect is responsible for all kinds of interesting phenomena. It makes hurricanes spin differently in the northern and southern hemispheres, it makes toilets go counterclockwise in the northern hemisphere and clockwise in the southern, and makes it really difficult to play catch on a spinning merry-go-round. Or does it?
At the time of writing, Wikipedia says this about the Coriolis force:
"In physics, the Coriolis force is an inertial force (also called a fictitious force) that acts on objects that are in motion relative to a rotating reference frame. In a reference frame with clockwise rotation, the force acts to the left of the motion of the object. In one with anticlockwise rotation, the force acts to the right."
That didn't help me much because I lost steam partway through due to jargon. I think of the Coriolis effect as "things acting weird because the world is rotating underneath them." For example, if two friends are sitting on opposite sides of a spinning merry-go-round playing catch, the ball veers unpredictably.
This happens because once you release the ball it no longer cares about what the merry-go-round is doing beneath it, it just travels along the path it was thrown. If you're on the merry go round in a different frame of reference that straight line looks curved. The "force" moving the ball is what is responsible for the Coriolis effect.
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| In this case, you are the red dot, and the black ball is the object being thrown |
So far so good, right? The curving of the ball is a bit odd, but here's where it gets really interesting. In our example, the Coriolis effect was quite dramatic because the merry go round was moving quickly relatively to the motion of the ball; the merry-go-round moved about 25% of the way through its rotation while the ball was in the air. Let's consider a larger rotating frame of reference: our planet.
The earth rotates once per day (I think). The a equator has a much larger circumference (40,076 km) than do the poles (0 km). Over the course of one day, the poles just spin in place around the earth's axis, while a point on the the equator needs to travel the full 40,076 km in that same time. This means that the speed the Earth travels at differs at different latitudes.
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| *assuming a spherical Earth, not a oblate spheroid (I can't do the math for the real shape, but these should be really close to the actual numbers) |
If you throw a ball on the earth, the earth will rotate beneath the ball as it is in the air. The Coriolis effect doesn't doesn't show up very often, because many things aren't in the air long enough to matter; it does however show up in a few interesting places: long distance shooting and field goal kicks in (American) football.
In the field, snipers work in teams of two, a marksperson and a spotter. The spotter has a very interesting job: they must consider a whole spectrum of effects to preserve accuracy. A spotter must accurately find the range to ensure the parabolic trajectory hits the target at the right point (including shooting uphill or downhill where gravity acts at an angle, making the math much harder). Other factors the spotter must take into account include the wind conditions both at their location and downrange, updrafts caused by radiant heat, the curvature of the earth, a potentially moving target, humidity, and you guessed it, the Coriolis effect. If you are shooting directly North, the bullet will deflect to the right by a matter of a few inches. If you are shooting directly South, the bullet will deflect to the left. An East or West facing shot will have a smaller effect, but will travel either farther or less far respectively.
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| The red lines in the
middle are the cardinal directions, and the blue lines above are the apparent trajectories of bullets fired in those directions. |
In football field field goal kicks, the Coriolis effect may make a small difference, a matter of centimeters, if the ball is kicked either North(ish) or South(ish). This is such a small effect that it has probably never made a difference in an actual game, but it may have in cases where a few centimeters might be the difference between bouncing off the uprights and going through the goalposts:
On to hurricanes and toilet bowls. Hurricanes do rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern. The structure of a hurricane looks roughly like this:
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| Winds are trying to rush into the low pressure zone in the middle |
The Coriolis force, shown in yellow, displaces these arrows to the right in the Northern Hemisphere. This means the winds move to the right, and the effect is that the whole thing begins to spiral:
This is why hurricanes spin counterclockwise north of the equator and Clockwise south of the equator.
On to the topic at hand (finally): toilets. As we have seen, the Coriolis force shows up in large-scale systems, where objects are either in the air for a long time, travel very long distances, or are massive in scale. A toilet bowl fits none of these criteria. The fact is that the Coriolis force is vanishingly small and insignificant on the scale of one or two feet. The direction a toilet bowl flushes is dominated by the shape of the bowl and the direction of the water jets that are released from the top of the bowl.
Phil Plait, in his book Bad Astronomy talks about a small Kenyan village right on the equator. For a few bucks you can pay someone to give you a "demonstration of the Coriolis
effect." There are two basins, each in different hemispheres, one that drains clockwise, the other drains counterclockwise. This is because the one to the North has right-facing jets, and the the one in the South has left facing jets. Simple as that.
The interesting thing is that you actually can see the Coriolis effect in a draining water basin, but you have to control variables very carefully or random effects will trump the Coriolis effect. Enjoy:
Note* It's cool to sync the videos, but not necessary. If you don't want to, just watch the "North" video. If you want to sync them up, grab your phone and your computer, or two computers, or two tabs. Here are the individual links:
- Scott
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