Shandrew (shandrew) wrote,

Warning: fun-destroying boring fluid-mechanics science spoilers follow

How does the equator sink experiment trick work?

A popular false myth told by many (including one of my middle-school science teachers) is that the Coriolis effect causes the rotation direction of draining water in a sink or toilet. If you want to learn why it's false, look up "bad coriolis", which is probably better to read than listening to me babble about physics, fluid mechanics, and meteorology classes that i took a decade ago.

Anyway, Teresa told us at lunch today about tourist areas on the equator demonstrating the "Coriolis effect" by showing a sink drain on the equator, north of the equator, and south. What sort of trickery do they use? Well in this case:

They start with the sink basin placed on the equator, already filled with water. I'm guessing that it's had plenty of time to sit in the sink and settle. The water drains straight down. To me, that demonstrates that the sink is relatively fair and there's no significant irregularity causing circular draining. Next, they move the sink to the southern hemisphere. They fill the sink using a bucket, pull the drain, and show the water draining clockwise. The sink is moved to the northern hemisphere and the procedure is repeated, showing the water draining counter clockwise.

The astute observer will notice that on the southern hemisphere, the water is poured into the sink in the lower left of the sink and in the direction towards the upper left. In the northern hemisphere, the water is poured into the sink in the lower right of the sink and in the direction towards the upper right. Pouring the water on different sides causes water to circulate in the sink in the direction that the water was moving after being poured.

Some other ways to produce the same (though probably more subtle) effect might be to tilt the basin in different directions in the two locations, or to induce a current with your hand as you pull out the drain.

Sometimes science is a downer, when it's like finding out secrets to how magic tricks work. At least this trick isn't all that harmful; it's rather fun and not the end of the world if people are misled a bit.
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