## Does the Coriolis force determine which way my toilet drains? Does it affect black holes? (Beginner)

A toilet in the northern hemisphere rotates counterclockwise when flushed. A toilet in the southern hemisphere rotates in a clockwise direction (so I've heard). I'm wondering if this phenomonon is related in some way to the rotation of black holes. Thank you, and I'm sorry for such a weird question!!!

First to clear up a very common misconception: the Coriolis effect is not the determining factor in which way your toilet drains. In a system that is as small as and rotating as slowly as the water in a basin or toilet, the effect is miniscule when compared with factors such as the the initial motion of the water and the shape and orientation of the basin. This is especially true in a toilet, where there are jets of water shooting in! Theoretically, you could detect the Coriolis effect by building a perfectly round, flat basin and making sure the water is initially perfectly still and the drain is opened very carefully, &c. I wish you luck. You might also be tempted to try a large statistical study, polling many people on which way their basins drain and looking for correlations with latitude; here there would be many factors to eliminate from the habits of basin-builders, plumbers and the like that make the non-Coriolis effects not completely random--in other words, basin-builders near the equator may build basins differently than those near the North Pole. (Of course this may already have been done; people do the strangest things.)

At any rate, there are physical systems that are strongly affected by the Coriolis effect (despite the fact that it's named after a mathematician). There are two ready ways to detect it. First, the measurement may take place over a long period of time, so that other factors may cancel out (for example, over many years river beds are often dug deeper on one side due to the Coriolis force). Second, the deflection itself may be large (such as in a hurricane, where there are large distances for deflection). Black holes fall into the latter category, and the Coriolis effect must be taken into consideration when studying the motion of objects around rotating black holes.

Here's some math to give you more confidence in my answer: the Coriolis force is

F = -2*m*(w X v)

where m is the mass of the deflected object (divide the water in the basin into small volumes, and consider each an object), w is the angular velocity of the rotating object (for Earth, 360 deg./day or about 1E-5 radians/sec), v is the velocity of the deflected object, and X indicates a vector cross-product. You can see that a large mass, large angular velocity, large object velocity, an object velocity perpendicular to the angular velocity, and long distances for the deflection to take place all contribute to a large deflection.

This page was last updated June 27, 2015.

### About the Author

#### Sara Slater

Sara is a former Cornell undergraduate and now a physics graduate student at Harvard University, where she works on cosmology and particle physics.

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