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The Water In The Sink And The Coriolis Effect

The Coriolis Effect

French physicist Gaspard-Gustave de Coriolis was the first to explain it in his 1835 book Sur les équations du mouvement relatif des systèmes de corps (On the equations of relative motion of a system of bodies). If you have an education on sciences, you’ll remember it from your early physics or mechanics lessons (and you can jump to the next paragraph).

If you’re not so fond of physics, the easiest way to understand it is to recall your day to day experience in a car. When a car that is going straight suddenly turns right, you feel something pushing you towards the left. Well, nothing is really pushing you: this force is apparent because the only force actually acting is the one making the car turn right. But because, while you’re sitting in the car, your reference happens to be the car itself, so you feel an apparent force pushing you to the left.

The maths behind this might be a little bewildering though, so it should suffice to know that, in the case of rotating objects (e.g. the Earth), similarly to the force that pushes you left when the car turns right, an apparent force – the Coriolis force – is acting on objects that are moving with respect to the rotating object itself (i.e. the Earth).

The water in the sink

What happens when you release the sink cork? The water doesn’t go down straight but rotates around the hole! The Coriolis effect affects everything moving on the surface of the Earth is subject to the Coriolis effect due to the Earth rotation; this includes the water going down the sink. The point is how intense the Coriolis effect is.

Is it an example of Coriolis effect?

The short answer is that the Coriolis effect is not noticeable in that case.

The long answer requires to consider the latitude of where we are and how much of this movement happens along the rotation axis. To set up an order of magnitude for this, we would say that big masses of air and clouds moving very fast will face a measurable Coriolis effect: this is encompassed in meteorology while studying tornadoes. Meteorological phenomena like hurricanes are some hundreds of miles wide and last for weeks: not only they are subject to the Coriolis effect but this effect has enough time to influence the movement of these objects. But the water in the sink moves for a small time and a relatively small speed: the Coriolis effect can be easily ignored.

There is another proof of this: the sign of the Coriolis effect change from Northern to Southern hemisphere. This is also noticeable in meteorology, observing the clouds movements. So depending on which side of the Equator you are, the water in the sink should rotate in a different direction. Well: that’s not the case.

If it’s not the Coriolis effect, what is it?

The formation of a vortex over the plug hole can be explained by one of the fundamental laws of physics: the conservation of angular momentum: it is the same law used by dancers and skaters spinning, quicker and quicker as they pull their limbs in a more compact position.
Regarding the sink, the diameter of rotation decreases as water approaches the plug hole, so the rate of rotation increases. Any movement around the plug hole that is already present will accelerate as water moves inward. As explained, the Coriolis effect is significantly smaller than any other influence, such as the geometry of the container: as explained by The Book of General Ignorance by Lloyd, Mitchinson & Fry, the water rotating in the sink is better explained just by the shape of the sink….

Sometimes explanations are simpler that you think, aren’t they?

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