Continente Siete

Flush the toilet…

..and check which way water spins!

It may sound new to some people and it may ring a bell to others when it comes to the question: which way does water spin when you flush the toilet? The answer will come later.

There are several forces involved in the motion of an object, but there is one special effect that appears when the analysis takes place in a rotating reference frame. The latter term refers to a non-inertial reference frame rotating relative to an inertial one, in which case Newton’s laws cannot describe properly the forces involved unless fictitious forces are used.

We can now discuss, for instance, the centrifugal forces; but it is more interesting to learn about Coriolis Effect. It was in 1835 when a French engineer-mathematician called Gustave-Gaspard Coriolis described this “force”. Any object in motion in a rotating reference frame is accelerating by a force perpendicular to its motion. Even though this force does not exist because of a physical interaction, it creates a deflection on the path of an object due to the motion of the coordinate system, i.e. the Earth. This is why this kind of effect can only be appreciated at large scale, when the object´s  speed is greater than the ground’s.

Three aspects are related to the Coriolis Effect. First of all, the motion of the Earth is the rotating frame reference, but the effect weakens as we approach to the equator in a horizontal direction. That is why the latitude is another aspect to take into account. Finally, the motion of the object will also determine the magnitude of the Coriolis Effect.

Some examples can be seen both in natural and human developments. When it comes to natural sources, this effect is shown by the motion of great lakes and oceans, even the wind. On the other hand, the deflection planes or missiles evidence when travelling mostly from one hemisphere to another is also because of the mentioned effect. 

Image 1. Large-scale wind patterns

All in all, we now know there is an effect named Coriolis which impact depends on the reference time-frame and affects our everyday-life (and in greater magnitude if you are further the Equator). But, can we really see it if we flush the toilet at home?

I am sorry to disappoint you guys! But if you flush the toilet in the Northern hemisphere or in the Southern, the swirl (vortex) generated only depends on the lavatory itself and other irregularities of the surface, because the speed of the drain is faster than the one generated by the Coriolis Effect. If you really want to verify the effect, you have to use a bigger recipient and tons of water. In laboratory circumstances, i.e. certain temperature and latitude, letting the water set still; then the conditions are set to evidence the Coriolis Effect when the Earth’s movement is slower than the one of the water relative to its container.