Do sinks drain in different directions in the northern and southern hemisphere?

Our physics teacher told us that sinks drain in different directions in the northern and southern hemisphere due to the Coriolis’ effect but that’s apparently not true. Was my teacher lying to us? Is the Coriolis’ effect real or a hoax?

That’s an interesting question. Your teacher was not deliberately lying to you but most likely has a rather superficial and limited knowledge of the Coriolis’ effect. That is not really surprising since several physics textbooks, included some at university level, provide an explanation of the Coriolis’ effect which is poor at best.

Let’s try to keep this post relatively simple and avoid complicated formulas and excessive scientific jargon. How does the Coriolis force originate? Newton’s second law, F=m x a (mass times acceleration) works on inertial frames of reference, namely frames which are either stationary or move at constant velocity, namely with no acceleration. Is there any way that we can still apply Newton’s second law, or a modified version of it, if the frame of reference has a velocity which is not constant namely has non-zero acceleration?

We can, but we must invoke some new forces, known as fictitious or virtual forces. These forces can be calculated by using some rigorous mathematical procedures but calculating them is beyond the scope of this post.

Fictitious forces are not really real, you wouldn’t feel them if you were on an inertial frame of reference; we use them to make things ‘add up’ so that Newton’s second law still works in non-inertial frames of reference. The Coriolis’ force is one of such forces; because of Earth rotation on its axis, then if we want to be precise, since the Earth is not an inertial frame of reference, the equation F= m x a does not work in its original form, but still works if we add some fictitious forces. The modified equation would them be Fr+Fv =m x a, where Fr is the sum of all real forces, while Fv is the sum of all virtual forces. The Coriolis force is one of the virtual forces. Like other forces, virtual forces give rise to virtual acceleration.

To go back to the first question above, whether a sink would drain in different directions in the two hemispheres depends on how strong the Coriolis force is and how strong the Coriolis acceleration is with respect to the gravitational acceleration g. The video below puts some quantitative basis to the Coriolis’ effect and shows how small the Coriolis acceleration is compared to g, making it hard to detect the effect. To be more precise in order to determine how tangible the Coriolis effect is, one can use the Rossby equation Ro=U/(fxL), where U is the velocity scale, f is the Coriolis parameter and L is the horizontal length of the system. Typically, the larger the system, the easier it is to detect the Coriolis' effect.

Ordinary sinks in fact do not drain in different directions in the two hemispheres. Does that make the Coriolis’ effect a hoax? No, it does not. The video below shows a well-designed experiment used to prove the Coriolis’ effect.

In conclusion, the direction in which a sink drains is largely dependent on factors other than the Coriolis’ effect, such as the geometry of the sink, the momentum of the jet entering the sink, residual currents, all those factors can typically make the Coriolis’ effect undetectable under ordinary conditions. Only with a well-designed experiment the effect can be detected.

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Physics myths, Physics hoaxes, Coriolis' effect, how to detect the Coriolis effect on small scales.