Can you see the vortex of Coriolis force in the toilet?

One in ten people on Earth see everyday phenomena differently than most of us do.

For example, they cannot see the North Star, they see typhoons ("cyclones") that rotate clockwise, and they see steep river banks that tend to be on the left side of the river...

Because they live in the Southern Hemisphere . In contrast, the Northern Hemisphere continent is much larger than the Southern Hemisphere, carrying nearly 90% of the world's population.

Due to the fact that the earth belongs to the northern and southern hemispheres, some natural phenomena on the earth show opposite patterns. The most well-known reason behind this is the Coriolis force (Coriolis force).

The misunderstood Coriolis force

In geography textbooks, Coriolis force is also known as the Coriolis force.

When mentioning Coriolis force, many people tend to associate it with the spin of the Earth, but in fact, pure "Coriolis force" does not exist. These two forces have only one unified name in English: "Coriolis force".

In physics, it is believed that any rotating object, including the earth, may produce Coriolis force.

The Coriolis force originated from a French engineer, Gustave Gaspard de Coriolis, who discovered it while studying the energy conversion of water wheel rotation. The name of the Coriolis force also came from this. At first, the Coriolis force had nothing to do with the atmosphere and the rotation of the earth. They were applied in their respective fields.

The Coriolis force was first discovered by Coriolis on a water wheel (Image credit: amatterofind)

Similar to centrifugal force, in the strict physical definition, the Coriolis force is not an actual force. It is an effect (Coriolis effect) introduced to maintain consistency with the local reference frame.

Although it "does not exist", the concept of Coriolis force is easy to understand and therefore widely spread and used.

On Earth, almost all objects moving horizontally are affected by the Coriolis force (except objects at the equator). When it moves along a straight line, its trajectory gradually bends as the distance increases.

"I cannot see the true face of Mount Lu because I am inside it." In a sense, the straight line in the eyes of a moving object uses the ground under its feet as a reference system, but in the eyes of other people, due to the rotation of the earth, this straight line is actually a curve from the beginning.

Is it a bit hard to understand?

When Galileo proposed the principle of relativity, he gave an example: suppose there is a large ship moving in a straight line at a constant speed on a calm lake, with all the windows closed. Then, can the people on the ship tell whether the ship is stationary or moving in a straight line at a constant speed?

Galileo's thought experiment on the ship (Image source: Physics Central)

The result is obviously not. Due to the control of inertia, the results of all mechanical experiments conducted in the cabin are no different from those in a stationary cabin.

Similarly, when we are in an airplane, train or elevator that is moving at a constant speed, we often have the illusion that "they are stationary."

But the earth is not a ship moving at a constant speed, it is always rotating along its axis. In front of all objects on the earth, there are at least two reference frames, one is the own reference frame with itself as the origin, and the other is the earth reference frame with the center of the earth as the origin and always rotating.

Of course, you can also establish countless other coordinate systems, such as the solar coordinate system with the sun as the center.

As an inertial force, the Coriolis force is not a force at all. Figuratively speaking, it is more like a bridge spanning two reference frames.

For objects that are stationary on the earth, they will maintain the same state of motion as the previous moment. Whether in their own reference frame or in the earth's reference frame, they are in a stationary state. The Coriolis force can only "stare blankly" and cannot play any role.

Objects moving on the surface of the earth (not at the equator) can also choose these two reference frames, but their motion states are different in different reference frames. When it moves straight forward, its trajectory is a straight line in its own reference frame, but when its trajectory is projected on the surface of the earth, it is a curve. It is as if an invisible hand (Coriolis force) has bent the straight line. This curve is formed by the superposition of the rotating earth and its own straight trajectory. This is the true appearance of the Coriolis force on the earth.

Why is the direction of the Coriolis force in the northern and southern hemispheres opposite?

Because the earth is round. Although the earth rotates from west to east, when we look down from high altitude in the northern and southern hemispheres (the altitude must be high enough), we will find that the direction of rotation of the earth seen in the two hemispheres is opposite: counterclockwise rotation from the North Pole and clockwise rotation from the South Pole . This is also the reason why the Coriolis force in the northern and southern hemispheres is opposite in direction.

Coriolis forces are opposite in the northern and southern hemispheres (Image source: pressbooks)

The rotation speed is the largest in the high-latitude polar regions, and the Coriolis force is also strong. Although there is also a rotation speed at the equator, it is exactly the same as the direction of the Earth's rotation. The equator seems to rotate clockwise or counterclockwise, and this controversial area has become a forbidden zone for the Coriolis force (an infinitely thin loop).

A little whirlpool in the sink!

Of course, it's not just winds, ocean currents and airplanes that are affected by the Coriolis force. Almost any object moving horizontally on Earth is affected by the Coriolis force, even the water in the toilet.

The swirling water in the toilet (Image source: Mental Floss)

Under ideal conditions, the water flow in toilets, bathtubs and washbasins in the Northern Hemisphere can produce a counterclockwise vortex, but because the Coriolis force is extremely weak, coupled with interference from the direction of the water spray, the shape of the pool and other external factors, the direction of rotation is often extremely uncertain.

Therefore, it is extremely difficult to observe the "vortex caused by the Coriolis force in the toilet" with the naked eye. Almost all books, websites and even teachers will say that the vortex produced in the pool we see every day is not caused by the Coriolis force.

But in every corner of the world, there are always a few scientists who like to argue and get stuck in a rut, and Ascher Shapiro, a professor of fluid mechanics at MIT, is one of them.

He believes that if there is no interference from any factors, no matter how small the pool is, the Coriolis force will definitely leave its own vortex that can be captured by us!

Although many scientists understand this truth, few have the courage to conduct experiments to verify it, because this simple experiment, which seems to be completed in the kitchen at home, actually has some unpredictable difficulties.

Whirlpool in a pool (Image source: technologyreview)

In 1962, Shapiro decided to try to challenge this problem. The latitude of MIT is 42°, and when the flow rate is close to 5 mm/s, the Coriolis force is only 1/30 million of the local gravity. In order to eliminate the interference of all factors, he carefully designed every detail of the test.

First, he chose a cylindrical pool with a diameter of about 1.8 meters and a depth of about 0.15 meters. There was a drainage hole with a diameter of about 1 cm in the middle of the bottom, which was sealed with a stopper.

In addition, he tried to remove impurities from the water and adjusted the temperature in the room to control temperature changes. In order to prevent interference from airflow, he also covered the top of the pool with a layer of plastic film.

The most easily overlooked point is that after the pool is filled with water, the water will still have a tiny movement, which may even exist for several hours. In order to completely avoid the influence of this part of the movement, Shapiro stirred the water in the pool clockwise to offset the counterclockwise vortex generated by the Coriolis force in the northern hemisphere.

After 24 hours of settling, Shapiro carefully pulled out the plug.

In the first 12-15 minutes, he could hardly observe any signs of rotation. However, as time passed, the vortex gradually showed a counterclockwise rotation without him noticing.

Under the condition that all factors were strictly controlled, Shapiro finally confirmed that the Coriolis force in the northern hemisphere can indeed cause the vortex in the pool to rotate counterclockwise.

That's it? This experiment doesn't seem difficult, and we can probably do it in our own kitchen, but why didn't others succeed?

On the one hand, others may overlook the residual motion of the water. They think that the water in the pool is completely still after 3-4 hours.

In addition, since there was almost no sign of rotation in the first ten minutes of the experiment, and some experimenters may have designed the pool too small, the water in the pool had already been lost before the vortex appeared. Or some experimenters lost patience after observing for a while and gave up before the dawn of success.

To be more rigorous, three years later, scholars from the University of Sydney conducted a similar experiment in the southern hemisphere, and the result also showed a clockwise rotating vortex. Thus, the widely circulated misunderstanding that "the vortex generated by the Coriolis force cannot be seen in the pool" was completely shattered.

Slowly rotating vortex (Source: ffden)

The results of both experiments were published in Nature, which immediately triggered questions from readers around the world. In the era without the Internet, authors and readers could only communicate through letters. From the time of publication to more than ten years later, Shapiro still received letters from all over the world, almost all of which were about "pool vortexes."

Today, in the MIT archives, we can still see a faded folder filled with emails from readers and Shapiro's careful and detailed responses.

Coriolis force: More ubiquitous than you think

The Coriolis force does not have a big impact on our daily lives. It only fully manifests itself on objects moving at high speeds. But for snipers, a high-speed flying bullet can be fatal if it is affected by the Coriolis force.

In fact, sniping is not just a simple aiming and shooting in the game, and ultra-long-range sniping does not completely conform to the principle of "target-sight-eye".

Shooting from the perspective of foreign "flat earthers" who also deny the existence of Coriolis force (Image source: thetruthaboutguns)

In addition to the moment of pulling the trigger, the sniper's work is more about feeling the local temperature and humidity, wind speed and direction, and considering the influence of air resistance, gravity and Coriolis force at the local latitude.

When faced with extremely complex killing missions, snipers will even use pen and paper to perform mathematical calculations with their assistants (observers) and adjust the scopes in a timely manner, otherwise a slight difference may lead to the failure of the mission.

Photo of a galaxy taken by the Hubble telescope (Image source: bfmtv)

Of course, the Coriolis force does not only appear on Earth, but any planet will be affected by it. Because the Earth rotates slowly, the Coriolis effect is not obvious.

Jupiter is the fastest rotating planet in the solar system, with wind speeds as high as 610 kilometers per hour. Here, the Coriolis force thrives and can even transform north-south winds into east-west winds.

Mars is often called Earth's sister planet, but in fact, the planet that has the most common features with Earth is Venus. Venus is the closest to Earth, and the two are almost the same size, have similar structures, and both have thick atmospheres.

The only difference is that Venus rotates from east to west (reverse rotation). Therefore, the Coriolis force phenomenon in the southern hemisphere of Venus is exactly the same as that in the northern hemisphere of the Earth. Planets are different from each other, but they follow similar laws.

Innocent people caught in the crossfire: "When in doubt, use Coriolis mechanics"

After recognizing the existence of Coriolis force, some people tend to associate all phenomena in the world with Coriolis force, such as the traffic rule of driving on the right.

They say: Cars in the northern hemisphere tend to deviate to the sides of the road under the influence of Coriolis force. If they drive on the left, they are prone to collide with oncoming vehicles and cause traffic accidents.

This sounds reasonable, and many countries in the southern hemisphere do follow the traffic rule of driving on the left. But in fact, this has nothing to do with the Coriolis force.

There are 163 countries and regions in the world that drive on the right as a traffic norm, while 76 countries use the left-hand rule, including the United Kingdom and former British colonies such as South Africa, Australia and New Zealand.

The specific reason for the different driving rules can be traced back to the British knights in the Middle Ages. When they dueled, they held their weapons in their right hands, so their horses were closer to the left. Even after the Industrial Revolution, when horses were replaced by cars, this tradition was followed. In the 18th century, Britain was known as the "Empire on which the Sun Never Sets", and it also brought the traffic rules of driving on the right and walking on the left to its colonies.

Left-hand and right-hand driving rules (Image source: BrightSide)

In addition, there are some claims that sound outrageous, such as the Coriolis force causing the right soles of people in the northern hemisphere to wear more severely than the left. However, this comes from personal running habits and has nothing to do with the rotation of the earth.

The wear patterns on the soles of each person's shoes are unique, just like our fingerprints. However, the "shoe patterns" record our walking habits, and some criminal investigators can even accurately identify suspects in a crowd based on the shoe soles and shoe prints in the mud.

Three typical wear shapes on the sole (Image source: treadlabs)

When you change shoes, you might as well turn your sneakers over occasionally and spend a few minutes analyzing your soles. Understanding the shape of the wear on the soles can help you improve your walking and running posture, prevent injuries, and provide some guidance for buying your next pair of shoes.

References:

1.https://www.nationalgeographic.org/encyclopedia/coriolis-effect/

2.https://www.thoughtco.com/what-is-the-coriolis-effect-1435315

3.https://factfile.org/10-facts-about-coriolis-effect

4.https://www.technologyreview.com/2012/10/24/183079/verifying-a-vortex/

5.https://www.thenakedscientists.com/articles/interviews/can-you-detect-coriolis-effect-your-sink

6.https://www.nap.edu/read/23394/chapter/47