The Magic of Coriolis

Sangluozhou was originally a sandbar at the bend of the Yangtze River in the northeast of Jiujiang City, Jiangxi Province. According to research, it is now located in the jurisdiction of Huikou Town, Susong County, Anqing City, Anhui Province. It was once a famous ancient battlefield. In 410), the peasant uprising army led by Lu Xun defeated Liu Yi, the general of the Beifu army in the Eastern Jin Dynasty. However, in the Tang Dynasty hundreds of years later, great changes have taken place here. The meaning of Hu Ping’s poem is: Don’t ask about the change from the sea to the mulberry field, you only need to look at the mulberry land in the Yangtze River to understand. There are several newly built houses on it. Maybe you never thought that it used to be the place where the Yangtze River flowed. The ancient river bank is collapsing one after another, but the river beach on the other side is increasing day by day, and it seems that there is no trend to stop.

After reading this poem, a careful person may think about this question: Why is one side of the river collapsing while the other side is growing? In fact, the reason is very simple. The river water always moves towards the low-lying place in the process of flowing, so it always goes around looking for the most favorable flow path. As the river flows, it erodes the river bank, trying to make the river channel wider. Even if the river bank is a hard rock, it is difficult to resist the scouring of the flowing water for a long time. In particular, the concave bank of the river channel (the concave bank and the convex bank on the opposite side) received the greatest impact. At the concave bank, the surface water rushes to the bottom of the river along the slope, and then carries the sediment to the convex bank, forming a curved circulation. The sediment on the convex bank of the river channel mainly comes from the scour of the concave bank, and the siltation amount of the convex bank and the scour of the concave bank are basically balanced. As a result, the concave bank gradually retreats, and the convex bank pursues closely, and the riverbed becomes more and more curved.

Isn’t Hu Ping’s vivid poems showing us the trend of river development and change? In geological terms, it is the lateral erosion of the river, also known as lateral erosion and lateral erosion. The result is concave bank erosion and convex bank accumulation.

When we observe the lateral erosion of the river, we need to do further thinking. What is the force that causes it to treat the two sides of the river differently? After long-term observations, geologists have found that this phenomenon is common on the earth: the right bank of the river in the northern hemisphere (the right side of the river’s advancing direction) is relatively steep, while the left bank is relatively flat, and the opposite is true in the southern hemisphere. The reason is that there is an invisible force behind it, called Coriolis force.

The Coriolis force was first discovered by the French mathematician and mechanical engineer Coriolis in 1835, but it was not named after him by the international academic community until the early 20th century, also known as the geostrophic force. This is a magical force created by the Earth’s rotation, which can deflect horizontally moving objects to the right in the northern hemisphere and to the left in the southern hemisphere.

On the Rhone River in Avignon, a city in southeastern France, there is a stone bridge that is listed as a world cultural heritage – the Saint Benezer Broken Bridge, which has a long history of more than 800 years. Originally, the San Benezer Bridge was about 900 meters long and 4.9 meters wide, with a total of 22 arches, but after the floods of 1644 and 1669, only 4 arches survived. Strangely, the washed-out parts are all on the right bank of the river, and the broken bridge that survives is on the left bank, which is one of the important examples of Coriolis forces forcing rivers in the northern hemisphere to erode the right bank.

In 2016, Chinese scholars Song Xiangyang and others investigated the distribution of cities on both sides of five rivers, including the Yangtze River, Jialing River, Minjiang River, Xiangjiang River, and Ganjiang River, and found that most of the cities are on the right bank of the river, with a total of 22, with Shanghai, Nanjing and other large and medium-sized cities. There are mainly cities, there are only 11 cities on the left bank, and their economic development and population are not as good as those of the right bank cities. The reason is that the river scours the right bank more seriously, making the river bed deeper and more suitable for use as a waterway or for the construction of ports and wharves, which in turn drives the formation and development of cities. In contrast, in the Amazon River Basin in the southern hemisphere, there are more large cities on the left bank of the river, and the degree of economic development is also better than that of the cities on the right bank.

Coriolis force can only change the direction of motion of the object but not the speed of motion of the object. The magnitude of this force depends not only on the mass of the object itself and the speed of horizontal motion, but also on its position on the earth. It can play an important role in large-scale space on the earth. For example, in the atmosphere, it can roll up tropical cyclones. In the northern hemisphere, they are large vortices that rotate counterclockwise, and in the southern hemisphere, they rotate clockwise; In the ocean, due to the combined action of the prevailing wind and the Coriolis force, a huge clockwise ocean circulation is formed in the northern hemisphere, and a counterclockwise ocean circulation is formed in the southern hemisphere.

If we don’t understand Coriolis force, it can cause some trouble. During the First World War, Germany used artillery to attack Paris more than 110 kilometers away. The strange thing is that the shells fired always deviate from the target to the right. . Coincidentally, during the “World War I”, the German and British fleets fought large-scale naval battles in the Falkland Islands in the southwest of the Atlantic Ocean, and the British artillery shells always missed the German warships. It was only later understood that it turned out that the British had made a common-sense mistake, that the cannons they calibrated in the northern hemisphere were no longer applicable when they arrived in the southern hemisphere. The shells fired in the northern hemisphere are always deflected to the right, and in the southern hemisphere, they are always deflected to the left, and the deflection direction is completely opposite. If not recalibrated, the error will be doubled.

For sports such as rifle shooting and archery, the accuracy of shooting mainly depends on the influence of gravity, wind direction and wind speed, but it is also affected by geography. Some scholars believe that in order to achieve good results in sports competitions, athletes should arrive at the competition venue for adaptive training some time in advance, especially when the daily and official competition venues are located in the southern and northern hemispheres, respectively.

In many cases, the Coriolis force can often be ignored, especially for some local and microscopic phenomena, it is difficult to use it to explain. One day in 1962, Schepiro, a professor at the Massachusetts Institute of Technology in the United States, happened to find that the discharged bath water formed a whirlpool in the bathtub. Later, in his experiments, he found that every time the plug of the bathtub was unplugged, there would be vortices in the running water, and these vortices always rotated in a counterclockwise direction. Schepiro believes that this phenomenon is related to the rotation of the earth, and speculates that such a vortex rotates counterclockwise in the northern hemisphere, clockwise in the southern hemisphere, and does not appear at the equator. After Schepiro published his research results as a paper, it aroused the interest of many people, and everyone carried out experimental observations all over the world.

However, many experiments have shown that the “Shepiro phenomenon” is not entirely correct. Through calculation, it is found that the Coriolis force of the water in the bathtub is very small, and vortices in both clockwise and counterclockwise directions may appear. Scholars believe that this is mainly related to whether the drainage process is disturbed by external forces and the design of the structure of the drainage outlet itself. is not entirely determined by the Coriolis force.

Inspired by “the erosion on the right bank of the river is more serious”, someone once proposed a strange way of health preservation-the body is in a north-south posture when sleeping, which can enhance the scouring of the blood vessel wall by the flowing blood, so as to remove the deposited cholesterol and prevent blood vessels. hardening. It sounds plausible at first, but this completely exaggerates the role of the Coriolis force. There is also a saying that it is safer for cars in the northern hemisphere to drive on the right than on the left, because the deflection force of the ground rotation in the northern hemisphere is biased to the right, which can reduce the probability of collision with oncoming vehicles. In fact, this is not true, so far, there is no scientific evidence to say which way of driving is safer. Japan and the United Kingdom, both in the northern hemisphere, are used to driving on the left. In fact, this situation has little to do with the Coriolis force. It is mainly a legacy of historical habits. For example, in the era of knights in the Middle Ages, the United Kingdom used the right hand for convenience. Use weapons to engage with the enemy and keep your heart away from the opponent, so get used to the left-handed principle.