Imagine that someone, while at the North Pole, throws a ball to someone at the equator. While the ball was flying, the Earth turned a little on its axis, and the catcher managed to shift to the east. If the thrower, aiming the ball, did not take into account this movement of the Earth, the ball fell to the west (or to the left) of the catcher. From the point of view of a person at the equator, it turns out that the ball flew more to the left than it should, from the very beginning – as soon as the thrower released it – and until it landed.
According to the laws of Newtonian mechanics, in order for a body moving rectilinearly to deviate from the initially specified trajectory, some external force must act on it. This means that the one who catches at the equator must conclude that the thrown ball has deviated from the rectilinear trajectory under the influence of some force. If we could look at a flying ball from space, we would see that in fact no force was acting on the ball. The deviation of the trajectory was caused by the fact that the Earth had time to turn under the ball while it was flying in a straight line. Thus, whether some force acts in a similar situation or not, it entirely depends on frame of referencein which the observer is.
And such a phenomenon inevitably occurs when there is some kind of rotating coordinate system – for example, the Earth. To describe this phenomenon, physicists often use the expression bogus force, bearing in mind that the force is “really” absent, it just seems to an observer in a rotating frame of reference that it acts (another example of a fictitious force is centrifugal force). And there are no contradictions here, since both observers are unanimous about the real trajectory of the ball and the equations that describe it. They differ only in the terms they use to describe this movement.
The fictitious force that acts in the above example is called Coriolis force – in honor of the French physicist Gaspard Coriolis, who first described this effect. Interestingly, it is the Coriolis force that determines the direction of rotation of cyclone vortices, which we observe in images obtained from meteorological satellites. Initially, air masses begin to rush in a straight line from areas of high atmospheric pressure to areas of low atmospheric pressure, but the Coriolis force causes them to spiral in a spiral. (You might as well argue that the air currents continue to move in a straight line, but as the Earth rotates beneath them, it seems to us on the surface of the planet that they are moving in a spiral.) Let’s go back to the example of throwing a ball from the pole to the equator. It is easy to understand that in the Northern and Southern Hemispheres, the Coriolis force acts on a moving body in directly opposite directions. That is why in the Northern Hemisphere, cyclone eddies seem to be swirling counterclockwise, and in the Southern Hemisphere – clockwise.
Hence comes the popular belief that the water in the sewer openings of bathtubs and sinks in the two hemispheres rotates in opposite directions, allegedly due to the Coriolis effect. (I remember when I was a student myself, the whole group, including one Argentine, spent more than one hour in the men’s closet of the Faculty of Physics at Stanford University, watching the streams of water in the sink, hoping to confirm or disprove this hypothesis.) In fact, Although it is true that the Coriolis force acts in the opposite direction in the two hemispheres, the direction of the swirling of the water in the drain funnel is only partly determined by this effect. The fact is that water flows for a long time through water pipes, while currents are formed in the water flow, which, although they are difficult to see with the naked eye, continue to swirl the water stream even when it pours into the sink. In addition, similar currents can be created when water flows into the drain hole. They determine the direction of movement of water in the funnel, since the Coriolis forces are much weaker than these currents. In ordinary life, the direction of swirling water in the drain funnel in the northern and southern hemispheres depends more on the configuration of the sewer system than on the action of natural forces.
However, nevertheless, a group of experimenters was found who had the patience to repeat this experiment in “clean” conditions. They took a perfectly symmetrical spherical sink, removed the sewer pipes, allowing water to pass through the drain hole freely, equipped the drain hole with an automatic shutter that opened only after any residual currents in the water had calmed down – and they saw the Coriolis effect in action! Several times they even managed to see how the water, first under a weak external influence, twisted in one direction, and then the Coriolis forces took over, and the direction of the spiral changed to the opposite!