July 28 Whoops, I am six (6!) articles behind! Lots happening in these last few weeks, but that's no excuse. I promise not to inflict all six (6!) on you right now, though.
Maybe 3. So this column takes a look at something I grew up taking for granted, and maybe you've heard it too: the water in a flushed toilet swirls clockwise in the northern hemisphere and anti-clockwise in the southern. (Or maybe vice versa, don't quote me.) This is the Coriolis Effect (a name that invariably reminded me of the Aurora Borealis). So I made it a point to stare pointedly, several years ago, at the first toilet I ever encountered south of the Equator. I can't say I was convinced. But I eventually did some reading and ... well, take a look at my Mint column for June 16: The way of the water in tea, toilets, https://www.livemint.com/opinion/columns/the-coriolis-effect-misconceptions-relativity-and-the-tea-leaf-paradox-explained-by-dilip-dsouza-11686848414660.html cheers, dilip PS: If you liked the book Joy Ma and I wrote, "The Deoliwallahs"; if you think the incarceration of Chinese-Indians after 1962 was a tragedy; if you care about justice ... please consider supporting the film Voices of Deoli: https://www.kickstarter.com/projects/voicesofdeoli/voices-of-deoli --- The way of the water in tea, toilets Growing up, I heard innumerable times about the Coriolis effect. I wager you did too. Due to the Earth's rotation, it suggests, water draining from a flushed toilet swirls clockwise in the Northern Hemisphere, and anticlockwise in the Southern. Or vice versa, I can never remember and I wager you can't either. Whichever it was, I couldn't wait to get south of the Equator. When I finally did - Windhoek, Namibia - I looked deeply into every flushing toilet I could find. Regardless of the image that puts in your mind, I couldn't decide whether the swirls in there were opposite to those from north of the Equator. Upshot: I wasn't persuaded of the Coriolis effect, at least as manifested in flushes. Let's try to understand what's happening here. Imagine you're looking down at a large disc that's rotating clockwise. You see a little ball start at the centre and move in a straight line across the rotating disc, to its edge. Nothing very dramatic there. But now imagine your friend Archana standing on the edge of the disc as it rotates. What does she see the ball doing? Not making a straight line, that's for sure. Instead, she will see it curving away from her, to her right. That's the Coriolis effect. Two things to note. One, if the ball was at rest, it would rotate clockwise with the disc. But you see it move in a straight line. Thus there is clearly a force that pushes it to the left of its direction of motion - or anticlockwise. That's what we call the Coriolis force. Two, Archana gets a different impression of what the ball is doing. That's because she has a different frame of reference from yours - on the disc's edge and rotating with it, as opposed to being static above it. Mention of frames of reference, and therefore seeing something happen relative to such frames, might bring Albert Einstein and relativity to mind. Sure enough, Einstein makes an appearance in this story. Hold on. The reason we associate the Coriolis effect with our planet is because our planet spins. If you were stationed directly above the North Pole, you'd see the Earth spinning anti-clockwise below you. Imagine watching a river flow in a straight line from the Pole. To make that straight line, the Coriolis force pushes the river to the right as it flows. But now station yourself directly above the South Pole. Below you, you see the Earth spinning clockwise. A river flowing in a straight line from the South Pole will also be subject to the Coriolis force. It will push the river to the left as it flows. This difference between the two hemispheres is the Coriolis effect. Unfortunately, the force is insignificant at the scale of toilet flushes and the forces at play in there. Any differences in how they drain north and south of the Equator is really because of the shape of the toilets and the force with which water enters them. Not the Coriolis effect. And speaking of rivers. In the 1850s, a French physicist called Jacques Babinet and a German scientist called Karl Ernst von Baer used the Coriolis force to propose the idea that because of the Earth's rotation, erosion would tend to be greater on the right banks of rivers than the left, in the Northern Hemisphere. In the Southern Hemisphere, the reverse: the left banks would be more eroded. This became known as Baer's Law of Stream Deflection. They found various rivers that, they suggested, followed the Law. But their Law has always been treated with some scepticism. For just like with toilets, the Coriolis force is far weaker than the other forces - wind, or snowmelt-driven increased flow of water - that work on the soil on the banks of any of those rivers. As a result, Baer's Law is more or less ignored today, just a relic of history. Cut to another phenomenon that you may have observed - that I did just a half hour before writing these words. Take a cup of hot water and drop some tea leaves into it. To help the brewing along, stir the water with a spoon. You'd expect that centrifugal force, caused by the whirling of the water, would push the leaves to the edge of the cup. After all, when you tie a stone to a long string and whirl it around your head, and then let go, the stone flies away from you because of centrifugal force. Surely similar with tea leaves? But get ready for the tea leaf paradox. As you stir, the leaves move to the centre of the cup, and settle on the bottom. (I just saw this happen with the sugar I was mixing into a fresh-lime and water solution.) Why so? The water certainly does whirl around in the cup. But it doesn't do so evenly: at the bottom, friction makes it spin more slowly than at the top. The centrifugal force, then, is weaker near the bottom than at the top. This causes a different, invisible flow in the water: from the top down along the walls of the cup, inward at the bottom and back to the top. "The tea leaves are swept into the center," wrote Albert Einstein in a 1926 paper, "by the circular movement ... The same sort of thing happens with a curving stream." (Curving to the left, I should point out.) Einstein went on: "The particles of liquid in most rapid motion will be farthest away from the walls [or] in the upper part ... [These] will be driven by the circulation toward the right-hand wall, while the left-hand wall gets the water which comes from the region near the bottom and has a specially low velocity. Hence in [a river curving to the left] the erosion is necessarily stronger on the right side than on the left." ("The Cause of the Formation of Meanders in the Courses of Rivers and of the So-Called Baer's Law", Die Naturwissenschaften, 1926, https://www.ias.ac.in/article/fulltext/reso/005/03/0105-0108). Einstein also considered the Coriolis effect. He wrote that the Earth's rotation, and thus the Coriolis force, can cause this same circular motion even where the river runs straight, but "on a small scale." Again, minimal compared to other factors. All in all, pay no attention to fancy claims about the Coriolis effect. But watch your tea when you next stir it. -- My book with Joy Ma: "The Deoliwallahs" Twitter: @DeathEndsFun Death Ends Fun: http://dcubed.blogspot.com -- You received this message because you are subscribed to the Google Groups "Dilip's essays" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/dilips-essays/CAEiMe8oTEnuHzhftPEW7DXfWvoPLCQ5Fy-h59WMB4E77yYyZhA%40mail.gmail.com.
