As I was mixing my coffee this morning, wishing that it would do it on its own, I started wondering: What makes my coffee stop swirling? I mean what exactly? Is it the walls of the cup or the coffee itself or both? Or something completely else that I am not aware of? Is it the same thing that makes it too sweet as well?
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3$\begingroup$ Read up on viscosity, it'll help! $\endgroup$– Hritik NarayanCommented Dec 15, 2015 at 4:34
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1 Answer
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The friction produced by relative motion makes the swirling stop. If the coffee is uniform then it is the wall of the cup that makes the swirling stop. If the coffee is composed of very different parts that swirl at very different speed, then their relative motion will contribute to the friction as well. Since you are mixing your coffee with some sweet things, the second situation fits better.
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$\begingroup$ I also used the non-soluable type of coffee that stays at the bottom of the cup thanks to which I noticed another thing. There were two whirls in the cup at one point, both going in opposite direction. Would those slow each other down as well? $\endgroup$ Commented Dec 15, 2015 at 9:39
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$\begingroup$ Yes, as long as there is relative motion. $\endgroup$– amasicsCommented Dec 15, 2015 at 12:24