This morning I made myself some coffee. There was cream on it as how I liked my coffee. I lifted my mug and rotated it so I can sip. Then I realized the cream on coffee wasn't rotating with the mug. It was in rest, the mug was rotating but the pattern of cream on my coffee was looking exactly the same. So the coffee had to be in rest or it was rotating the opposite direction I rotated my mug but why? Can anyone explain this phenomena or say the name of it?
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1$\begingroup$ As an interesting and semi-relevant link here is an example of what can happen when the liquid in the container is incredibly viscous and DOES rotate when the container rotates... youtu.be/j2_dJY_mIys $\endgroup$– FogmeisterApr 10, 2020 at 10:11
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$\begingroup$ It does rotate, it just takes awhile. $\endgroup$– FattieApr 10, 2020 at 11:20
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2$\begingroup$ This is also the source of much frustration when drinking hot chocolate with marshmallows in it $\endgroup$– Luke SawczakApr 10, 2020 at 18:20
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2 Answers
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Suppose your cup was full of a lump of stuff that had no friction. Then as you rotate your cup there is no force between the cup and the stuff, and so it would not rotate. In the case of coffee - the friction is from the viscosity of the coffee. But, as your cup pulls some of the coffee around with it right at the edge, the rest of the coffee slips on that layer, rather like if you slip on a patch of coffee on the floor. Eventually, if you keep rotating the coffee, say on a turn table, the small friction will rotate all the coffee. Then, if you stop rotating the cup, the coffee will keep rotating from its inertia, until the small amount of friction has brought it to a halt.
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10$\begingroup$ i.e. it does rotate...just not very quickly in response $\endgroup$– BradApr 8, 2020 at 15:54
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19$\begingroup$ Or from the other direction, if you stir your coffee with a spoon so that the liquid rotates, why doesn't the cup rotate with it? Now suppose you put the cup on a frictionless turntable and stir it, what happens? $\endgroup$– jamesqfApr 8, 2020 at 15:54
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15$\begingroup$ @jamesqf Just to really drive it home; now replace the low-viscosity liquid (coffee/tea/water) with a high viscosity liquid like a syrup and stir the liquid (or spin the cup), and you would see that the effect is much more apparent. $\endgroup$– JMacApr 8, 2020 at 19:16
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2$\begingroup$ I think this answer might be improved if you mention the name of this phenomenon: laminar flow. en.wikipedia.org/wiki/Laminar_flow $\endgroup$ Apr 8, 2020 at 22:40
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2$\begingroup$ @Stian : You are describing a (false) contrast between transient laminar flow and fully developed laminar flow. $\endgroup$ Apr 9, 2020 at 16:30
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I would think inertia is the primary reason the liquid doesn't start moving immediately. There will be a point where, once the container is rotating fast enough, the liquid will start to rotate (vortex). Once inertia is overcome, the liquids rotation will accelerate and eventually approach the speed of the container. Has a bit in common with the photoelectric effect, there needs to be an initial energy input that overcomes inertia (analogous to the work function).