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cfhalcyon t1_iu4pkcb wrote

Tea is not very viscous (as in it is not very thick/sticky consistency) so when you turn the cup, the liquid stays in place, while the outer rim (the cup) rotates. If you did the same thing with a cup of honey, the hair would rotate. You can make the hair rotate in tea by doing repeated circular motion movements (imparting rotational momentum in the liquid over time) or by rotating the cup really slowly so that the liquid flow doesn't detach from the surface.

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Alas7ymedia t1_iu4q0eu wrote

The wall drags only the layer of liquid in contact with it. That layer drags along another layer a little less, that next layer drags along another layer even less and so on. The intensity of the dragging at a certain distance of the wall is the literal definition of viscosity.

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chazwomaq t1_iu4x7vz wrote

The fundamental answer is Newton's first law of motion: things tend not to move unless a force acts on them, in other words the principle of inertia.

In this case, not much force acts on the hair because the water is not very viscous and so does not transmit the rotational force much over distance (compared to more viscous fluids).

If you want to know where Newton's law comes from, I think it "just is" i.e. it's an axiom.

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