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Whenever I wash the dishes, I realize that water always tend to acquire some kind of rotation with respect to the axis passing through the hole of drain. I'm not sure if my observations account for only one direction of rotation, but the rotation is certain every time. I think about a box with sand and a hole in the bottom, and I'm pretty certain that it should fall off in (almost) straight lines, so I want to know if this is a unique behaviour shown only in fluids, and why it is that way.

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it is because of the conservation of angular momentum.

https://www.youtube.com/watch?v=M6PuutIm5h4

The water in the drain is always spinning very slowly, the only way it wouldnt rotate if it is small enough to not be affected by the coriolis effect of earth and if nothing disturbs the water. What you see when it is draining is that the linear rotation speed becomes evident as the water concentrates in a small area, because the revolutions per minute increase

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    $\begingroup$ Then it applies in the same way with sand or some gas, but only to a greater or lesser extent (harder to measure) ? $\endgroup$ – Pablo Navarrete Jun 30 '19 at 21:48
  • $\begingroup$ it isnt observed in sand because sand damps the rotation, you cant revolve sand as if it was water, but the conservation of angular momentum is observed in gases, for example in a tornado or hurricane $\endgroup$ – Sartem Cacartem Jun 30 '19 at 22:18
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It is caused by the Coriolis force. It is caused by the earths rotation. There are pretty good explanations when you google it.

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    $\begingroup$ it is not the coriolis effect, it is the initial speed given by the tap, the coriolis effect is very low compared to that $\endgroup$ – Sartem Cacartem Jun 30 '19 at 21:42
  • $\begingroup$ You are totally right. I missed the whole point of the question. Sorry to all people that i confused. $\endgroup$ – roran_physician Jul 1 '19 at 7:28

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