Why doesn’t it just stay still and drain? What causes it to spin clockwise or counter-clockwise?
In: Physics
Ever have two people try to walk through the doorway at the same time? They get stuck.
All of the water molecules can’t fit down the drain at once. So while some can go down the drain, most of it can’t. But it has to go somewhere. So if it is moving toward the drain, and can’t go down the drain, then it has to go *around* the drain.
At a molecular level, water isn’t perfectly uniform, and the material of the drain isn’t perfectly smooth. So there will be some bias in which direction around the drain the molecules will “choose” to go around. This creates a circular motion that then will cause all the molecules to choose to go around in that direction and, viola, the vortext is formed.
The rotation of the earth all by itself creates a “Coriolis force” that imparts a tiny bit of rotation to all things on the planet, including water. That tiny rotation reinforces itself when water is drained at a certain rate.
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