Part of the show Avian Flu, How Flu Spreads and How to Avoid It
Beccy from Huntingdon asked:
When you let the water out of the bath, why does the water spiral down the plug hole?
The shape of the plug has something to do with it, because you get a kind of whirlpool effect. The size of the whirlpool is affected by the size of the the plug hole. Some people believe that the direction of the whirlpool is down to the Coriolis Effect, the conservation of angular momentum which applies to moving things - like air and fluids - as the Earth turns in space. But simple calculations show that actually the Coriolis force is too small to account for the direction that water goes down the plughole, so the whirlpool produced when the plug comes out must be a product of the shape of the sink and the drain hole.