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Physics, Astronomy & Cosmology / Why is the vortex of water clockwise in the Southern hemisphere and anticlockwise in the northern?
« on: 24/05/2010 14:10:42 »
I'm surprised no-one has mentioned the Coriolis effect. I'm assuming that's the natural force that might influence the direction of flow. It works something like this:
If you were standing on a spinning planet (such as Earth) and you shot a cannon then, from the perspective of someone on that planet, the cannon ball would travel in a straight line. However, because the ball is travelling through a medium which offers resistance (the atmosphere), the cannon ball is actually being pulled round in the same direction the planet is rotating in. So from the perspective of an observer sitting in space the cannon ball will follow a curved trajectory.
The same applies to water falling down a plughole. The water wants to travel in a vortex but it needs something to kick-start it off and decide which direction the vortex will spin. In the absence of any other outside force the Coriolis effect will be the deciding factor - clockwise in the southern hemisphere and anticlockwise in the north. In practise, the planet Earth takes so long to make a revolution, the water is such a small volume and the effect is oblique (unless your bath is actually sitting on the North or South Pole) so it's effect is negligible. Other influences, such as small eddies within the bath water, are what really count.
If you were standing on a spinning planet (such as Earth) and you shot a cannon then, from the perspective of someone on that planet, the cannon ball would travel in a straight line. However, because the ball is travelling through a medium which offers resistance (the atmosphere), the cannon ball is actually being pulled round in the same direction the planet is rotating in. So from the perspective of an observer sitting in space the cannon ball will follow a curved trajectory.
The same applies to water falling down a plughole. The water wants to travel in a vortex but it needs something to kick-start it off and decide which direction the vortex will spin. In the absence of any other outside force the Coriolis effect will be the deciding factor - clockwise in the southern hemisphere and anticlockwise in the north. In practise, the planet Earth takes so long to make a revolution, the water is such a small volume and the effect is oblique (unless your bath is actually sitting on the North or South Pole) so it's effect is negligible. Other influences, such as small eddies within the bath water, are what really count.