Zinnia O'Brien, of Paparoa School, asks :-

Sarah Smith, of Ardgowan School, asked:- Melissa Robertson, of Kingswell High School, asked:-

Why does water always go clockwise down the plughole? Is it true that water always goes anti- clockwise down the plughole in the southern hemisphere but clock-wise in the northern hemisphere?

Keith Dawber, a wind energy physicist at Otago University, responded.

The main feature which affects the way the "vortex" in the plug-hole rotates is the way the water flows before it reaches the hole. This can be affected by initial motion remaining in the water from a person washing their hands or from non-symmetry in the shape and roughness of the surface over which the water is flowing.

There is also an effect caused by the rotation of the earth called the "Coriolis Effect" and it produces an anti-clockwise rotational motion in freely moving fluids in the southern hemisphere and clockwise rotational motion in the northern hemisphere. Within the confines of the bathroom or kitchen, the magnitude of the effect is very small and is usually negligible, but it is very large and important in the oceans and atmosphere where huge masses of fluid and long time scales are involved.

Perhaps of more interest in plug-hole mechanics is the reason why the water goes round at all, instead of flowing directly down through the hole. As the water moves slowly towards the plug-hole it has rotational motion with respect to the centre of the hole. As the radius of motion of the water decreases as it enters the plug-hole its speed increases (because its angular momentum must stay the same) and so it swirls rapidly round in a spiral as it flows through the hole.

In my observations of plug-holes at the equator and in the northern and southern hemispheres, location seems to have no effect. Sometimes I have even seen the direction of rotation change during the discharge, possibly due to the shape and depth and possibly due to the initial angular momentum of the particular bit of water before it reached the hole.