Walter McCahon, of Cashmere High School, asks :-

Why is it that toilets flush and plug holes drain in different directions in different hemispheres?

AAS p104c

Chelsea and Hannah, of Balclutha Primary School, asked:-

Is it true that the plug hole water spins the way that earth spins? We have looked at loads of web sites and some say that it is a myth. The last web site we looked at said that it was the way that the earth spins.

Ken Morison, a chemical and process engineer at the University of Canterbury, responded.

Plug hole water doesn’t spin in the same direction as the Earth. Such a phenomena is expected because of the Coriolis Effect but it would happen only in a perfect world. It does not happen in practice.

The theory considers the inertia that moving objects (and water) have. When they rotate this causes interesting results which we can see in the following experiment.

Make a hole about 6 mm in diameter in a flat bottom round container and take it to a playground round-about. Hold the container filled with water at the centre of the round-about with your finger plugging the hole from underneath and get someone to rotate the round-about. You can put some small pieces of dry grass on top to see how steady the water is. When the water is rotating at the same speed as the round-about remove your finger. The water will drain out in the same direction as the round-about. Try rotating in the other direction too. Before you leave the round-about please wipe the water off it.

The Earth is just like a large round-about but it rotates in space only once per day. In the northern hemisphere it rotates clockwise but in the southern hemisphere it rotates anticlockwise. If there were no other forces or disturbances, the same effect would occur, but in practice the shape of the sink or basin and very small flows in the water determine the direction of draining.