My birthday was spent at a campsite in Kampala, and after an early departure from there, we stopped on the equator for breakfast. This was coming north to south – we'd already crossed it once in Kenya , but they make more of a thing about it here. Either side of the road are large white rings, with the line running at a slight angle across the road; I marked the occasion by standing in the northern hemisphere and pissing into the southern.
There is a well known, and enduring myth that water drains in a clockwise direction in the northern hemisphere, and a counter-clockwise direction in the south; in reality the Coriolis force, which in principle would cause such an effect, is far too weak, but this does not stop some physics textbooks stating it as fact (the authors are either too stupid or too lazy to actually do the maths). Also some entrepreneurial guys on the equator, are ready with conical shaped water containers to 'demonstrate' the effect.
Now it's time for me to put on my geek hat (OK I never really take it off). The Coriolis force is a so called pseudo or fictitious force, which simply means that it arises (along with the better known centrifugal force) when one transforms Newton's laws from an inertial frame (i.e. non-accelerating) into a rotating frame, such as the Earth. It is a 'real' force from the point of view of an observer in the rotating frame, and on the Earth it produces real effects, such as the prevailing winds and ocean currents, and is also an important factor in long range gunnery calculations. Formally it produces an acceleration on an object that is perpendicular to both the angular velocity of the frame and the velocity of the object relative to the frame, with a magnitude that is proportional to twice the angular velocity of the frame; the speed of the object relative to the frame; and the sine of the angle between the angular velocity and the object's velocity.
Possibly the simplest example to give is that of two people on opposite sides of a counter-clockwise spinning merry-go-round, trying to throw a ball to each other. If they throw the ball straight at each other, the ball with always veer to the left of the catcher. From the point-of-view of an observer standing in the park, the ball has simply travelled in a straight line and missed the catcher. However on the merry-go-round, the angular velocity is vertical, the ball's (initial) velocity is horizontal towards the centre, so if the merry-go-round rotates once every, T, seconds and the ball is thrown with a speed, v, it will have an acceleration to the right of 4πv/T - putting in some plausible numbers of T = 3 seconds, and v = 20m/s, then the acceleration will be approximately 80m/s2, which is quite large at more that 8 times acceleration due to gravity.
Now back to draining bowls of water; at the north (or south) pole the geometry is simplest and the effect largest. The Earth's angular velocity is vertically upwards, and water molecules will travel towards the centre of the bowl, so the set-up is similar to the merry-go-round. Again putting in some numbers, we have T = 1 day, and we assume v = 0.1m/s, this gives an acceleration of 1.45×10-5m/s, or about one seven-hundredth that of acceleration due to gravity. To put it another way, by the time the molecule has travelled 30cm, it will have acquired a velocity to the right of 3.8 metres per day – some glaciers move faster that this. The point is that the Coriolis force, when applied to a bowl of water, is far too small to affect its direction of drainage – other factors such and the shape of the bowl; the way the water is poured in; and the way the plug is removed will have a far great effect.
But there's more: On the equator, the surface of the water is parallel to the Earth's axis, so there will be no Coriolis forces acting in a way to spin the water. A few metres either side of the equator, the geometry means that the Coriolis forces will be less that a millionth of their (already weak) strength at the poles. So ironically, the worst place in the world to 'demonstrate' this effect is near the equator. Now the reason I go on about this, is that many people, on seeing the 'demonstration' believe it in the face of long established scientific knowledge, and if people are so easily convinced by a little slight-of-hand, peppered with some pseudo science, then they'll believe in crystal healing, homoeopathy, and, most worrying of all, that the MMR vaccine causes autism. One person on our truck even said “we can believe it if we want, perhaps science is wrong”. Sigh.
