David McNaughton

(From A Science Miscellany, chapters 10-11; Vantage Press, New York, 1992)

Centrifugal Effect

Everything on a revolving platform behaves as if there was a mysterious force pulling outwards along all radii of the circle. Tall objects tend to topple over outwards; flatter ones try and slide off. Some people refer to this as "Centrifugal Force", but it is better to call it the "Centrifugal Effect", because there is really no force there at all; it is just a type of illusion created by the rotation.

When studying circular motion, you will also hear about "Centripetal Force", which pulls in towards the centre. This is a genuine force, necessary to keep an object in a circular path instead of flying off at a tangent (i.e. in a straight line). For example, a heavy weight tied to a rope and being whirled round is constantly being pulled towards your hand by the rope, but deviating only just enough to remain travelling in a curve. More force would be required to bring it in closer. Similarly, if you wish to stay on a large rotating platform then some kind of Centripetal Force has to hold you – either its bars or friction from its surface – always pulling you inward towards the middle.

Thus, a semi-intelligent insect living in the woodwork might conclude that there was sometimes a type of gravity there trying to attract everything outwards (quite independently of Earth's gravity, which pulls downwards).

The Centrifugal Effect is used to separate cream from milk, and to drain water out of clothes in a spin-drier. It could also be utilised to create artificial gravity in a large doughnut-shaped space station. To simulate Earth's gravity, that structure would need to rotate at the appropriate speed.

Coriolis Effect

Imagine a gun in the Arctic, trying to bombard an enemy position at the North Pole. A projectile fired from that gun will acquire a slight sideways velocity – because the Earth’s rotation is carrying the gun to its right. Thus, it will be necessary to aim slightly left of the target. (Of course this ignores the wind, which can blow from any direction and is usually the more important factor).

A similar requirement holds when shooting from the North Pole back at the other people – because in this second instance the target will become displaced while the projectile is in flight, making it vital for this gun too to aim to its left.

There is a law applicable to any object travelling in the Northern Hemisphere: it behaves as if there was a mysterious force pulling to its right – always acting perpendicular to its direction of movement. It is often called the "Coriolis Force", but there is not actually any force present – only the illusion of one (as with the Centrifugal Effect). Thus, we should really refer to the "Coriolis Effect". That mysterious displacement appears to happen only because it is referred to Earth's surface - but it is of course really the surface which is undergoing the displacement.

In the Southern Hemisphere the Coriolis Effect appears to pull the opposite way, to the left. Why is it different there? As our planet spins, a distant observer looking down on the Antarctic would see it rotating clockwise (instead of counterclockwise as at the North Pole). Remember too that someone standing on the South Pole is upside down from the Northern Hemisphere viewpoint.

There is an easy experiment which demonstrates the Coriolis Effect. One person sits near the outside edge of a revolving platform and tries to throw a ball to someone near the middle. Both people will soon discover that it is no use aiming directly at the other.

The Coriolis Effect gradually decreases as you travel away from the poles, which is why it is most easily visualised over the Arctic or Antarctic. On the equator its value is zero. It is never noticed when walking or driving a car, because you get carried with the ground as the Earth revolves. However, this phenomenon does mean that rifles whose sights have been set correctly for Alaska or Norway, should really be adjusted when taken to the Falkland Islands or to New Zealand (where they will need to aim very slightly to the right instead of to the left).

Meteorologists would not be able to explain windflow without the Coriolis Effect. If the Earth was not rotating, air would tend to blow from regions of high pressure towards low-pressure centres. The Coriolis Effect causes air pockets to swing round as they move; (once again, Earth's surface is taken as the reference-frame). The wind-pattern cannot settle down until the pressure force balances the Coriolis Effect. Thus, equilibrium is not achieved until the wind blows parallel to the isobars (which are lines of equal pressure). That is indeed what we tend to find in Nature.

 David McNaughton
E-mail: DLMcN@yahoo.com

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