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Foucault's pendulum the spinning Earth


This really simple experiment, which you could carry out easily if you school has a stair well, proved that the Earth was spinning on its axis. If you lived at the North Pole you could also show that it takes one day to make one revolution.

It was first performed on a large scale by the French scientist Jan Bernard Foucault in 1851. He hung a 28 kg brass sphere with a spike on the base from the roof of the Pantheon in Paris by a 67m long wire and on 31st March set it swinging in one plane. At each end of the swing the spike grazed through a heap of sand.

As time went by it looked as if the plane of swing of the pendulum was changing but this is not what was happening. The pendulum with its heavy bob stays swinging in the same plane because of its large inertia while the Earth rotates beneath it.


The apparent movement of the pendulum is in a clockwise direction in the northern hemisphere but it would move in the opposite direction in the southern hemisphere. The rate of rotation measured using this experiment depends on where it is done. If the experiment had been done at the pole the rotation period would have been 24 hours, while at the equator it would have looked as it the Earth was not rotating at all. At latitudes between the pole and the equator the rotation period would have been more than 24 hours (32 hours in Paris and 31 hours in London.

The time rotation in seconds (T) for a latitude A is:
T = 86164/sin A therefore for latitude 51o the time period is: T = 86164/0.7771 = 110872s = 30 hrs 48 mins.

However in Nairobi (Kenya) (1.3o S) the period would be an enormous 91 hrs 26 mins

It is very easy to see this from the following diagrams.



 

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© Keith Gibbs 2020