Polar Pendulum
For a pendulum swing at the North Pole it is shown that the surface velocity vectors of the Earth underneath one side of the pendulum swing are directed in opposition to the velocity vectors underneath the other side of the swing (see Figure 1). The Earth's rotation can then be observed in relation to the pendulum swing. At the North Pole the support-point of the pendulum swing is located on the axis of rotation of the Earth. No matter what vertical orientation is established by the plane of the swing, the relative velocity vectors of the Earth's surface on opposite sides and equidistant from the center point of the swing will be in opposition. The Earth turns underneath the plane of the pendulum swing that is established.
The support-point of the connection turns with the Earth and is depicted as freely suspended above the Earth. The plane of the pendulum swing does not turn with the support point and is not affected by the turning of the support point. The bob is free to swing in any direction in relation to the support (refer to a Charron chuck). For a pendulum at the pole, the plane of swing remains aligned with a star that the bob is swinging toward. A spot on the bob, however, turns with the support since the bob is not independent of the support point. The spot on the bob does not remain aligned with a star that the bob is swinging toward. This occurs whether the pendulum is swinging or not. If the bob is not allowed to turn with the support-point one twist of the connecting wire will occur in one day at the pole. If the connection apparatus of the pendulum to the bob and to the support were in a fixed relationship (and the connection not allowed to twist, for example, using a rigid rod pinned into place between two sides of the support), then once the plane of the pendulum swing is established in one direction, that plane would be forced to turn with the support and connection. The Earth's rotation would not be observable in relation to the pendulum swing with this type of connection.
Read more about this topic: Foucault Pendulum Vector Diagrams
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