The latter corresponds to a tilting of the floor and can be observed as the rate of the passing of stars viewed directly overhead. It has an effect on the vertical motion of objects; a stone dropped down a mine shaft will land eastward of straight down as defined by a plumb line because in the time it has taken the stone to fall the mine shaft has tilted. On the other hand, for predominantly horizontal motion such as that of air in the atmosphere or the bob in the Foucault pendulum its effect is negligible. We ignore this term in what follows.

The vertical component we' = 10.36º/hr can be observed as the horizontal passing rate of the stars on the horizon. It is important for dynamics involving horizontal
motion. Objects thrown in any direction will appear to drift to the right when viewed from behind because the earth has rotated to the left. In particular, the plane of
oscillation of the Foucault pendulum will precess clockwise at exactly 10.36º/hr when viewed from above because the earth is rotating counter-clockwise at this rate.
In Fig. 2. the motion of the pendulum when released from rest as viewed from above. The magnitude of the Coriolis force and rotation are exaggerated; the correct
scale is b = we' a/w where w is the pendulum frequency.

a) As seen in the earth fixed frame. Both the restoring force Fr and the Coriolis force Fc contribute to the motion.

Fig. 2
b) As seen in the frame of the fixed stars only the restoring force Fr acts in.

Fig. 3

The effect can also be interpreted as being caused by the force introduced by Coriolis and now named after him. Its magnitude for a mass m travelling at speed V is
Fc = 2 m we' V and it always acts at right angles to the velocity vector. An exaggerated view of how it affects the Foucault pendulum is shown in Fig. 2 for a
special case in which the pendulum bob is released from rest. The effect as Foucault rightly guessed is cumulative.

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