Foucault Pendulum

Plane of pendulum oscillation appears to change due to rotation of Earth.

What it shows:

Due to the rotation of the Earth, the plane of oscillation of a pendulum will rotate with respect to the surface beneath it. We expect a rotation of about 10˚/hr at our latitude of 42.˚

How it works:

Here the observer standing on the Earth resides in the reference frame, with the swinging pendulum oscillating in a rotating frame. From the pendulum's point of view, it keeps oscillating in the same plane, but the Earth spins below it. The deflection from its original plane of oscillation as far as the observer is concerned is due to the coriolis force given by

a = -2ωx v

where v is the horizontal velocity of the pendulum bob and ω is the angular velocity of the Earth. It will act to divert the direction of the pendulum bob as it swings (see figure 1), rotating the plane of oscillation clockwise in the northern hemisphere and anti clockwise in the southern.

figure 1: The changing trajectory of the pendulum bob with each swing (angle of deviation greatly exaggerated).



The final proof of a rotating Earth was performed by Jean Leon Foucault in 1851. He suspended a 67m long pendulum from the dome of the Pantheon, Paris. The canon ball bob etched a trail in the sand below it as it swung, recording a rotation speed of 11° 15' per hour for that latitude.


1. H. H. Gottlieb 1981 The Physics Teacher, Sept. 1981, 421-423
2. W.A. Hilton 1978 Am. J. Phys. 46(4), 436-440
3. R. W. Sinnott 1980 Sky and Telescope, Oct. 1980, 330-332
4. C. L. Strong 1958 Scientific American, June, 1958, 115-124