From the Cabinet of Physics: Riding Along with Foucault’s Pendulum

The pendulum named for Léon Foucault (1819–1868) can be found in museums and tall buildings here and there across the world.

(Here in Illinois, there was for many years a Foucault pendulum swinging in the majestic 15-story atrium of Wilson Hall, Fermilab's headquarters building. When I worked in that building, I saw it every day. But it's no longer there; the pendulum was removed in 2010.)

For best effect, a Foucault pendulum usually is usually suspended from a long cable. At a glance, the pendulum is moving in a flat vertical plane. Each moment, the straight line formed by the cable is a line in this plane; each moment, the location of the pendulum bob, at its bottom, occupies a point in this plane.

As one watches the pendulum swing slowly, back and forth, back and forth, there is a subtle change. The plane of its movement appears to shift very slightly. Watch it for hours (or come back hours later), and a considerable movement will be apparent. A swing that started out, for example, moving north-to-south and swinging back south-to-north will, many hours later, become east-to-west.

The reason for this slow shift is this: While the pendulum is moving in an unchanging plane, the Earth beneath it is rotating, as are the walls and floor of the room.

In this video, we see a small Foucault pendulum in the Cabinet of Physics collection. It's mounted on a rotating platform. We can clearly see the behavior of the pendulum. The effect becomes more dramatic when our video camera is attached to the platform, riding along with the rotation. Then, from this new point of view, the plane of the pendulum's appears to shift relative to the platform—just as the big Foucault pendulums in museums do.

At the time the Foucault pendulum was invented, nobody really doubted that the Earth rotates. Nevertheless it remains an enjoyable demonstration, and can perhaps get students of physics and astronomy thinking about rotating coordinate systems.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

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