Experiments · High school

Foucault pendulum

Pick a latitude and swing the pendulum: its plane slowly turns, the pins fall and the Earth’s rotation shows itself.

In the learning section below the figure, predict, try, complete the tasks and check yourself with short questions.

Fig. 1Foucault pendulum

The floor stays still; the plane of swing turns.

Pick a latitude, press play and watch the pins fall. Switch the view to see the same event through an eye that turns with the pendulum.

From space: the Earth and your latitudeLatitude 41.01°. The floor turns about the local vertical by sin φ of the Earth's spin.

Period T: 16.4 s
Turning per hour: 9.87°/h
Full turn: 36.5 h
Direction (from above): clockwise
Pins down: 0 of 36
Time passed: 0 min
Angle the plane has turned: 0.0°

Places

Latitude (south negative)

Pendulum length

Time speed

Learn

Progress is kept on this page only; no points are given.

1Predict
2Try
3Explain

If you shorten the wire from 67 m to 2 m, what happens to how fast the plane of swing turns each hour?

Purple: the plane of swing and the latest mark in the sandGold: the angle the plane has swept since the startThe numbers come from the model in this article: the Earth turns once in a sidereal day (23 h 56 min 4 s), the plane turns against the floor by 15.04° × sin φ an hour, and the period is T = 2π√(L/g) (g = 9.81 m/s²). The pins stand every 10°, the first one 5° from the starting plane; because the ball swings to both ends of the plane, they are all down once the plane has turned 180°. Changing the latitude starts the experiment again. The drawing is not to scale; when time is sped up, the swing is shown slowed down so the eye can follow it, and the true period is in the readout. The view from space shows only the floor turning about the local vertical (Ω · sin φ).

The Foucault pendulum: seeing the Earth turn with your own eyesRead the article