Turning Floor
Here's how it works — the pendulum keeps swinging in the same plane, and the floor (attached to Earth) turns underneath it.
Dashed: same plane 6 h later
You are hovering above the Earth. The swing plane holds still. The floor — bolted to the planet — turns underneath it. The dashed line is that same plane six hours later.
T = 24 h / sin(45°)
= 34 h
Only the vertical piece of Earth’s spin shows up as the floor turning: ω × 0.71.
A day of Earth time in 8 seconds. The swing stays at a visible pace on purpose.
The key physics in three points
- 1
The room is bolted to Earth
The building, the floor, and the observer turn with the planet. Nothing in the hall is a fixed reference — it only feels that way because you are riding along.
- 2
The pendulum is not
A free pivot lets the bob swing any way. Once it is moving, inertia keeps it in the same plane, set against the distant stars — not against the tiles under your feet.
- 3
Latitude decides how fast the floor turns
Only the piece of Earth’s spin around the local vertical shows up: ω sin φ. At the poles that is the whole day. At 45° it takes about 34 hours. On the equator the vertical component is zero, so the plane never appears to drift.
Try this
- Set the pole. In Space view the floor completes one turn in 24 hours — pegs fall all the way around.
- Drag to the equator. The floor stops turning. Only the two pegs along the first swing ever fall.
- Switch to Room view at 45°. Now it looks as if the pendulum is wandering. It is not. You are.
Léon Foucault hung a 67-metre bob in the Panthéon in 1851. The swing plane held still; Paris turned. The period at latitude φ is 24 h / sin φ — about 32 hours in Paris, 34 hours at 45°, and never on the equator.