eli5: how Foucault pendulum works
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It’s a pendulum with the added feature of a frictionless ball bearing where the rope attaches to the ceiling.
What this means is that the ceiling can spin in circles and not transmit that spinning to the rope of the pendulum, so the pendulum swings back and forth in a straight line forever.
Since the Earth is spinning on it’s axis, and the building is attached to the Earth, and the ceiling is attached to the building, the ceiling is spinning. But the ceiling cannot transfer that spinning to the pendulum because of the special connection.
So the typical experiment is array 24 cylinders around the room in a circle at the ends of where the pendulum swings. As the Earth spins, the room spins, and the pendulum stays in the same place. So eventually a cylinder will move into the path of the pendulum and get knocked down.
It’s a way of showing that the Earth spins on an axis and is not stationary with the universe spinning around it.
It “works” just like any pendulum. An initial force pushes the pendulum up on one side, gravity pulls it back down and its momentum carries it back up the other side. Eventually friction will cause it to lose momentum and become still again, so some sort of continuing force will be necessary to keep it in motion.
Foucault used pendulums to demonstrate the rotation of the Earth. The Earth is rotating under the Pendulum. So when it swings from side to side it will cross over different parts of the ground since the ground is moving relative to the pendulum.
The Earth rotates, including the building containing the pendulum. The bearing suspending the pendulum prevents the building from transferring that rotation to the pendulum. As a result, the apparent rotation of the pendulum reveals the rotation of the Earth.
The ‘bearing’ referred to elsewhere is not really required for this demonstration. The rotation of the earth will ‘twist’ the suspending wire by just 1 turn (or less) per day. The effect of this (if any) would be to rotate the pendulum mass **about the axis of the wire,** NOT to tend to change the plane swept by the motion.
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