* Inertia: The pendulum's massive bob has a lot of inertia, meaning it resists changes in its motion. Once set in motion, it wants to keep swinging in the same plane.
* Conservation of Angular Momentum: This is the key to the Foucault pendulum's magic. The Earth rotates beneath the pendulum, but the pendulum's angular momentum (a measure of its rotational inertia) is conserved. Since the pendulum is free to swing in any direction, the Earth's rotation causes the plane of the pendulum's swing to appear to rotate relative to the Earth.
* Friction: Of course, there's always some friction in the system (air resistance, the pivot point), which will eventually slow the pendulum down. This is why Foucault pendulums often have an electromagnetic system to gently "push" the bob, keeping it swinging for longer periods.
Think of it this way: Imagine you're on a spinning carousel, holding a string with a weight attached. If you let go of the string, the weight will continue to move in a straight line, even though the carousel is rotating. The Foucault pendulum acts similarly, the Earth's rotation is like the spinning carousel, and the pendulum's plane of swing remains relatively fixed in space.
It's not that the pendulum is actively *moving* due to the Earth's rotation; it's that the Earth is rotating *beneath* the pendulum, causing the apparent change in the pendulum's swing plane.
