Based on function/movement:
* Static: If you're talking about the pendulum's movement, "static" is the opposite of its back-and-forth motion.
* Fixed: This implies something that is not moving or swinging.
* Immovable: Similar to "fixed," it emphasizes the lack of movement.
Based on purpose:
* Clockwork: If the pendulum is part of a clock, "clockwork" could be an antonym in the sense of something not regulated by a pendulum.
* Quartz: This is a common alternative timekeeping mechanism.
Based on concept:
* Stability: If the pendulum represents uncertainty or oscillation, "stability" is the opposite.
* Equilibrium: This emphasizes a state of balance and lack of movement.
Ultimately, the best antonym will depend on the specific context in which you're using the word "pendulum."