Why do pendulum clocks keep time?

Answer: At a fixed location, small swings of a fixed-length pendulum take nearly equal times

Air resistance always sets exactly the same speed for each swing — Air resistance dissipates energy rather than setting an exact swing speed. The bob speeds up and slows down within every swing; it is the nearly repeated period that provides the clock’s regular beat.

At a fixed location, small swings of a fixed-length pendulum take nearly equal times ✓ — Small swings of a fixed-length pendulum take nearly equal times at a given location. The clock counts those swings, while its mechanism replaces energy lost to resistance. Swing time also depends on gravity, and large swings break the small-angle approximation.

The clock’s gears impose a fixed swing time regardless of pendulum length — The gears do not override the pendulum’s natural swing time. The pendulum regulates the mechanism’s release, so adjusting its length changes how fast the clock runs.