Science
How does a pendulum work?
A pendulum works by swinging back and forth under gravity: when you lift it to one side, gravity pulls it back down, and its momentum carries it up the other side. This steady, repeating swing makes it a natural timekeeper.
See it in motion.
Watch a 2-minute animated lesson that shows exactly how a pendulum works.
Step by step
- 1Lifting the bob to one side gives it stored (potential) energy.
- 2Gravity pulls it back down, converting that into motion (kinetic energy).
- 3Momentum carries it up the far side, then the cycle repeats.
- 4For small swings, the time of each swing depends on its length, not how far it swings.
- 5Friction and air slowly drain its energy, so real pendulums need a push to keep going.
Frequently asked questions
- Why does a pendulum keep a steady beat?
- For small swings, the time of each swing depends almost only on the pendulum's length, so it ticks at a constant rate — which is why it works in clocks.
- What makes a pendulum eventually stop?
- Air resistance and friction at the pivot gradually remove its energy, so without a push it slows and stops.
- Does a heavier bob swing faster?
- No. The swing time depends on length and gravity, not the bob's mass — a heavy and a light pendulum of the same length keep the same beat.