Gravity provides a restoring effect when the bob is displaced sideways. The bob speeds up toward the centre and slows toward either end. The rope keeps the bob at the same distance from the support. Our small-angle model excludes air resistance, rope stretch and friction at the pivot.
Forces, motion and energy · Discover & try
What changes a pendulum’s swing time?
Reading level 2 of 5 · DevelopingChange
Word help
Period
The time for one complete repeating cycle.
Amplitude
The largest displacement from equilibrium in an oscillation.
Pivot
The fixed support about which the pendulum swings.
Watch · Try · Explain
See how it works
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Ideal small-angle pendulum with a massless fixed-length string; no damping or large-angle correction.
What to look for
- Count a complete cycle, not one crossing.
- Change mass while keeping length fixed.
- Compare period with the square root of length.
Explain what you noticed
A separate ideal small-angle pendulum has its length increased from 0.5 m to 2 m at the same gravity. How does its period change?
Keep this idea: For small swings in the ideal model, length and gravity set period; bob mass does not.
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