Simple and Physical Pendulums: What the Swing Actually Times

You'll learnwhat actually sets a pendulum's period — length and gravity, not mass or amplitude at small angles.

A heavy bob, a wide swing — surely one of those slows the clock? At small angles, neither does. A simple pendulum keeps time by T = 2π√(L/g); a rigid body needs T = 2π√(I/(mgd)) with I taken about the pivot.

8 STEPS · 6 QUICK CHECKS · v2

PENDULUM LAB

Before you start

What you're looking at

Two pendulums of equal length swinging side by side, one with a light bob at a small angle, one heavier and wider.

The question

Released together, which pendulum takes longer to complete a swing?

Watch for

Both swings keeping the same time despite different bob mass and amplitude, at small angles.

What the symbols mean

L is the string length, θ the swing angle; small angles (θ ≲ 15°) make the motion simple harmonic.

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