A pendulum's period answers to exactly two things: $T = 2\pi\sqrt{\dfrac{L}{g}}$ — read aloud: T equals two pi, root L over g. Length upstairs, gravity downstairs, and the mass nowhere: hang a brass bob or a potato from the same string and they keep identical time. Galileo spotted it in a swaying cathedral lamp, timing it against his own pulse — the heavy lamp and a light one would have agreed.

Keep one anchor in your pocket: a 1 m pendulum swings once in almost exactly 2.0 s. Every answer in this lesson should orbit that fact — quadruple the length only doubles the period, because $L$ lives under a square root. Estimate first; the arithmetic then has a rail to run on.