What the time period depends on

The book asks you to think like a scientist. Would every pendulum have the same time period? How would you test it fairly?

Repeat Activity 8.2 with the same bob but strings of two or three different lengths. Does the time period change? Then keep the length fixed and try bobs of different mass. Change only one thing at a time, so you know which change caused the result.

The result: the time period of a simple pendulum depends on its length, but not on the mass of the bob. All pendulums of the same length have the same time period at a given place.

So a pendulum of a given length always swings with the same time period at one place. That steadiness is exactly what makes it useful for measuring time.

It is said that Galileo Galilei, sitting in a church, watched a lamp swinging from the ceiling and timed it with his own pulse, finding that each swing took the same time. Inspired by Galileo's work, Christiaan Huygens invented the pendulum clock in 1656 and patented it in 1657.

All clocks, old or new, work on the same idea: some process that repeats again and again at equal intervals. Modern clocks use tiny, very fast vibrations instead of a swinging bob — a quartz crystal in quartz clocks, or particular atoms in atomic clocks.

The gain in accuracy is huge. Huygens' early pendulum clocks could gain or lose 10 seconds a day. Today's atomic clocks lose only one second in millions of years.