CoachA mechanical oscillator is a system that, displaced from a stable equilibrium position and released, swings back and forth around it: these are free oscillations. The motion repeats after the period (); the frequency (, Chapter 20); the amplitude is the maximum displacement.
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Example 28.2 (A zoo of oscillators)
A playground swing: . A guitar’s A string: . A quartz crystal: . The cesium oscillation defining the second: about . Ten orders of magnitude, one job: counting.
Example 28.14 (Resonance for good and ill)
A parent pushing a swing pushes at its own frequency — resonance put to work. In the new Millennium Bridge in London swayed alarmingly when the crowd’s steps locked onto a natural frequency near ; it closed two years for dampers. Every quartz watch keeps its crystal singing by driving it at resonance, .
Example 28.16 (Three centuries of tick)
Huygens built the first pendulum clock in : isochronism took clocks from a quarter hour of drift per day to seconds, and compensated pendulums to fractions of a second per day. The quartz wristwatch () swapped the rod for a crystal flexing at : tenths of a second per month. Since the second itself is defined by an atomic oscillation: the duration of periods of the microwave radiation of an internal transition of cesium-133 (Chapter 34). The best cesium clocks drift one second in a hundred million years.