Why does a swing go higher only when you push at the right moment?
Tiny, well-timed pushes can build a huge swing, shatter a wine glass, and, since 1831, make soldiers break step on bridges.
▶ Start the storyA swing goes higher only when you push it in time with its own natural rhythm. Every swing has a favourite tempo, set by how long it takes to swoop back and forth. Push at exactly that tempo and each shove adds a little more energy than the last, so the arcs grow and grow. Push faster or slower and you fight the motion, and the arcs stay small. Physicists call this resonance: when something is driven at its own natural frequency, it soaks up energy and vibrates with a bigger and bigger amplitude.
The trick is that the energy is stored, not lost. A swing trades height for speed and back again, and only a little leaks away each cycle through friction, which physicists call damping. So small, well-timed pushes pile up into a big result.
The same idea is everywhere. Strike one string of an instrument and a neighbouring string can start to hum on its own, the sympathetic resonance Galileo wrote about. The quartz crystal in a watch keeps time by resonating, and a crystal wine glass can shatter when it hears a note at exactly the right pitch.
Resonance can also be dangerous. In 1831 the Broughton Suspension Bridge near Salford, in England, collapsed while soldiers marched across it in step, and the British Army has told its soldiers to break stride on bridges ever since.
1831
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Recap
Resonance is about timing, not strength: small pushes at the right rhythm add up to huge motion.
Surprising fact · Since the Broughton bridge collapse of 1831, British soldiers break step when they march across bridges.
Sources (2)
No source, no claim. Every fact in this lesson (14 claims) cites at least one of these.