Why a falling ruler slips through your fingers: gravity as a reaction-time stopwatch
30seconds
Drop a ruler without warning and you will not catch it at the top. The place where your fingers close is a stopwatch reading: the distance it fell is ½ g t², so a catch at 20 cm means about 0.2 seconds.
Catch the ruler yourself, then switch to a steady beat and see how guessing beats reacting. Find out what happens in that gap between eye and fingers, why athletics treats anything under 0.1 s as a false start, and why a quarter of a second at 100 km/h is nearly 7 metres of road.
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