A physical pendulum finds stability in its inverted position when driven at the proper frequency and amplitude combination.
The physical pendulum seen here is mounted on a ball-bearing pivot and can rotate 360 degrees; the pivot is driven at about 50 Hz with an amplitude of about 1 cm (3/4" per stroke) by a Sears Craftsman Auto Scroller Saw (model 315.172090); the length is 45 cm and the center of mass is slightly above 15 cm from the pivot; the rotational inertia is roughly 4x10^(-4) kg*m^2; the mass is about 100 grams.
For more details and references for further study see:
Shot in 24 and 300 fps. Thanks to Rob, Fu, and Daniel for their help.
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