Negative damping — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
Also named here as self excitation, stability threshold — the same set of essays touches all of them, so they are one junction rather than several.
The bridge that was pushed by its own sway
A crowd walking on a bridge that moves sideways adjusts its footing to stay balanced, and the adjustment pushes the bridge the way it is already going. The crowd is a damper with the sign reversed, and past a certain number of people the total damping is negative.
The motion that feeds itself
A steady wind contains no frequency at all, and it can destroy a bridge. The force that does it is manufactured by the structure's own movement, so there is no excitation to resonate with — there is a wind speed above which the equilibrium is unstable, and below which nothing happens.
Named alongside it
The objects these essays reach for when they reach for this one.
DampingSelf excitationStability thresholdAeroelasticityCritical velocityCrowd loadingFlutterFootbridgeGallopingLateral vibrationTuned mass damper