  The cause of the collapse of the Tacoma Narrows Bridge has been a topic of
much debate and confusion since the day it fell. Many mischaracterizations of
the observed phenomena have limited the widespread understanding of the
problem. Nevertheless, there has always been an abundance of evidence in favour
of a negative damping model. Negative damping, or positive feedback, is
responsible for many large amplitude oscillations observed in many
applications. In this paper, we will explain some well-known examples of
positive feedback. We will then present a feedback model, derived from
fundamental physics, capable of explaining a number of features observed in the
instabilities of many bridge decks. This model is supported by computational,
experimental and historical data.
