Introduction. (a) General Range of the Theory. The theory to be given relates solely to dynamical systems which, apart from the presence of solid friction, obey linear laws. The term solid friction is interpreted to mean a resistance between two bodies, due to tangential surface actions, which follows the simple idealised law that its magnitude has a constant value so long as relative motion between the bodies occurs. Whenever the forces tending to produce relative motion fall short of the foregoing value, the resistance adjusts itself to balance these forces, and no relative motion results. These simple laws, which are adopted as a mathematical convenience, take no account of the difference between static and dynamic friction, and thus only approximately represent true conditions.

The influence of friction on the oscillatory behaviour of a dynamical system is often somewhat unexpected. For instance, the introduction of friction may cause an otherwise stable system to develop maintained oscillations.

(b) Experimental Illustration of the Influence of Friction. Some wind tunnel experiments carried out at the National Physical Laboratory in connection with tail flutter of an aeroplane illustrate some of the effects of solid friction. The model used had a rigid front fuselage but a flexible rear fuselage and tail unit, and it was suspended by a sling of wires from a turn-table locked to the roof girders of the wind tunnel.