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A nonlinear equation describing the transverse vibration of an axially travelling string with constant and time-varying length is obtained by developing a new finite element model described by quadratic shape functions. A novel nonlinear coordinate transform is introduced with regard to its nonlinear terms. Subsequently, a new hybrid Newmark-Beta / time varying degree of freedom method, which can adjust the element number automatically according to the change of string length, is proposed to improve accuracy. The proposed method as well as normal numerical methods are compared with an analytical solution. Results show that the proposed method is in good agreement with the Newmark-Beta method for the case of small variations in string length, whilst superior in accuracy to the latter for the case of large variations. Complex mode theory is adopted firstly to obtain the modal components as well as the modal energy for a travelling string. A phenomenon is observed where the free vibration energy leaks from one mode to the others in a travelling string. The higher the speed of translation and the modal order, the more energy that is leaked into the modes close to the initially excited mode.

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