On the equations of motion of the folded inextensible string

In a number of excellent, even recently published, books in Theoretical Mechanics the equations of motion of a folded inextensible string, for example to explain the crack of a whip, are derived using either the energy principle, if a system of one degree of freedom is considered, or Lagrange's equations for conservative systems, if a system of more than one degree of freedom is studied. However, it will be shown in this paper that the resulting equations are incorrect because physically a nonconservative system is given. Hence, both methods mentioned above based on energy conservation must not be used. A correct derivation of the equations of motion is given by means of the linear momentum balance.