The design and testing of a wire velocimeter

The basic principles underlying the design of a velocimeter based on an unwinding wire, for use in athletics research, are discussed. It is shown by theoretical analysis that, in order to avoid runaway effects, the tension on the wire should be either high or low but not of intermediate strength. The low tension regime is shown to be theoretically the most favourable as it combines high accuracy of speed measurements in decelerated motion with insensitivity to resonance oscillations of the wire. Practical considerations concerning the ruggedness of the apparatus, however, favour the high tension regime. A modern apparatus incorporating microprocessors and working with thin nylon wire stretched by a force of the order of 1 N, i.e. in the high tension regime, has been constructed and tested. The test results show that the velocity of decelerated motions (up to decelerations of the order of 10 m s-2) can be faithfully recorded in the velocity range 0-15 m s-1. The relative error for the measurement of constant speed up to 15 m s-1 is about one in a thousand, which is very small and practically unattainable by other methods. An application to the study of the long jump is demonstrated and validated by the use of film analysis.