Driving fatigue

Fatigue is a common experience usually associated in the public mind with prolonged work. Research on the subject over some sixty years has, however, produced conflicting evidence on the nature of fatigue and it is therefore not surprising that scientists still differ on its definition. Nevertheless, political decisions of far-reaching importance have to be made in the hope of controlling the effects of fatigue on road safety. An attempt is made to present a scientific understanding of fatigue, from which it appears that some of our current legislation on professional drivers' hours of work is actually counterproductive.     

The vibrational mode structure of a golf ball

In this paper, we report the discrete frequencies at which golf balls can vibrate, the mode patterns of these vibrations and how these modes can be excited. There are two broad classes of modes: those that radiate sound waves and those that do not. Both silent and acoustic modes are excited by tangential (i.e. spin-producing) impact forces; only acoustic modes are excited by radial impact forces. Exact analytical results for a homogeneous ball core are compared with finite element numerical results for both a core and a model two-piece ball. Correspondences are readily established for the important low-frequency modes, and the good agreement suggests the validity of these results for real golf balls. The results potentially provide the basis for a rapid, simple and non-destructive method of measuring the effective high-frequency elastic shear modulae of balls (and ball cores) as well as a method for 'tuning' the performance of balls for specific clubs. Some of these aspects are explored further in our companion paper in this issue.