Why? The Science of Athletics

SOME MORE MATHEMATICS FOR ATHLETES 279 the last ounce of energy in projection, right through until it comes to earth, there are two main movements which affect its flight. These are an even balance of the missile itself and a perfect parabola of flight. To make this matter more clear, let us imagine the discus as an ordinary fly– wheel, uniform in weight and shape, and having a true centre, round which centre the discus revolves at the greatest speed which can be generated by the final spin imparted by the fingers, as the final phase of a cumulative effort, at the instant of projection. The diagram in Fig. IOI represents a discus with d centre c, and abc a segment of it, while the black portion, ab, repre– sents the outer portion of the segment which rotates around the centre. a Any body moving round a centre in a circular path has a tendency to leave that path along the tangent b ad, but there is a binding force that reacts to hold the body to its centre. The _Rull seeking to drag the defect- FrG. 101 ing segment, ab, away from its centre, c, and against the binding power working along the lines ac, be, is called centrifugal force. To stabilize matters and thus obtain a perfect flight the discus must be thrown in such a manner that the centre of gravity will coincide with the centre of rotation. The same conditions do not obtain in throwing the cricket ball or putting the shot, as already noted, because both projectiles are spherical in shape and thus have all points in the circumference equidistant from the centre of gravity. Consequently these missiles must always travel with an even centre and with no forces reacting to make them eccentric in flight. The discus, on the contrary, is so shaped that it has every tendenc;y towards eccentricity in flight. Therefore it must be thrown truly and made to spin fast if it is to travel gyroscopically fiat.