Why? The Science of Athletics

262 WHY?-THE SCIENCE OF ATHLETICS and the two hands, touching, will consolidate their grip. He is . then in process of converting his forward (running) momentum into upward momentum by ·means of spring. Two forces operate simultaneously, one due to his weight and the momentum of his run, the other due to the force of his spring in taking-off. The resultant of these forces gives the magnitude and direction of the force existent at the instant of take-off. Of the two forces at work simultaneously, the one along the line AB, in Fig. 83, represents an energy of 937 foot-lb ., due to his weight and the velocity of his approach run; while the other along the line AE is equal to 6oo foot-lb. due to the energy of his spring. If the two lines referred to are joined to make the parallelogram ABDE, we get a resultant along the line AD representing a kinetic energy of 1,120 foot-lb., and its direction, which is the force existent at the instant of take-off, along the arc AD. Again, the diagram serves to emphasize the importance to the vaulter of building up maximum approach speed commensura:te with a strong spring and a well-balanced take-off, for it will be readily followed that the longer the the lines AB and AE, the greater will be the resultant AD, and consequently the greater will be its value on scale, and the more easily will the vault be made. Mention has been made already of the important mechanical movements carried out by the arms, both in pulling and pushing, to raise the body over the bar, and if it is postulated that a man weighing abou1 I I stone raises his body 2 ft. by the pull and another I ft. by the push, then the work he does has been calculated approxi– mately as follows : On pulling-raising body say 2 ft. = 300 foot-lb. On pushing- , ,; , r ft. = I 50 foot-lb. Total 3 ft . = 450 foot-lb. Or a force equal to about three times the weight of his own body.