Athletics of To-Day 1929

The Long Jump 213 also the straight-ahead foot placement with the spring starting from the heel, the high pick-up of the knee of the free leg, and the continuation of the arm action of the sprinter. Now let us examine the diagram in Fig. 17. The point A indlcates the wooden joist, sunk flush with the surface of the ground, from which the spring is made. AB shows the correct sprint angle of 75 degrees formed between the line of the body AB and the ground line AD at the instant of take-off. The line AC indicates the correct direction of take-off and with the ground line AD forms an angle of 45 degrees and this, in its turn, produces the true parabola of flight AHD, which is the path of the jumper's centre of gravity. If, however, the body is projected from the take-off at an angle greater or less than 45 degrees the range will be reduced as shown at E. His the maximum height attained in the true 45 degree parabola of flight, and this height can be calcu– lated for any given distance of jump. In order not to complicate the issue by the introduction of speed velocity and increased acceleration, let us-----~"---- FJO. x8. assume that a man weighing r68 lb. is going to jump ro ft. from a standing take-off. (See picture in Fig. r8.) In this case the body will be at full stretch (standing jump only) as shown in the accompanying sketch. He will leave the board with a velocity of 17.8 ft. per second and his initial horizontal and vertical velocities will both be 12.6 ft. per second, the compressed force x rted being 93·45 lb. and the kinetic energy 83r.6 ft.-lb.* If he jumps at the correct angle of 45 degrees, according to all laws of dynamics, the highest point in the parabola of flight will be reached at 2 ft. 6 ins. midway between take-off and landing. During the :first part of the jump from A to H (Fig. 17) the • force is that which changes, or tends to change, the state of rest or uniform motic;m ?fa body. Kinetic energy is the energy due to the motion of a body, ~nd lS mcreased by the amount of work the body can perform against the llnpressed forces before its velocity is destroyed.