Mechanics of diving

At the moment of take-off, two critical aspects of the dive are determined, and cannot subsequently be altered during the execution. One is the trajectory of the dive, and the other is the magnitude of the angular momentum. The speed of rotation Ц and therefore the total amount of rotation Ц may be varied from moment to moment by changing the shape of the body, in accordance with the law of conservation of angular momentum. The center of mass of the diver follows a parabolic path in free-fall under the influence of gravity (ignoring the effects of air resistance, which are negligible at the speeds involved). [edit]Trajectory Since the parabola is symmetrical, the travel away from the board as the diver passes it is twice the amount of the forward travel at the peak of the flight. Excessive forward distance to the entry point is penalized when scoring a dive, but obviously an adequate clearance from the diving board is essential on safety grounds. The greatest possible height that can be achieved is desirable for several reasons: the height attained is itself one of the factors that the judges will reward. a greater height gives a longer flight time and therefore more time to execute maneuvers. for any given clearance when passing the board, the forward travel distance to the entry point will be less for a higher trajectory. [edit]Control of rotation The magnitude of angular momentum remains constant throughout the dive, but since angular momentum = rotational velocity ? moment of inertia, and the moment of inertia is larger when the body has an increased radius, the speed of rotation may be increased by moving the body into a compact shape, and reduced by opening out into a straight position. Since the tucked shape is the most compact, it gives the most control over rotational speed, and dives in this position are easier to perform. Dives in the straight position are hardest, since there is almost no scope for altering the speed, so the angular momentum must be created at take-off with a very high degree of accuracy. (A small amount of control is av

ilable by moving the position of the arms and by a slight hollowing of the back). The opening of the body for the entry does not stop the rotation, but merely slows it down. The vertical entry achieved by expert divers is largely an illusion created by starting the entry slightly short of vertical, so that the legs are vertical as they disappear beneath the surface. A small amount of additional tuning is available by 'entry save' techniques, whereby underwater movements of the upper body and arms against the viscosity of the water affect the position of the legs. [edit]Twisting A twisting dive performed from a 10 meter platform. Dives with multiple twists and somersaults are some of the most spectacular movements, as well as the most challenging to perform. The rules state that twisting 'must not be generated manifestly on take-off'. Consequently, divers must use some of the somersaulting angular momentum to generate twisting movements. The physics of twisting can be explained by looking at the components of the angular momentum vector. As the diver leaves the board, the total angular momentum vector is horizontal, pointing directly to the left for a forward dive for example. For twisting rotation to exist, it is necessary to tilt the body sideways after takeoff, so that there is now a small component of this horizontal angular momentum vector along the body's long axis. The tilt can be seen in the photo. The tilting is done by the arms, which are outstretched to the sides just before the twist. When one arm is moved up and the other is moved down (like turning a big steering wheel), the body reacts by tilting to the side, which then begins the twisting rotation. At the completion of the required number of twist rotations, the arm motion is reversed (the steering wheel is turned back), which removes the body's tilt and stops the twisting rotation. An alternative explanation is that the moving arms have precession torque on them which set the body into twisting rotation. Moving the arms back produces opposite torque which stops the twisting rotation.