Mechanics of Diving
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 – and therefore the total amount of rotation – 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.
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.
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 available 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.
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.
The rules state that the body should be vertical, or nearly so, for entry. Strictly speaking, it is physically impossible to achieve a literally vertical position throughout the entry as there will inevitably still be some rotational momentum while the body is entering the water. Divers therefore attempt to create the illusion of being vertical, especially when performing rapidly rotating multiple somersault movements. One technique is to allow the upper body to enter slightly short of vertical so that the continuing rotation leaves the final impression of the legs entering vertically. Another is to use "entry save" movements of scooping the upper body underwater in the direction of rotation so as to counteract the rotation of the legs.
The arms must be beside the body for feet-first dives, which are typically competed only on the 1m springboard and only at fairly low levels of competition, and extended forwards in line for "head-first" dives, which are much more common competitively. It used to be common for the hands to be interlocked with the fingers extended towards the water, but a different technique has become favored during the last few decades. Now the usual practice is for one hand to grasp the other with palms down to strike the water with a flat surface. This creates a vacuum between the hands, arms and head which, with a vertical entry, will pull down and under any splash until deep enough to have minimal effect on the surface of the water (the so-called "rip entry").
Once a diver is completely under the water they may choose to roll or scoop in the same direction their dive was rotating to pull their legs into a more vertical position.