DEFINITIONS :
MASS :- The quantity of matter in a body.
WEIGHT :- A measure of the effect of the force of gravity
on the mass of an object.
GRAVITY :- The force exerted by the pull of the earth on
all matter.
CENTRE OF GRAVITY :- The point through which the force of
gravity seems to act (the balance point).
FIRST LAW - "THE PRINCIPLE OF INERTIA" A body
at rest resists being set in motion, and when set in motion by outside
forces, equally resists attempts to alter or stop its motion. The
resistance or reluctance to change in the existing state of a body
moving in a STRAIGHT
LINE (linear movement) is termed INERTIA. When the body
is
ROTATING about an axis this resistance to change is termed
the MOMENT OF INERTIA.
SECOND LAW - "THE PRINCIPLE OF ACCELERATION"
A force acting on a body produces either an acceleration (constant
increase of velocity) or a deceleration (constant decrease of velocity).
The force of gravity produces an acceleration (or deceleration)
of 32 feet(9.80m) per second for every second it acts on a body.
The change in velocity of the body is proportional to the magnitude
of the force acting on it.
THIRD LAW - "THE PRINCIPLE OF EQUAL AND OPPOSITE REACTIONS"
Every action produces an opposite reaction of equal momentum. An
outstretched arm can only be moved across the body by virtue of
the fact that the muscle responsible for moving the arm is attached
to the trunk, which will react by moving towards the arm with equal
MOMENTUM. When the feet are fixed to the earth (or diving board)
the trunk and the earth rotate in theory towards the arm. The mass
of the earth is so great however that its velocity is so small as
to be for all practical purposes non-existent.
THE CENTRE OF GRAVITY OF A DIVER
A diver's mass is the matter of which it is composed. A unique
point is associated with every diver, around which the diver's mass
is equally distributed in all directions. This is known as the centre
of mass of the diver. Since the diver is subject to gravitational
force, the centre of mass may also be referred to as the centre
of gravity (CG), the point about which a diver's weight is equally
balanced in all directions. It can also be viewed as the point at
which the entire weight of the diver's body may be considered as
concentrated.
A knowledge of the location of the diver's CG for various body
positions is useful to the teacher/coach for the following reasons:
The flight path of the diver can be predicted at the instant of
take-off.
The amount of rotation possessed by the diver can be assessed at
the instant of take-off.
It is the main reference point used to describe a diver's linear
and angular motion.
CREATING ROTATION
During the flight of a diver there is only ONE force acting on
the body, the force of gravity, so the diver is always balanced
in the air. He cannot overbalance or set himself rotating.
In order to rotate, a diver must employ some other force. The
only force available to him is the "reaction" of the earth's
surface, it is opposing the force of gravity acting downwards on
his body. His muscular system kept him upright. When standing too
long, his muscles tire and he falls over, ie. he overbalances.
The earth's surface reacts upwards in effect through his feet,
and as he topples over, his centre of gravity passes outside the
base (his feet). He rotates through 90 degrees from vertical to
horizontal, ie, "falls flat on his face".
When this is performed from a diving board, the extra height will
enable him to enter head first.
The body movements needed to provide the turning force necessary
to produce the angular momentum required during the flight must
be started whilst the feet are in contact with the board, that is
during the take-off, and must be completed just before the feet
lose contact with the board.
Angular momentum there are basically four methods of producing
it:
1 Overbalancing (Lean).
2 Momentum Transfer (Jerk).
3 Eccentric Leg Thrust (Hips bent).
4 Eccentric Board Thrust (Springboard only).
OVERBALANCING (LEAN)
The simplest method of creating rotation, particularly for the
beginner.
All forward and back dives require the body to overbalance just
prior to the feet leaving the board. This is to ensure that the
centre of gravity is set in motion away from the board for reasons
of safety. This slight lean at take-off will inevitably provide
some angular momentum, however if lean were necessary to create
rotation, reverse and inward somersaults would not be possible.
The greater the lean at take-off the greater the angular momentum
created, but only at the expense of height and therefore time. It
should be reduced to a minimum consistent with the requirements
of safety and aesthetics.
Any lean during the take-off for a reverse and inward dive will
induce angular momentum in the wrong direction, which will need
to be overcome by extra angular momentum in the correct direction
if the dive is to be performed successfully.
MOMENTUM TRANSFER (JERK)
When free in the air, an arm swing will result in the trunk moving
"towards" the arm; when the arm stops moving, the trunk
stops. If the trunk is prevented from reacting, the momentum stored
in the arm as a result of the arm swing will be transferred to the
rest of the body, pulling it round in the same direction.
Momentum from the PART has been transferred to the WHOLE.
For the given mass, the momentum stored in the PART of the body
set in motion during the take-off will depend on the speed with
which it is moving in the required direction at the moment the feet
leave the board.
The faster the PART is moving at the moment of take-off, the greater
the amount of angular momentum stored in it.
Double the speed produces twice the momentum.
The angular momentum stored in the PART during this important phase
of the take-off will be transferred to the WHOLE during the flight,
rotating it in the same direction.
MOMENTUM = MASS times VELOCITY
Therefore a MASS moved a SHORT distance at HIGH speed will produce
MORE MOMENTUM than when moved a greater distance at a slower speed.
As the initiation of rotation must be co-ordinated with the final
leg drive at take-off, this becomes of vital importance, especially
in multiple somersaulting dives.
In practice, momentum transfer from the arms can be combined with
that of the upper trunk to provide a strong turning force when performing
multiple somersaulting dives.
Especially effective with back and reverse movements, as the normal
upward arm swing is in the same direction as the body rotation.
With forward and Inward dives this same advantage can be achieved,
providing the arm swing is made directional, ie, changes direction
when above the head.
ECCENTRIC LEG THRUST (HIPS BENT)
When the legs straighten at take-off, the thrust is along a line
projected upwards from the toes passing through the hip joint. If
the body is straight at take-off, so that this line passes through
the body's centre of gravity as well as the hip joint, the body
will rise, but there will be no rotation from this action. If, during
the take-off for a forward or Inward dive, the body is bent forward
at the hips or the back is rounded so that the body's centre of
gravity lies in front of the hip joint at the completion of the
leg thrust, an off-centre or eccentric turning force will be applied,
causing rotation of the body about its centre of gravity, in addition
to the upward motion.
The GREATER the distance between the hip joint and the centre
of gravity, the GREATER will be the angular momentum created, but
the LESS force there will be available to project the body upwards
and vice versa
ECCENTRIC BOARD THRUST (SPRINGBOARD ONLY)
The line of thrust from the end of a rising springboard is at right
angles to its surface. If at the moment of take-off, this line passes
outside the body's centre of gravity, then a turning force will
be imparted. In forward and backward take-offs, this will assist
the general rotation of the body, but in reverse and inward dives,
it provides rotation in the wrong direction.
Xiong Ni (CHI)
METHODS USED FOR SPECIFIC DIVES :- METHOD DIVING GROUP
1.OVERBALANCING FWD/BK/ARMST SS
2.MOMENTUM TRANSFER ALL GROUPS
3.ECCENTRIC LEG THRUST FWD/INWD
4.ECCENTRIC SPRINGBOARD THRUST FWD/BK
DIVING GROUPS METHOD
FORWARD DIVES 1 / 2 / 3 / 4
BACK DIVES 1 / 2 / 4
REVERSE DIVES 2
INWARD DIVES 2 / 3
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