Do we as coaches fully understand the influence of gravity on the events we coach and manage it to the advantage of our athletes. If you fill a bucket with water and you swing it in a circle at a certain speed the water will stay intact. However, if the line of movement or speed is not right the water will spill. Should the same bucket be kept static and upside down the water will keep spilling. This is due to gravity. Gravity has a great influence on our daily lives, since it keeps us on the earth and also allows us to move about where we want to be. Gravity is what causes our weight. How much mass an athlete has will determine the attraction of the body towards the centre of earth. Thus, all our movements are about the proper management of the influence of gravity. Therefore, it is imperative for a coach to know the basic laws of gravity and have some understanding of the influence it has on the event he /she coaches. This is a topic that we can dwell on for a couple of days, but I am going to try to give you some insight into it in a couple of minutes hoping that I inspire you to study this topic in more detail since it will assist you greatly in your coaching. Together with these laws we should also understand something about movement, force and motion.

NEWTONS LAWS OF GRAVITY

a. Newton's first law = Law of inertia. Every material body continues in its state of rest (inertia), or uniform motion in a straight line, except when forces are applied to change that state. It comes down to the body's resistance to change.

b. Newton's second law = Law of acceleration. The rate of change of momentum is proportional to the applied force and takes place in the direction in which force is applied.

c. Newton's third law = Law of action and reaction (counter force) For every action there is an equal and opposite reaction. (In the air this turns around)

The centre of mass(CG) of a human being is extremely sensitive to movement of the body parts and the shape of the body. It is situated between the navel and spinal cord. When the arms are lifted above the head the CG rises, when an arm is lifted laterally the CG moves towards the side the arm is lifted etc.



PRINCIPALS RELATED TO THE LAW OF INERTIA

Combining transferring and rotary motions

The combined motions, if performed correctly with proper timing and sequence, will produce maximum final velocity of 'an object' in the desired direction of release (e.g., discus throw, long jump, pole vault, high jump, etc.) An interesting phenomenon is that the Centre of Gravity (CG) follows the line of the big toe of the foot on the ground. Thus, in managing the movement of the CG it is extremely important to manage foot placing in the run-up and especially at take-off.

Continuity of motion

To accomplish the first motion a certain amount of inertia must be overcome, therefor hesitation prior to the next motion will result in a loss of some or all the advantage gained in the previous motion.

Transfer of momentum

The momentum generated by part of a body can be transferred to the total body provided the body is in constant contact with the supporting surface. This rule applies whether the athlete is direct contact with the ground or he/she is in indirect contact with the ground when the vaulter is in the air but touches the pole.

PRINCIPALS RELATED TO THE LAW OF ACCELERATION

Acceleration is proportional to the force causing it.

A jumper can increase acceleration by increasing the forces that are applied backwards and down against the run - up. So, if the force applied is made bigger the acceleration will be faster. Evenly, if a jumper accelerates the speed of the swing leg, the force applied through the take - off will be quicker resulting in a better jump.

Maximum acceleration and efficiency of motion.

To obtain maximum acceleration all forces should be applied sequentially and with proper timing and in line with the intended motion.

Effects of the body's length on angular velocity.

The longer the body is the less the rate of angular rotation will be.

Momentum in swinging movements

By shortening the upswing of the swing leg in a jump will build momentum. The down swing should however be long to conserve the momentum built.

Movements while unsupported.

Once a body has left the suri'ace of take-off no movement will enhance the flight path. Thus, the maximum height or distance that the body will travel is determined by the rhythm and velocity in the run - up and take off. It is however important that the speed at take - off must be such that the athlete has full control over the movements of his / her legs and centre of mass.

PRINCIPALS RELATED TO THE LAW OF COUNTER FORCE

Surface variation

The counter force (reaction) is equal to the force applied provided that the surface used is stable.

Direction of the counterforce.

It is directly opposite to the direction of the force applied. The counterforce is most effective when it is applied more vertical to the surface.

Temporarv stored counterforce ( kinetic energy)

In Pole Vault, due to the fact that the poles can bend, they store force (kinetic energy) due to thie fact that the pole bends and the stored force increase the propulsive force.

Surface contact.

The maximum force applied in any take - off or delivery of an implement will diminish drastically if one or both feet do not keep contact with the supporting surface until the force providing motion is completed

A body resists motion in terms of the law of inertia and if it has to turn the length of a body also has an effect the degree of difficulty for the body to turn - this is known as angular inertia. The longer the body, the more difficult it becomes to turn. So, now there are two factors that institute the angular inertia - mass and length.

Angular momentum will be produced when enough force is applied to a body. The total amount of angular momentum will be dependent on the speed and the magnitude of angular inertia that exists.

It is important to understand that torque is created. It can be created by "tripping". If a body moves at high speed and all of a sudden the feet become stationary a angular force forward will develop and if not managed the person will literally fall flat on his/her face. The axis of rotation becomes the point where the feet are in contact with the running surface. However, if the person pushes the legs downward on the running surface the body will tend to go into the air. What will happen if it goes into the air, the body will continue to go forward because of its running movement, go into the air because of the push down of the legs and rotate forward as a result of the tripping effect.

It is also possible to rotate the body by applying eccentric force (force not directed through the CG of the body). Should one balance a stick on your finger and contact with the stick is directly below the CG and flick the stick upwards it should move straight up and down without rotating. However, if you place your finger a reasonable distance away from the CG and repeat what was done above the stick will go up but will also rotate. The further you place your finger away from the CG the more the rotation will be and the less vertical the stick will travel.

It is also possible to rotate the body by applying eccentric force (force not directed through
the CG of the body). Should one balance a stick on your finger and contact with the stick is
directly below the CG and flick the stick upwards it should move straight up and down
without rotating. However, if you place your finger a reasonable distance away from the CG
and repeat what was done above the stick will go up but will also rotate. The further you
place your finger away from the CG the more the rotation will be and the less vertical the
stick will travel.

Of great importance is the fact that angular momentum is only created when a body is still in
contact with the ground, in other words at the time of take-off. Once in the air it is impossible
to create angular momentum.

FORCE

General principles of Force

•	A total force is the sum of forces of all the parts of the body contributing to the act.
•	In order to conserve energy a force should be constant and use the minimum amount
        of energy to obtain the set goal.
•	Force should be applied as directly as possible in the direction of the intended
        motion.
•	The greater the distance of movement the greater the velocity that will be developed.
•	Self produced additional forces -
o	Correct muscle selection.
o	Stability of movement.
o	Correct angle of application.
o	Initial muscle tension - placing muscles on stretch before contraction
        increases force.
•	Force is also present in angular motion. The force of rotation is called torque.

MOTION

In athletics we find different types of motion that have an influence on the execution of the
event and ultimately performance. When the static balance of a static body is put into motion
it simply means that it changes position. However, there are different types of motion that
are important for coaching.

Linear motion
The whole body moves in the same forward direction which can per definition should be
straight forward (rectilinear). However, in athletics linear motion also occurs round a curve
(curvilinear) due to the nature of the track and different events. In the case of curvilinear
movement there is apart from forward forces also an angular force that pulls inward. These
forces can be applied by a push action or a pull action.

In the human body muscles are responsible for generating most of the forces to move the
body and determine its direction once it is moving. How well the body moves in a certain
direction will largely be determined by how much force the muscles can generate through
the application of strength and power and how much inertia there is to overcome. Coaches
should therefore have a good knowledge the main muscle groups that are used for the
jumps that he/she coaches and hoe to develop these muscles without adding mass.

Angular motion
Whereas linear motion is forward, angular motion is rotational by nature and is more
common in athletics as pure linear motion. When an object / body rotates, it always happens
around an axis. An axis may be real like in a wheel or imaginary as in the human body.
When a body travels through the air the exact centre of mass becomes the axis around
which it will rotate.

The above mentioned is the reason why it is of the utmost importance for a jumps coach to
understand the principals so that he/she can coach athletes so that the CG can be
managed. Because of the fact that angular momentum is only created while contact is still
held with the run up, it must be understood that the swing leg must be in the ultimate
upswing position before the CG passes the front of the take-off foot, it will not be possible to
get the CG into the required parabola for a good jump.

Whilst it is not possible to change the mass of a body in the air it is possible to change the
shape of the body at will. The longer the body is stretched in the air, the more forward
angular rotation will be. This is of great importance for instance in the flight phase of the
Long and Triple Jump, but it will also have relevance to Pole Vault and High Jump.

Pendulum swing
This activity is most important for the Pole Vault coach to understand. However it is also of
importance in coaching the other three jumps, since it is of importance in the movement of
the swing leg.

What is of great importance is the fact that by shortening the radius of rotation (bring swing
leg closest to axis of rotation) on the up - swing, upwards velocity will be generated.
However, it is also important to understand that as the pendulum action continues the height
of the swing will reduce because of gravity and air resistance. To this end it is very important
to obtain maximum controllable speed on take - off so that gravity has a longer time to act
and start pulling the body back to the centre of earth

Centripetal and centrifugal force
These two forces are of great importance to the high Jump coach since to a great extent it
facilitates the flight over the bar which is the area where most high jumpers tend to make
costly mistakes.

When an athlete runs on a curve and leans into the curve the pulling action is called
centripetal (pulling to the centre of earth) force.

On release the centripetal force will create centrifugal (outward force away from the centre of
earth) that will propel the athlete over the bar. To this end it is important to understand the
principal of flight after release.

The important thing to notice in this animation is that the bail continues in the direction it is going
when the string breaks (direction A) rather than in the direction of the "centrifugal force" (direction B).
The significance of this is that the run up curve of a high jumper should be so devised that the cross
bar is approached at an angle which will fling the athlete over the bar and not parallel to the bar.

CONCLUSION
In all the jumps maximum controllable speed should be attained at take-off in order for the CG to be
effectively displaced into height or distance.

In the take-off action the free leg and arms should be moving at maximum vertical velocity at the
instant of take-off, since further acceleration after the body has left the ground is not possible. It is
imperative that the approach speed is such that the take-off movements occur as close to
simultaneous as possible. All of this will be dependent on the level off control that the athlete has over
his/her centre of gravity.

All forces that create movement are at their optimum when the centre of gravity of the body is neutral
which means that it must be in the longitudinal axis that runs through the length of the body in a
straight line. If this axis is contorted in any place the effectiveness of the application of force through
the feet will be diminished. Maximal force can be applied when the line in which gthe force is applied
is as close to 90° as possible followed by a quick swing leg that swings through close to the body so
that the pendulum radius is as small as possible.