MECHANICS, MOTION AND MOVEMENT

Motion

Motion is movement and is divided into three main categories:

1. Linear motion

2. Angular motion

3. General motion (bola)

LINEAR MOTION

When a body moves in a straight line or curved line, with all its parts moving the same distance, in the same direction and at the same speed.

For example:

Skiing (in a straight line) - all parts of the person and skis are moving in a straight line, the same distance, in the same direction at the same speed.

Javelin – all parts of the javelin are moving in a curved line in the same direction at the same speed.

Angular Motion

When a body or part of a body moves in a circle or part of a circle about a particular point called the axis of rotation.

To produce an angular motion, the movement must occur around a fixed point or axis, for example, like a wheel turning about the axle.

When relating to the body, we are including movements such as spinning, circling, turning and somersaulting ie. movement through a circle or part of a circle about a particular point.

An obvious example of angular motion is our limbs, as they move around fixed points called joints. For example, when bowling the arm completes a complete circle about the axis of rotation, the shoulder joint.

General MotionA combination of linear and angular motion.

Most movements in sporty are a combination of angular and linear motion.

For example:

Javelin – on the approach the torso is moving in a linear fashion but before and during release there is angular motion (rotational movement) about various joints.

Wheelchair athletes – in the chair, the body is linear yet the arms are moving in an angular fashion.

Force

A push or a pull that alters, or tends to alter, the state of motion of a body.

A force can:

• Cause a body to move (eg. penalty)

• Cause a moving body to change direction (eg. return of serve in tennis)

• Cause a moving body to accelerate (eg sprint finish in 1500m)

• Cause a moving body to decelerate (eg. parallel stop in skiing)

• Change an object’s shape (eg. when trampolining the force of our own body weight causes a distortion on the bed)

Clearly, the size, the direction and where the force is applied will all influence the above.

Newton’s Laws of Motion

There is a direct link between motion and force. Without force there can be no motion.

This close relationship is explained by Newton’s Laws of Motion.

Newton’s First law

A body continues in a state of rest or of uniform velocity unless acted upon by an external force. The Law of Inertia.

This means that, for example, an athlete or the shot will remain stationary until a force is exerted upon them.

As Newton stated, an object may also travel at a constant speed in a straight line if there is no resultant force to change it.

Therefore, for an athlete to be running at a constant speed, the resultant force must be zero.

Newton’s Second Law

When a force acts on an object, the rate of change of momentum experienced by the object is proportional to the size of the force and takes place in the direction in which the force acts.

The Law of Acceleration.

Acceleration= force------mass

Therefore, for an object with a constant mass to have a change in acceleration, the force acting upon it will have to change and the greater the net force, the greater the acceleration.

A stationary athlete at the start of a race responds to the gun and moves forward (accelerates). Therefore, the resultant force must be greater than zero and in a forward direction.

Newton’s Third Law

This law states that for every action there is an equal and opposite reaction.

Whenever an object exerts a force on another, there will be an equal and opposite reaction exerted by the second on the first.

For example, a gymnast who is hanging stationary on the parallel bars exerts a force of 540N. The bar exerts a downward force of 540N on her hands and so the gymnast and the bars remain stationary.

This same law of reaction also applies to impact sports such as football. So, when the player strikes the ball with a certain force forwards, the ball will exert an equal and opposite force backwards on the foot.

Centre of Mass

The point at which the body is balanced in all directions.

The centre of mass of a body is the point where all of its mass could be considered to be concentrated.

In a uniform symmetrical object the mass is evenly distributed, the centre of mass is found at the geometrical centre of that object. For example, shot, tennis ball or discus.

However the centre of mass is not always as simple to locate, as it is an imaginary point that can lie outside the body.

In the human body, the centre of mass is not a fixed point located in a specific part of the human body. Its location will vary depending on body position.

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Relationship between Centre of Mass and Application of Force

The direction of the application of a force in relation to the centre of mass will determine whether the subsequent motion of a body is linear or angular.

Linear motion

If the line of action of the force passes through the body’s centre of mass, the resulting motion will be linear. For example, a vertical jump.

A force that passes through the centre of mass of a body is called a direct force. A direct force is a force whose line of application passes through the centre of mass of a body causing the resulting motion to be linear.

Angular motion

If the line of action of the force passes outside the body’s centre of mass, the resulting motion will be linear. For example, a somersault.

A force that passes outside the centre of mass of a body is called an eccentric force. An eccentric force is a force whose line of application passes outside the centre of mass of a body causing the resulting motion to be angular.

StabilityStability relates to how difficult it is to disturb a body from a balanced position.

Stability is important in sport as a stable body position will enable an athlete to resist motion.

The stability of an athlete is determined by a number of mechanical principles that depends on the following:

• Position of athlete’s centre of mass

• Position of the athlete’s line of gravity

• Athlete’s base of support

• Mass of the athlete

Line of gravity – a line extending from the centre of mass vertically down to the ground.