Motion is divided into 3 main categories.
Linear Motion - when a body part moves in a straight line or curved line, with all its part moving the same distance, in the same direction and at the same speed.
Eg. Skeleton bob event.
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.
Eg. Front crawl in swimmming.
General Motion - A combination of linear & angular motion.
Eg. Javelin throw.
- during the approach the javelin & torso of the athlete are showing linear motion by moving in a straight line.
- the arms & legs of the athlete are showing an angular motion as the non-throwing arm rotates around part of a circle about the shoulder joint, the upper legs about the hip joints, lower legs about the knee & feet about the ankle.
Force - a push or pull that alters, or tends to alter, the state of motion of a body.
A force can;
- cause a body at rest to move.
- cause a moving body to change direction,
- cause a body to accelerate or decelerate.
- cause an object to change shape.
1. The ball at rest in the bowlers hand, moves when thrown
2. The ball being bowled at the batter, once its hit will change direction into the field.
3. The ball will ahve been thrown quite fast but the batter should be able to hit the ball with a greater speed. The ball will decelerate when it is caught by a feilder, or hits the floor.
4. As the ball is hit, it will change shape slightly with the power from the bat.
Newton's Laws of Motion
Law of Inertia - A body continues in a state of rest or uniform velocity unless acted upon by an external force.
Eg. Penalty kick in football, the ball will remain still until kicked.
Law of Acceleration - 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 & takes place in the direction in which the force acts.
Eg. Penalty kick in football, the greater the force the more the ball will accelerate.
Law of Motion - For every action there is an equal & opposite reaction.
Eg. Penalty kick in football, the ball hitting the cross barm the bar provides the equal force to the opposite driection for the rebound.
Centre of Mass - The point at which the body is balanced in all directions.
Line of Gravity - A line entering the centre of mass vertically down to the ground.
Base/Area of Support - Points of contact on the ground.
Stability - How difficult it is to disturb the body from its balanced position.
The stability of an athelte depends on the
- position of an athlete's centre of mass.
- position of the athlete's line of gravity.
- size of the athlete's base/area of support.
Relationship between centre of mass & application of force.
The direction of the application fo force in relation to the centre of mass will determine whether the following motion of the body is linear or angular.
Linear motion: To generate linear motion the line of action of the force passes through the body's centre of mass. This is called a direct force.
Direct Force - A force whose line of application passes through the centre ofmass of a body causing the resulting motion to be linear.
Angular motion: To generate angular motion, the line of action of the force passes outside the body's centre of mass. This is called an eccentric force.
Eccentric Force - A force whose line of application passes outside the centre of mass of a body causing the resuluting motion to be angular.