MECHANICS, MOTION AND MOVEMENT Motion LINEAR MOTION - When a body moves in a straight or curved line, with all its parts moving the same distance, in the same direction and at the same speed. E.g. Linear motion in a straight line = Tobogganist. Linear motion in a curved line = shot in shot put 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. The movement must occur around a fixed point/axis. E.g. when an athlete spins, turns, circles or somersaults. The fixed point in the human body is usually joints. Angular Motion of a body part = the arm when swimming front crawl shoulder acts as the fixed point. Angular Motion of the whole body = when a Gymnast swings around the high bar GENERAL MOTION - A combination of Linear and Angular Motion. E.g. Javelin the approach shows Linear Motion with the Javelin and torso of the athlete; while showing Angular Motion through the movement of the legs and nonthrowing arm Force This is a push or a pull that alters, or tends to alter, the state of motion of a body. A force can perform the following functions: Cause a body at rest to move Cause a moving body to: o Change direction o Accelerate o Decelerate Change an objects shape 1
The effects of force on a penalty kick in Football are given in the table below: EFFECT OF FORCE A force can cause a body at rest to move A force can cause a moving body to change direction A force can cause a moving body to accelerate A force can cause a moving body to decelerate A force can cause a body to change its shape EXAMPLE FROM A PENALTY KICK IN FOOTBALL The Football will remain at rest on the penalty spot until the force of the footballer s boot on the ball causes it to move If the ball is travelling towards the goal and the goalkeeper makes a save, the force exerted by the hands of the goalkeeper will push the ball away in the opposite direction The penalty taker can accelerate faster towards the ball, after initially starting to move, by applying a larger force on the ground The force of the net at the back of the goal will cause the ball to slow down. Or the force of the goalkeepers hands on the ball will cause it to stop moving At the point where the player s boot contacts the ball, the ball will be slightly deformed and no longer truly spherical. The force of the ball on the net will also cause the net to change shape YOU WILL NEED TO BE ABLE TO EXPLAIN THE FOLLOWING WHEN RELATING TO SPORTING EXAMPLES: The Effect of Size of Force Direction of the Force Position of Application of Force on a Body Link between Force and Motion Without Force, there can be no Motion Newton s Laws of Motion 1 ST LAW OF MOTION - A body continues in a state of rest or ( Law of Inertia ) of uniform velocity unless acted upon by an external force E.g. Hockey ball during the penalty flick. A body (the Hockey ball) continues in a state of rest (on the penalty spot) unless acted upon by an external force (the muscular forces on the Hockey stick). 2
2 ND LAW OF MOTION - When a force acts on an object, the rate ( Law of Acceleration ) 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 E.g. Hockey ball during a penalty flick When a force acts on an object (the muscular forces applied to the stick on the ball), the rate of change of momentum experienced by the object (the acceleration of the Hockey ball) is proportional to the size of the force (the ball will accelerate faster with a greater push of the stick) and takes place in the direction in which the force acts (the ball accelerates towards the Hockey goal). 3 RD LAW OF MOTION - For every action there is an equal and ( Law of Action/Reaction ) opposite reaction E.g. a Rugby player swerving to the right to avoid a tackle For every action (the Rugby player pushes the ground to their left with their feet) there is an equal and opposite reaction (the ground exerts an equal force to the right allowing the player to swerve in that direction). Centre of Mass (CoM) CENTRE OF MASS - The point at which the body is balanced in all directions. Also the point at where all of its mass could be considered to be concentrated MASS - The amount of material of which a body is made With symmetrical objects in which the mass is evenly distributed, the centre of mass is found at the geometrical centre of that object e.g. shot put Sometimes the CoM can lie outside the body. E.g. the CoM of a ring will be at the centre outside the object itself In humans the CoM is not a fixed point; as it depends on the position of the body and can also lie outside of the body CoM of a male standing upright with their arms to the sides is about 2-3cm above the naval and slightly lower for a female (as males tend to have more weight in their shoulders and women tend to have more weight on their hips) As soon as an athlete moves from that position, so does the CoM. E.g. if the arms are raised above the head then the CoM will raise as well In other body shapes the CoM can move outside the body. E.g. Pike Jump in Gymnastics or Trampolining (CoM will be outside the stomach as the arms and legs move forwards) or the Fosbury Flop in High Jump (CoM comes out the back of the body as it bends backwards) 3
Stability Relates to how difficult it is to disturb a body from a balanced position This is determined by a number of mechanical principles that depend on the following: Position of athlete s CoM Athlete s base of support (e.g. feet) Mass of the athlete Position of the athlete s line of gravity (a line extending from the CoM vertically down to the ground) PRACTICAL EXAMPLES OF THE EFFECTS OF CHANGES IN THE POSITION OF THE CoM AND THE AREA OF SUPPORT YOU NEED TO BE ABLE TO APPLY THIS TO PRACTICAL EXAMPLES FROM VARIOUS SPORTS Relationship between CoM and Application of Force LINEAR MOTION - If the line of action of the force passes through the body s CoM, the resulting motion will be linear. E.g. an athlete performing a vertical jump. A force that passes through the CoM of a body is called a Direct Force (a force whose line of application passes through the CoM 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 CoM, the resulting motion will be angular. E.g. forward somersault on the trampoline. A force That passes outside the CoM of a body is called Eccentric Force (a force whose line of application passes outside the CoM of a body causing the resulting motion to be angular) PRACTICAL ANALYSIS OF TYPICAL PHYSICAL ACTIONS YOU NEED TO BE ABLE TO COMPLETE A PRACTICAL ANALYSIS OF A NUMBER OF PHYSICAL ACTIONS INCLUDING THE FOLLOWING: Kicking a Football Passing a Netball Long Jump Shot Putt 4
EXAM QUESTIONS JANUARY 2003 1 d) Knowledge of force can be beneficial when analysing physical activities. Using practical examples explain how the size of force, direction of the force and position of application of the force can affect performance. (3 marks) MAY 2003 No Questions JANUARY 2004 1 b) With reference to the centre of mass, explain why a headstand is an easier balance to hold than a handstand. (3 marks) MAY 2004 1 c) During an analysis of practical activities, movement can be described as linear, angular or general motion. Use a practical example to describe how angular motion could be produced. (2 marks) JANUARY 2005 No Questions MAY 2005 1 b) When hitting a ball in Tennis an understanding of force is important. Explain how force can cause the ball to: JANUARY 2006 No Questions MAY 2006 (i) Move straight (ii) Spin (2 marks) 1 c) Describe how the position of the centre of mass can affect a balance. (3 marks) 5
JANUARY 2007 1 b) Use a practical example to describe how angular motion is produced. (1 Mark) MAY 2007 1 b) iv) Apply Newton s 3 Laws of Motion to a strength training exercise. (3 Marks) JANUARY 2008 1 c) Explain, using a practical example, how either size or direction of force can affect performance in PE and sport. (2 Marks) MAY 2008 1 b) Movement can be described as linear, angular or general motion. (i) (ii) Use a practical example to describe how linear motion can be produced. (2 Marks) A knowledge of centre of mass in physical education and sport can improve performance. Using a practical example from PE or sport, explain how the position of centre of mass enables a performer to resist motion or external forces. (3 Marks) 6