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Distance and displacement Distance and displacement are quantities used to describe the extent of a body's motion. Distance is the length of the path a body follows and displacement is the length of a straight line joining the start and finish points e.g. in a 400m race on a track the length of the path the athlete follows (distance) is 400m but their displacement will be zero metres (they finish where they start). Speed and velocity Speed and velocity describe the rate at which a body moves from one location to another. These two terms are often thought, incorrectly, to be the same. Average speed of a body is obtained by dividing the distance by the time taken where as the average Velocity is obtained by dividing the displacement by the time taken e.g. consider a swimmer in a 50m race in a 25m length pool who completes the race in 60 seconds - distance is 50m and displacement is 0m (swimmer is back where they started) so speed is 50/60= 0.83m/s and velocity is 0/60=0 m/s Speed and Velocity = distance traveled ÷ time taken EDU2MP Movement Perspectives Acceleration Acceleration is defined as the rate at which velocity changes with respect to time. average acceleration = (final velocity - initial velocity) ÷ elapsed time From Newton's 2nd law: Force = Mass x Acceleration Acceleration = Force ÷ Mass If the mass of a sprinter is 70kg and the force exerted on the starting blocks is 700N then acceleration = 700 ÷ 70 = 10 msec² Acceleration due to gravity Whilst a body is in the air it is subject to a downward acceleration, due to gravity, of approximately 9.81m/s² EDU2MP Movement Perspectives Uniformly accelerated motion When a body experiences the same acceleration throughout some interval of time, its acceleration is said to be constant or uniform. In these circumstances, the following equations apply: Final velocity = initial velocity + (acceleration x time) Distance = (initial velocity x time) + (½ x acceleration x time²) Moment of force (torque) The moment of force or torque is defined as the application of a force at a perpendicular distance to a joint or point of rotation. EDU2MP Movement Perspectives Linear Kinetics Kinetics is concerned with what causes a body to move the way it does. Momentum, inertia, mass, weight and force Momentum: mass x velocity Inertia: the resistance to acceleration - reluctance of a body to change whatever it is doing Mass: the quantity of matter of which a body is composed of - not affected by gravity - measured in kilograms (kg) Weight: force due to gravity - is mass x gravity (9.81m/s²) Force: a pushing a pulling action that causes a change of state (rest/motion) of a body - is proportional to mass x acceleration - is measured in Newtons (N) where 1N is the force that will produce an acceleration of 1 m/s² in a body of 1kg mass The classification of forces, external or internal, depends on the definition of the 'system'. In biomechanics, the body is seen as the 'system' so any force exerted by one part of the system on another is known as an internal force all other forces are external EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives So weight is the force of gravity between two bodies, usually some small object in contact with the earth. EDU2MP Movement Perspectives A cricket ball that has a mass of 0.2 kg has the same mass wherever it is ? Look at 3 different scenarios: 1) In Bendigo ? 2) On top of Mount Everest ? 3) On outer space ? Explain. The weight of a cricket ball is the same wherever it is ? Look at 3 different scenarios: 1) In Bendigo ? 2) On top of Mount Everest ? 3) On outerspace ? Explain. EDU2MP Movement Perspectives Do a force acceleration example. EDU2MP Movement Perspectives Refer to the following information for the next six questions. Static Equilibrium Little Nellie Newton wishes to be a gymnast and hangs from a variety of positions as shown. Since she is not accelerating, the net force on her is zero. This means the upward pull of the rope(s) equals the downward pull of gravity. She weighs 300 N. Enter the scale reading, in newtons, for each case. EDU2MP Movement Perspectives In the example below, the action-reaction pair is shown by the arrows (vectors), and the action-reaction described in words. For the remaining situations, discuss with your neighbor the direction of the "action" vectors. In the first form provided, verbally state the "reaction" to the given "action". In the second form provided, state the direction of the "reaction" vector. EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives Force Production Mass, Weight Types of Force Force Summation Action Reaction Inertia COG Force, Gravity EDU2MP Movement Perspectives Forces • Internal forces – produced by the muscles • External forces – gravity, air resistance etc. EDU2MP Movement Perspectives Forces “something that causes an object to be deformed or moved.” (Roberts & Falkenburg, 1992) Force can: • Get objects moving • Stop objects moving • Change the direction of a moving object • Change the speed of a moving object • Balance another force to keep an object still EDU2MP Movement Perspectives External Forces Worksheet • Get objects moving Gravity Friction Air resistance Ground reaction force Point of application Direction Resultant action • Stop objects moving • Change the direction of a moving object • Change the speed of a moving object • Balance another force to keep an object still EDU2MP Movement Perspectives Summation of Joint Forces In whole body sports skills: • Using (recruiting) joints in the order, big to small, will make objects move faster. (acceleration). • Using (recruiting) joints in the order, small to big, will usually result in deceleration. EDU2MP Movement Perspectives Implications for the Coach There are two practical principles that apply specifically to running, jumping and throwing where the athlete is concerned with creating optimal force and speed: ● Use all the joints that can be used ● Use every joint in order Use All the Joints That Can be Used The forces from each joint must be combined to produce the maximum effect. This is best done when all joints that can be used are used. This will help to get the most speed or acceleration out of a movement. In the shot put, for example, the knee, hip, shoulder, elbow, wrist and finger joints should all be used to exert the greatest force on the shot. Beginners frequently miss out early joint movements such as the knee or hip action, or fail to complete a movement fully by not using the wrist or fingers. EDU2MP Movement Perspectives EDU2MP Movement Perspectives Use Every Joint in Order When several joints are used in a skill, their sequence and timing are important. This principle tells us when the joints should be used. Movement should begin with the big muscle groups and move out through the progressively smaller muscles, from big to small. This pattern produces optimal forces and flowing, continuous movement. The continuous, flowing movement produces a summation of forces, forces adding together. The force generated by one part of the body is built on by the force of subsequent joints. In the well timed shot put, the hip action commences just as the leg extension decelerates. The shoulder action commences as the hip rotation decelerates and so on. The release velocity of an implement depends on the speed of the last part of the body at release. The correct sequence and timing allow the athlete to attain maximal release velocity. EDU2MP Movement Perspectives Force Summation • • Combination of forces to produce a maximal force 2 types – Simultaneously: explosive action of all body parts occurs at the same time • Eg: high jump take-off, vertical jump for rebound in basketball – Sequentially: body parts are moved in sequence to generate a great force • Eg: throwing, striking, kicking EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives • What are the 4 parts to a sequential force summation technique (page 54) • Write down (or draw) the steps in force summation for a throw from the outfield in softball EDU2MP Movement Perspectives Homework Task • Answer the following questions – 1. Pick one benefit of biomechanics for a sport of your choice and discuss in detail – 2. For the same sport analyse and discuss in detail one form of equipment that has used biomechanics – 3. Describe the sequence of body parts in the force summation for a tennis serve EDU2MP Movement Perspectives EDU2MP Movement Perspectives Locating Centre of Gravity Single segmental objects have their Centre of Gravity directly above the base of support. Sometimes the Centre of Gravity can be found outside the body. (Arc) EDU2MP Movement Perspectives Gravity: A Constant Force • The Earth’s rotation produces a force called gravity. • Gravity provides a constant force on matter, which is commonly understood as weight. • Weight is mass multiplied by gravity. • Gravity causes objects to travel toward the earth at a constant rate of acceleration. 10m/sec2 EDU2MP Movement Perspectives Human Body Centre of Gravity • Standing still – centre of gravity is located in the abdominal cavity, about 6 inches above the pubis symphysis. • As your position changes – so does your centre of gravity. • The position of the centre of gravity will determine whether the body is in balance. EDU2MP Movement Perspectives Definition The centre of gravity is defined as the point around which a body’s weight is equally balanced in all directions. (Hall, 1999) EDU2MP Movement Perspectives Centre of Gravity, Stability & Balance Stability and balance will be easier if: • The mass is large • The base is large • The centre of gravity is low • The centre of gravity is located over the base Headstand Handstand EDU2MP Movement Perspectives Centre of Gravity • When the force of gravity acts on a body, it acts through the centre of gravity and always moves towards the centre of the earth. • Symmetrical objects like balls and cubes have their CoG in the exact centre of the object. • Objects are 3 dimensional, so the CoG will be at the point where the axes of all 3 planes meet. EDU2MP Movement Perspectives Inertia - Newton’s First Law Newton’s first law of motion states that: “a body will continue in a state of rest or in a straight line of uniform motion unless acted on by an external force.” (Roberts & Falkenburg, 1992) • Inertia is the resistance of an object to change. • An object at rest will remain at rest unless acted upon by a outside force. EDU2MP Movement Perspectives EDU2MP Movement Perspectives Center of Gravity Gravity is a force which is always present and is a pulling force in the direction of the centre of the earth. This force acts on every body through an imaginary point called the centre of gravity (GG). A solid object like a shot or discus has its CG in the centre and this is a fixed point. The human body is a complex and constantly changing shape. The centre of gravity now moves according to the positioning of the body and limbs. The CG may be inside the body, for example, when standing or it may be outside the body as in the pole vault and flop high jump bar clearances. EDU2MP Movement Perspectives Types of Forces Force without motion – isometric force Force with motion – isotonic force Sub-maximal force Maximal force EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives EDU2MP Movement Perspectives