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**Sports Mechanics Principles**

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**Sports Mechanics Principles**

Forces Centre of Gravity Newton’s Laws Impulse Reaction Momentum Acceleration Linear Motion Curvilinear Motion Projectile Motion Drag Force Lift Force Spin Angular Motion Axes and Planes

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**Forces Internal forces – produced by the muscles**

External forces – gravity, air resistance etc.

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**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

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**External Forces Worksheet**

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 Gravity Friction Air resistance Ground reaction force Point of application Direction Resultant action Discuss links between the phrases on the right and the external forces on the left. Use examples from sports to help your understanding of the links.

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**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. Think about tennis serve starting in legs, moving up body, to final wrist snap - acceleration Think about landing a jump, joints of foot, ankle, knees then hips - deceleration

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**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)

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**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 2 balls, golf ball and indoor shot Drop both at same time – both will fall at same rate.

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**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.

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Definition The centre of gravity is defined as the point around which a body’s weight is equally balanced in all directions. (Hall, 1999)

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**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 Show with person standing in middle of circle – trust exercise. Fwd rotation, backwd rotation. Relate to gymnastics. Headstand, handstand, headstand more stable why? What happens with tucked headstand and lower cog? Balance in flight – cog tends to follow flight pathway no matter what arms legs etc are doing. Top class sprinter cog remaining level all way down track – beginner sprinter cog moves up & down.

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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. This is of interest to us when we study human movement because when a human body moves in flight, it acts as if all of it’s mass is concentrated at the Centre of Gravity.

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**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. Book on top of a paper – jerk paper out suddenly. This shows that it is easier to move the paper than the book. This shows the greater inertia of the larger mass. Sit on chair that spins Spin chair quickly Hold heavy book in each hand and extend arms out straight What happens? Spinning slows down Try moving books in toward you as you spin What happens? The spinning speeds up

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Impulse When a force is applied to an object, the product of the force (F) and the length of time (t) that the force is applied, is called the impulse of the force. Impulse = Ft Impulse is equal to Force x time, measured in Newton Seconds.

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**Impulse Depends on: The time for which the force acts**

The size of the force applied

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**Reaction - Newton’s Third Law**

Newton’s third law of motion states that: “for every action there is an equal and opposite reaction.” (Roberts & Falkenburg, 1992) Discuss practical examples of this third law – get ideas from class.

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**Momentum Refers to the quantity or amount of motion**

Momentum = Mass x Velocity The runner has a mass of 75 kg and is running at 5 m.s-1. What momentum does he have? 375 kg.m.s-1

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**Acceleration - Newton’s Second Law**

Newton’s second law of motion states that: “When a force acts on an object, the object accelerates in the direction in which the force is acting.” Acceleration is the rate of change of velocity and is determined by force. Acceleration = The final velocity minus the initial velocity divided by time. A = v – u t Blow football - everyone with a straw and a TT ball. Blow through the straw and measure how far the ball moves, (Laws 1 and 2) repeat, blowing again before the ball stops. PLAY THE GAME MAKING PAPER GOALS What happens when partner blows the other way? What happens when a third person plays too? Have some fun – play the game between 4!

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Linear Motion Movement in a straight line of a body or body part. (Translatory motion) The sprinter runs from A to B. The pathway of the runner is a straight line BUT it happens because of angular motion of the joints of the limbs. Linear motion can be considered in terms of quantities of: Distance – displacement – velocity – acceleration and therefore force. REMEMBER A body’s inertia is it’s resistance to change in motion. With linear movement, mass is the only measure of that inertia. IN OTHER WORDS The greater the mass, the greater the resistance to change, and therefore the greater the inertia.

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**Curvilinear Motion Movement along a curved line.**

Most jumps are along a curved line.

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Projectile Motion Motion of an object or person in flight.

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**Factors Affecting Pathways**

The pathway of an object is determined by: The angle of release. Drag force is a resistant force Lift force Type of spin

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Drag Force Drag force is a resistance force - a force that slows the motion of a body through a fluid (air or water). Sportsmen reduce drag by using specialised techniques, equipment or clothing. Streamlining reduces form drag. Form drag results from the suction-like force created between the positive pressure zone on the leading edge and the negative pressure on the trailing edge when turbulence is created by moving through the air or water. The effect of streamlining is a reduction in the turbulence created at the trailing edge of a body in a fluid. "Photos courtesy of and copyright Free Range Stock,

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**Direction of fluid flow**

Lift Force Lift force is a force generated perpendicular to fluid flow. Direction of fluid flow Although LIFT implies that the force is directed upwards – it may assume any direction as determined by the direction of the fluid flow and the orientation of the body. The angle of attack is the angle formed between the primary axis of a body and the direction of the fluid flow. Spin also generates lift force e.g. magnus effect on a backspinning dimpled golf ball. Lift force

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**Types of Spin Spin is the rotation of a body or object.**

Torque is the force which creates spin. The type and amount of spin depends on how much torque is applied and where it is applied. TOPSPIN Discuss examples of spin in sport. SIDESPIN BACKSPIN

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Outcome of Spin Players learn to control the amount of spin to achieve different effects. Draw diagrams to represent the outcome the different types of spin.

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Angular Motion Sporting activity cannot be defined purely as linear motion. Even for the sprinter there is considerable angular motion. The legs and arms are rotating about their particular axis. With angular motion, resistance to change in motion depends not only upon mass, but also on the DISTRIBUTION OF THE MASS AROUND THE AXIS. This is called the moment of inertia of the object. The closer the mass is to the axis (centre of gravity), the easier it is to turn. RUNNING It is easier to run by bending the legs on recovery, because it is easier for the quadriceps to lift a bent leg (lower moment of inertia) than one that is straight. TENNIS Children use smaller racquets because they can swing the smaller racquet more easily than the larger one. The junior racquet has a lower moment of inertia because it has a lower mass. DELTOID RAISE STRAIGHT ARM VERSUS BENT ARM Bent arm is easier because centre of gravity is closer to the axis making rotation easier. TRY IT AND SEE!

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**Movement Axes When standing:**

The vertical axis runs from head to foot through the body’s centre of gravity. The frontal axis runs from side to side through the body’s centre of gravity. The sagittal axis runs from front to back through the body’s centre of gravity. Use one cocktail stick for each axis and show it on jelly baby sweets. Eat afterwards mmmm. If the axis (cocktail stick) were not a pole but a sheet of card – discuss planes. The transverse plane is a horizontal plane which divides the body into upper and lower halves. The sagittal plane is a vertical plane which divides the body into right and left halves. The lateral (coronal) plane is also a vertical plane and divides the body into front and back. DRAW!

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Movement Axes There are three axes of movement around which movement can occur. Vertical Horizontal (Frontal) Anterior/Posterior (Sagittal)

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