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Sport studies biomechanics Some definitions… • Velocity – speed in a given direction • Acceleration – rate of change of velocity • Force – a push or pull that changes the state of motion of an object • Inertia – resistance to action or change • Momentum – the quantity of motion that occurs Newton’s first law of motion Law of inertia • A body at rest or moving with constant velocity in a straight line, will continue in that state until a force of sufficient magnitude occurs to overcome its inertia • Basically, an object will not move until you apply a force that is large enough to move it • If it is moving, it will remain on that path, unless a force great enough is applied in a different direction Newton’s second law of motion Law of acceleration • This law states that the acceleration of a body is proportional to the force acting on it and that the acceleration takes place in the direction in which the force acts • Basically.. the greater the applied force, the greater it’s acceleration will be • This law also takes into account the fact that the accelerating effects will vary depending on the mass of the body which experiences the force • Therefore.. For a greater mass, a lager amount of force is needed to provide the same acceleration Newton’s third law of motion Law of action/reaction • When on object exerts a force on a second object there is force equal in magnitude, but opposite in direction exerted by the second object on the first • So.. For every action there is an equal, but opposite reaction • Eg. A sprinter on the blocks experiences a force propelling him forward. As the athlete pushes backwards and downwards on the blocks, the blocks push the athlete upwards and forwards out of the blocks – ground reaction force forces • Internal forces – produced by the muscles • External forces – gravity, air resistance etc. • 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 Gravity • Gravity is a constant force • Gravity provides a constant force on matter, which is commonly known as weight • Weight is mass multiplied by gravity • Gravity causes objects to travel towards the earth at a constant rate of acceleration (10m/sec²) Centre of gravity • The centre of gravity is defined as the point around which a body’s weight is equally balanced in al directions • Single segmental objects have their own centre of gravity directly above the base of support. • Sometimes the centre of gravity can be outside the body (eg.Arc) Human body centre of gravity • Standing still – the 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 • 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 Impulse • Impulse is the product of the force that is applied to an object and the length of time the force is applied • 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 • it depends on: • The time for which the force acts • The size of the force applied Momentum • Refers to the quantity or amount of motion • Momentum = mass X velocity Impact • The size/force applied between objects during collision • The greater the momentum, the greater the impact Frictional force • Acts on the interface of surfaces which are in contact and acts in the opposite direction of motion • The co-efficient of friction indicates the ease of momentum i.e. the larger the coefficient of friction, the harder the movement is Projectiles • Height of release • Shotputters release the shot well above shoulder height, with the shot landing at ground level • If the shotputter could release the shot from ground level the optimum angle of release would be at 45° • Angle of release • The shape of the objects flight path is dependant on the angle it is released at • If ball is thrown straight up it will go directly upward and pulled straight back down by gravity • If ball is thrown at an angle above 45° it will have a trajectory in which height dominates over distance • Speed of release • If you throw with a faster arm action, it will go further • Air resistance doesn’t really have an affect on the shotput, because it is heavy Net force • All force acting upon a body or object • Ice puck example, • Net force is 0 when puck is resting on surface of ice because vertical forces acting on puck cancel each other out • When the hockey stick strikes the puck, an additional force is introduced, causing the puck to accelerate during the period of time the stick and puck are in contact • Once contact between the puck and stick is broken, there is a zero net force situation again. Therefore, according to Newton's 1st law of motion the puck will continue to travel at a constant velocity