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PEShare.co.uk Shared Resource PEShare.co.uk Shared Resource Presentation Transcript

  • Mechanics of movement A2 Sports Studies Mr Jennings
  • What you need to know…..
    • Mechanics of movement
    • Vectors and scalars; velocity, acceleration
    • Momentum/impulse in sprinting
    • Newton’s Laws applied to movements
    • Application of forces in sporting activities
    • This is a daunting topic
    • However, the questions always focus on very similar scenarios
    • Very quickly you will become familiar with the key terms and concepts
  • Forces
    • A force is:
    • “ A force is that which alters or tends to alter a body’s state of rest or of uniform motion in a straight line.”
    • If a body changes direction or velocity, a force has been applied to it
    • All performance are affected by forces
    • How is this athlete affected by forces?
    • Forces cause us to move, change direction and to stop. Gravity keeps her on the floor
  • Key Terms
    • Displacement
    • Velocity
    • Acceleration
  • Displacement or Distance A B Length of journey in meters = Straight line from start to finish in meters = distance displacement
  • Displacement and distance
    • In a 400m race?
    • 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 or velocity
    • Speed is distance / time
    • Velocity is displacement divided by time
    • Displacement has direction
    • Most biomechanics refers to displacement and velocity
  • Speed and velocity
    • Consider a swimmer in a 50m race in a 25m length pool who completes the race in 60 seconds
    • Work out her speed and her velocity
    • 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
  • Physics of sprinting
    • An athlete runs 100m in 15 seconds
    • We know their speed is 100/15 = 6.67(ms-1)
    • Velocity is a similar concept to speed but includes the idea of ‘direction’
    • All velocities take place in a certain direction
    • Velocity = displacement/time
    • An athlete running 100m in 12s: Velocity = 100/12 = 8.33(ms-1)
  • Calculating velocity
    • Use 100-metre sprinting to calculate athlete’s velocity
    • Plot the velocity curve
    • What do you notice?
  • 12.6 10 100 11.0 10 90 9.6 10 80 8.3 10 70 7.1 10 60 6.0 10 50 5.0 10 40 4.0 10 30 2.9 10 20 1.7 10 10 0 0 Start Time to reach this point (seconds) Distance covered (Metres) Timing point (displacement in metres)
  • What happened?
    • Velocity varies during race
    • Slow at start (0-40m)
    • Fastest in middle
    • Decrease from 60-100m
    • Why?
      • Energy sources- PC system 6 seconds
      • Lactate system is slower hence deceleration
  • Velocity/time graphs Velocity Time Velocity changes
  • Acceleration
    • Average velocity changes
    • Change in velocity over a period of time is called
    acceleration final velocity – initial velocity time taken Acceleration =
  • Velocity Time On a velocity-time graph acceleration is shown by the of the line steepness (gradient)
  • Velocity Time Highest acceleration? zero acceleration? deceleration?
  • Vectors and scalars
    • Displacement, velocity and acceleration have direction as well as magnitude =
    • Temperature, time, speed, etc do not have direction =
    • http://www.grc.nasa.gov/WWW/K-12/airplane/vectors.html
    vectors scalars
  • Typical question
    • The Figure shows a velocity/time graph for an elite 100-metre runner.
    (i) Use the figure to determine the velocity of the sprinter after 3 seconds, and identify the period of time when the sprinter’s acceleration was the greatest. (2 marks) (ii) What is happening to the sprinter between 6 and 11 seconds? Explain why this occurs. (3 marks)
  • Answer
    • (i)
    • 1. 9.1 ms-1 (accept 9.0-9.2);
    • 2. 0-1 seconds/s. 2 marks
    • (ii)
    • 1. Deceleration/decrease in velocity; (Do not credit slowing down)
    • 2. Lack of ATP;
    • 3. CP breakdown to ATP slowing/limiting;
    • 4. Due to lack of stored PC;
    • 5. Change to slower lactic acid/alactic/anaerobic system max3 marks
    • From a sport of your choice:
    • Identify 2 examples of when a body’s state of motion gets quicker
    • Identify 2 examples of when a body’s state of motion gets slower
    • What causes these changes in speed and direction?
    • There are 2 types of force
      • Internal and external
    • Internal – contraction of muscles
    • External – air resistance, gravity and friction
    • Forces are Vectors so have
      • Magnitude and direction and are represented by arrows
  • External forces
    • Gravity - force pulls objects back down to earth
    • Friction – objects moving against each other in opposition cause friction. Basketball shoes have ‘extra grip’ which actually means more friction whereas ice skates are designed for minimal friction
    • Air resistance – friction caused by air moving over a surface. Very different to wind resistance
    • Inertia – the reluctance to change state of motion. Pushing a car is hard at first but not too hard once its moving. Once moving though it would be difficult to stop!
  • Newton’s Laws
    • An exam question on these is extremely straight forward once you have a basic understanding of how they apply to different scenarios
  • Newton’s 1 st Law
    • The Law of Inertia
      • “ a body will remain in it’s state of motion/rest until affected by a force acting upon it”
    • A body will be reluctant to change its state of motion
    • Applying the law:
    • A football being kicked, a sprinter in the start blocks and a snooker ball prior to being hit
  • Momentum
    • Every moving object has mass and velocity
    • Momentum = mass x velocity
    • If two rugby players with same mass collide, the one with higher velocity wins
    • If they have same velocity, the one with larger mass wins
    • Both have larger momentum
  • Newton’s 2 nd Law
    • The Law of Acceleration
    • “ The rate of change of momentum is directly proportional to the force causing the change, and the change takes place in the direction in which the force was applied”
    • In sport, mass remains constant and so momentum equates to acceleration
  • Applying the 2 nd Law
    • The magnitude and direction of the force applied by the sprinter in the blocks will determine the magnitude and direction of the force received (acceleration)
    • Acceleration is proportional to force applied (F=ma)
  • Newton’s 3 rd Law
    • The Law of reaction
      • “ for every action there is an opposite and equal action force”
    • In sport this is usually the performer and the ground
  • Applying Newton’s 3 rd Law
    • The performer cannot move the earth but receives significant acceleration
    • This is called Ground Reaction Force
    Action force of muscle contraction Equal and opposite force
  • Using Newton’s Laws explain how an athlete accelerates out of the blocks at the start of a race.
    • The athlete remains at constant velocity, at rest, in his blocks at the start of a race due to Newton’s First Law – the Law of Inertia. In order for him to accelerate an external force must be applied.
    • As the athlete uses his muscles to generate a force into the blocks/ground there will be an equal an opposite reaction force pushing him forwards, due to Newton’s Third Law – the Action-Reaction Law.
    • This resultant force is the external force required to overcome the inertia (Newton’s 1st Law) and the athlete accelerates from the blocks.
    • The acceleration of the athlete is in direct proportion to the size of the resultant external force due to Newton’s Second Law – the Law of Acceleration. The acceleration can be calculated using the formula F=ma.
  • Forces in running
    • List as many forces as you can think of acting upon the runner in the next picture
    • Label the force arrow to show the direction of the force
  • Forces acting on a sprinter Action force of muscular contraction Friction Gravity Air resistance Equal and opposite Ground Reaction Force
  • Forces in high jumping
    • List as many forces as you can think of acting upon the jumper in the next picture
    • Label the force arrow to show the direction of the force
    • Muscle force applied to ground on take off
    • Large vertical ground reaction force
    • Gravity
    • Friction between foot and ground
    • Air resistance to forward motion
  • Forces in kicking
    • Explain how Newton’s 3 laws will affect the ball when it is kicked
    • Label the force arrow to show the direction of the force
    • 1 st Law – ball is kicked, overcomes inertia and accelerates
    • 2 nd Law – size and direction of acceleration depend on size and direction of the applied force
    • 3 rd when ball is kicked a fore equal to the force applied to the ball is felt by the foot
    • Gravity and friction slow ball down
  • Typical question
    • Use Newton’s Three Laws of Motion to explain how a tennis player moves towards the ball in preparation to play a stroke. (5 marks)
  • Answer
    • First Law – reluctance to change state of motion/constant motion/ uniform motion/velocity;
    • Force required to change state of motion/overcome inertia of player;
    • Muscle contractions; (Sub max 2 marks)
    • Second Law – magnitude/size of force governs change in momentum;
    • Mass remains constant;
    • Force governs magnitude of acceleration given to player;
    • And direction; (Sub max 2 marks)
    • Third Law – equal and opposite reaction force;
    • Force applied to ground/ moves performer;
    • Ground Reaction Force. (Sub max 2 marks)
    • Max of 5 marks
    • Do not credit Force = Mass x Acceleration
    • Only credit responses that relate to the player not the ball.