A2 Biomechanics

1

• A2 Biomechanics
• Lesson 1 Extension Work

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MECHANICS OF MOTION

• LINEAR MOTION
• motion in a straight line
• examples the movement of the body as a whole in
  
• sprinting
• cycling
• swimming
• sports vehicles
• any motion in which there is no bulk rotation of
  the object or body in motion
• projectiles in flight

• ANGULAR MOTION
• motion in which there is a rotation of the body
• tumbling
• diving
• spinning skater
• turning during skiing
• spins and turns in dancing
• or part of the body
• forearm rotating about the elbow
• lower leg rotating about the knee
• any twisting or turning motion
• wheels on a bike or vehicle

3
MASS - INERTIA

• MASS
• the mass of a body or object is the same
  everywhere and is related to amount of matter and
  inertia
• INERTIA
• is the property of mass which means that it is
  hard to get a massive body moving, and also hard
  to stop it once it is moving
• measured in kilogrammes kg

• WEIGHT and MASS are DIFFERENT
• weight is produced by the gravitational force
  field acting on objects / bodies
• it is a force which acts downwards towards the
  centre of the Earth

4
DISTANCE - DISPLACEMENT

• DISTANCE
• means the total path length moved by a body
• example
• a 10,000 m race is run round and round the track
• 25 times 400 m, starting and finishing POSITION
  are the same
• distance travelled is 10,000 m
• unit the metre m

• DISPLACEMENT
• this means the vector distance from a fixed point
  (starting point or origin)
• the actual as the crow flies distance between
  start and finish (with direction included)
• example
• the start and finish of a long distance race
  (Stage 5 of the Tours de France)
• may be 190 km apart due West, but the distance
  travelled may be 250 km!
• unit the metre m

5
POSITION

• POSITION
• a way of explaining where a point is relative to
  some fixed point
• position is usually expressed in terms of
  coordinates (x and y) like a graph in maths
• example
• the centre forward takes a shot from a position
  20 m out from the goal line, and 10m to the left
  of the left hand post
• the left hand post is the fixed point or origin
  of measurement
• 20 m and 10 m are the coordinates of the position
  of the centre forward relative to that point.

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SPEED - VELOCITY

• SPEED
• distance moved v s unit ms-1
  time taken t
• scalar (no direction)
• VELOCITY
• speed in a given direction
• vector
• DISTANCE / TIME graph
• gradient of graph is velocity

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ACCELERATION

• ACCELERATION
• change of velocity a v - u unit
  ms-2 time taken to change t
• acceleration is in the same direction as net
  force
• acceleration is a vector (has direction)
• an object changing direction is accelerating,
  since the velocity changes
• example
• swerving rugby player
• direction of acceleration is along the radius of
  the curve (path of player)
• this is a radial acceleration
• DECELERATION
• is negative acceleration (slowing down)
• VELOCITY / TIME graph
• gradient of graph is acceleration
• area under graph is distance travelled

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The 100m SPRINT

• VELOCITY - TIME GRAPH
• steep slope for first part large acceleration
• this corresponds with a large forward net force
  applied at the start

THE START

• friction is large
• provides forward acceleration

• net force forwards (resultant) shown in black

9
The 100m SPRINT

• MIDDLE OF RUN
• the velocity time graph is almost level
• which means that acceleration is almost zero
• therefore forces cancel out

• END OF RUN
• the velocity time graph has a small negative
  slope
• which means that the sprinter decelerates
• therefore there is a net force backwards shown in
  black

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FORCE

• PROPERTIES OF FORCE
• force has direction and size (value)
• and is therefore a VECTOR
• when describing a force it is important to
  explain where the force acts (the point of
  action)
• as well as the direction

• NET FORCE
• net force is the result of all forces added
  together taking the direction into account (see
  VECTORS)
• net force forwards produces acceleration -
  positive
• net force backwards produces deceleration -
  negative
• net force sideways produces change of direction

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PIN MEN - FREE BODY DIAGRAMS

• DIAGRAM
• shows four forces acting
• 2 forces acting up on the foot and down on the
  body

• FORCES ACTING
• forces are represented by arrows
• in the direction of the force
• the point of action of the force should be shown
  where the force acts
• at the foot
• on the body
• on the hand
• the length of the arrow represents the size of
  the force

• 2 forces acting backwards on the body and
  forwards on the foot
• longer arrows mean greater force

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MOMENTUM

• MOMENTUM
• a concept derived from Newton's second law which
  says
• force rate of change of momentum
• (Linear) momentum mass x velocity
• linear means in a straight line
• momentum includes both mass and velocity
• so an object which has a lot of momentum requires
  a lot of force to stop it
• which is a good argument for fast heavy rugby
  players or American footballers
• momentum is a vector (and therefore has direction)