Biomechanics

• Linear motion is the movement of a body in a straight or curved line with all parts moving in the same direction, in the same distance and at the same time, e.g bobsleigh. Created when an external force is hit through the centre of mass.

  Angular motion is the movement of a body/part of a body in a circular path about an axis of rotation, e.g gymnast. Created when an external force is hit off the centre of mass.
• Newton’s laws of motion:

  1. Law of inertia - a body continues in a state of rest or of unit velocity unless acted upon by an external unbalanced force.
  2. Law of acceleration - a body’s rate of change in momentum is directly proportional to the size of the force applied and acts in the same direction.
  3. Law of reaction - for every action force there is an equal and opposite reaction force that is equal in size and opposite to the action force.
• Centre of mass is the point at which a body is balanced in all directions and weight is equally distributed, not a fixed position and follows a pre determined flight path and it can be outside of a body. Affected by shape and density of different parts of the body. E.g Fosbury flop.
• Stability is the measure of how easily a body is moved from a position of equilibrium or balance.

  Affected by - position of COM as the lower the COM the more stable the performer is, position of line of gravity as it needs to be over the base of support for more stability, size of base of support as the more contact points/the bigger the base of support the more stability, and mass of athlete as the more mass the more stability (inertia).
• Distance time graphs:

  Horizontal line - no change in position over time, stationary body.

  Straight line with an increasing gradient - constant change in position over time equals constant speed, the steeper the gradient the faster the speed.

  Curved line with a decreasing gradient - decreasing change in position over time equals decreasing speed.

  Curved line with an increasing gradient - increasing change in position over time equals increasing speed.
• Speed time graphs:

  Line with no gradient - constant speed, the higher the horizontal line the faster the speed.

  Line with a positive gradient - acceleration (constant if line is straight).

  Line with a positive but decreasing gradient - accelerating to a constant speed as the line reaches no gradient.

  Line with a negative gradient - deceleration.
• Velocity time graphs:

  Shows the same as speed time graphs but including a negative y axis. Shows horizontal and vertical velocity, if the graph crosses the y axis the body has changed direction.

  Line with increasing negative velocity - acceleration in a change direction.

  Line with decreasing negative velocity - deceleration in the changed direction.
• External forces:

  Vertical - weight, acts downwards from the COM and reaction, acts upwards from the point of contact with the body.

  Horizontal - friction, acts with the direction of motion from the point of contact to surface (parallel) and air resistance, acts in the opposing direction to motion from the COM.
• Balanced forces are when 2 or more forces acting on a body are equal in size and opposing in direction, create 0 net force and remain at rest/constant speed/uniform velocity.

  Unbalanced forces are when 2 or more forces acting on a body are unequal in size and opposing in direction, create a positive or negative net force and are accelerating/decelerating.

  Net force is the sum of all forces acting on a body, also known as the resultant force.
• Factors affecting friction

  • Roughness of contact surface
  • Roughness of ground surface
  • Temperature of surface
  • Size of reaction
  • F1 downforce created by the vehicles spoiler bar
• Increased roughness of contact surface

  Increases friction and increases acceleration
• Increased roughness of contact surface


  • Running spikes
• Decreased roughness of contact surface

  Decreases friction and increases acceleration
• Decreased roughness of contact surface

  • Wax skis
• Increased roughness of ground surface

  Increases friction and increases acceleration
• Increased roughness of ground surface

  • Tartan running track
• Increased temperature of surface

  Increases friction and increases acceleration
• Increased temperature of surface


  • F1 tyres
• Increased reaction force

  Increases friction and increases acceleration
• Increased reaction force

  • 100m runner
• F1 downforce created by the vehicles spoiler bar

  1. Area of high pressure above the car

  2. Low pressure below

  3. Concentration gradient

  4. High pressure pushes down

  5. Increases downforce

  6. Increases friction

  7. Increases acceleration
• Factors affecting air resistance

  • Speed
  • Frontal cross sectional area
  • Smoothness of surface (clothing/equipment)
  • Streamlining/shape
  • Air density/altitude
• Increased speed

  Increases air resistance
• Speed shouldn't be compromised
• Decreased frontal cross sectional area

  Decreases air resistance
• Decreased frontal cross sectional area

  • F1 car
  • Cyclist
• Increased frontal cross sectional area

  Increases air resistance
• Increased frontal cross sectional area


  • Cyclist slowing down
• Increased smoothness of surface (clothing/equipment)

  Decreases air resistance and increases acceleration
• Increased smoothness

  • Fully lycra suit
• Increased streamlining/shape

  Decreases air resistance and increases acceleration
• Increased streamlining

  • Teardrop helmet
• Decreased air density/altitude

  Decreases air resistance
• Force is a push or pull that tends to alter the state of motion of a body. Mass*Acceleration. Units are newtons (N).
• Momentum is the quantity of motion possessed by a moving body. Mass*Velocity. Kgm/s
• Acceleration id the rate of change in velocity. Change in velocity/change in time. Units are m/s^2
• Weight is the gravitational force that the earth exerts on a body. Mass*Acceleration (due to gravity). Units are newtons (N).
• Distance is the length of the path taken by a body moving from one position to another. Units are metres (M).
• Displacement is the shortest route between 2 positions. Units are metres (M).
• Speed is the rate of change in a body. Distance/Time. Units are m/s.