Biomechanics

• Newton established three laws of motion
• Newton's First Law (Law of Inertia)


  An object will continue in a state of constant velocity until compelled to change by an external force
• Constant velocity


  Can mean either moving at a set speed or rate, or stillness (velocity of zero)
• Changes when an unbalanced external force is applied
• Balanced forces


  All forces acting on the body are cancelling each other out, resulting in a net force of zero
• Balanced forces


  • Sprinter in set position on blocks
  • Middle of a 10k race
• Newton's Second Law (Law of Acceleration)

  An object will accelerate proportional to the force acting and in the direction of that force
• Force = Mass x Acceleration
• Applying more force


  • Causes the object to accelerate more
• Applying force in a direction

  • Causes the object to move in that direction
• Newton's Third Law (Law of Reaction)


  For every action, there is an equal and opposite reaction
• Judo player hitting the ground

  • The ground applies an equal and opposite reaction force back on the player
• Striking a ball


  • The ball applies an equal and opposite reaction force back on the racket
• Smaller objects move more than larger objects when an unbalanced force is applied
• Illustrating forces


  1. Draw weight vector from centre of mass downwards

  2. Draw ground reaction force vector from point of contact upwards, equal and opposite to weight

  3. Draw friction force vector forward

  4. Draw air resistance force vector backward
• The vector illustrations should be accurate and to scale
• Illustrating vectors and forces in a sprint finish


  1. Depict acceleration at the start of the race

  2. Depict constant velocity in the middle of the race

  3. Depict deceleration past the finish line
• Friction force


  The anti-slipping force that acts when two surfaces move across one another
• Air resistance


  The fluid resistance experienced by an object moving through the air
• Friction force > Air resistance


  Indicates acceleration
• Friction force = Air resistance


  Indicates constant velocity
• Air resistance > Friction force


  Indicates deceleration
• For a runner, vertical forces are generally considered negligible
• Factors affecting friction


  • Nature of the surface
  • Cross-sectional area of the surface
  • Force applied
  • Firmness of the reaction force
• Factors affecting air resistance


  • Surface characteristics
  • Frontal cross-sectional area
  • Velocity of the body
  • Weight of the body
  • Shape of the body
• The student's answer incorrectly identified the relationship between friction and air resistance, leading to the wrong conclusion about the state of motion
• Balanced forces


  Two or more forces equal in size but opposite in direction, resulting in no movement
• Unbalanced forces


  Forces not equal in size and/or not opposite in direction, resulting in acceleration or deceleration
• Balanced forces


  • Initial shove in a rugby scrum
• The student's answer only provided an example of balanced forces and did not include an example of unbalanced forces
• Center of mass


  The point where the mass of the body is distributed equally in all directions
• Factors affecting stability of center of mass


  • Height of center of mass
  • Mass of the object
  • Size of the base of support
  • Number of points of contact
  • Line of gravity in relation to the base of support
• The illustrations demonstrate how the position of the center of mass changes with different body positions
• Base of support


  The area over which the body's weight is distributed, providing stability
• More points of contact at the base


  Leads to more stability
• Line of gravity


  If the center of mass is above the base of support, the body is stable
• Factors affecting stability of a handstand


  1. Mass of gymnast

  2. Size of base of support

  3. Height of center of mass
• Lighter gymnasts have lower stability
• Larger base of support increases stability
• Lowering center of mass increases stability