2) 3.2 angular motion

• angular motion - movement of a body or part in a circular path about an axis of rotation
• eccentric force - a force applied outside the centre of mass, resulting in angular motion
• torque - a measure of the turning (rotational or eccentric) force applied to a body
• principal axis of rotation - an imaginary line that passes through the centre of mass about which a body rotates: longitudinal, frontal & transverse
• longitudinal axis - runs from head to toe through the centre of mass. e.g a spin or pirouette
• transverse axis - runs from left to right through the centre of mass e.g somersault
• frontal axis - runs from front to back through the centre of mass. e.g cartwheel
• sagittal plane and transverse axis

  for example - walking and squatting
• frontal plane and frontal axis

  for example - star jumps
• transverse plane and longitudinal axis

  for example - throwing and golf swings
• moment of inertia - a body's resistance to change its state of rotation
• Radian (rad) - a unit of measure of the angle through which a body rotates. 360 degrees = 2 pi radians and 1 radian = 57.3 degrees
• angular velocity - the rate of change in angular displacement measured in radians per second (rad/sec)
• angular velocity = angular displacement/time taken
• moment of inertia = sum of(mass x distribution of mass from axis of rotation)^2
• Mass:

  • the greater the mass of a body, the greater the moment of inertia
  • the low mass decreases the moment of inertia and the resistance to change state of rotation, so athletes can start routine, change state of rotation and stop rotation with relative ease

  low MI = fast rate of spin

  high MI = slow rate of spin
• moment of inertia is measured in kgm2
• if moment of inertia is high, resistance of rotation is also high therefore angular velocity is low = rate of spin is low
• if moment of inertia is low, resistance of rotation is also low therefore angular velocity is high = rate of spin is fast
• factors affecting moment of inertia

  • mass
  • distribution of mass from the axis of rotation
• Distribution of mass from the axis of rotation:

  • the further th mass moves from the axis of rotation, the greater the moment of inertia
  • e.g tucks lower the moment of inertia and the resistance to change state of rotation
• angular momentum - the quantity of angular motion possessed by a body
• angular momentum (kgm2rad/s) = moment of inertia (kgm2) x angular velocity (rad/s)
• conservation of angular momentum - angular momentum is a conserved quantity which remains constant unless an external eccentric force or torque is applied
• angular analogue of newtons 1st law - the angular equivalent of newtons 1st law of motion, which states: a rotating body will continue to turn about its axis of rotation with constant angular motion unless acted upon by an eccentric force or external torque