AS A Level Sport and Physical Education

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• AS / A Level Sport and Physical Education
• Centre of Mass

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CENTRE OF MASS (GRAVITY)
Stability

• CENTRE of MASS (CoM)
• this is the scientific term for centre of gravity
  - since the concept is not dependent on gravity
• CoM is the single point in a body which
  represents all the spread out mass of a body

• the weight acts at the CoM since gravity acts on
  mass to produce weight

• WHERE IS THE CENTRE OF MASS?
• position of centre of mass depends on shape of
  body
• this is how the high jumper can have his CoM pass
  under the bar
• but he could still clear the bar

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BALANCE and TOPPLING
Stability

• TOPPLING
• the CoM must be over the base of support if a
  person is to be on balance
• toppling would be caused by the weight acting at
  the CoM creating a moment about the near edge of
  the base of support
• this can be used by divers or gymnasts to
  initiate a controlled spinning (twisting) fall
  and lead into somersaults or twists

• BALANCE
• to keep on balance the CoM must be over the base
  of support

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CENTRE OF MASS - GENERATION OF ROTATION
Stability

• FORCE ACTING AT TAKE-OFF THROUGH CoM
• the line of action of a force on a jumper before
  take-off determines whether or not he rotates in
  the air after take off
• if a force acts directly through the centre of
  mass of an object, then linear acceleration will
  occur (Newton's second law), no turning or
  rotating
• example
• basketballer force acts through CoM therefore
  jumper does not rotate in air

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CENTRE OF MASS - GENERATION OF ROTATION
Stability

• FORCE ACTING AT TAKE-OFF NOT THROUGH CoM
• a force which acts eccentrically to the centre of
  mass of a body will cause the body to begin to
  rotate (will initiate angular acceleration)
• this is because the force will have a moment
  about the CoM and will cause turning
• example
• high jumper force acts to one side of CoM
  therefore jumper turns in air