KinesiologyUnit 8
Centre of Gravity & Stability
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BalanceDefinition of Balance:
An individual’s ability to control stability
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Balance (continues)
To achieve a balanced state, the two forces acting on the object – it’s
1. weight and 2. the vertical reaction provided by the support
–
must be equal in magnitude and must lie in the same vertical plane 3
Centre of Gravity
• Also called CENTRE OF MASS
• Body’s Balance or Pivot Point
• Definition of Centre of Gravity: The point about which a body’s weight is equally balanced in all directions
• In the human body the centre of gravity is not fixed because the body is made up of many segments that can move or stay stationary at any moment in time
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Centre of Gravity (continues)
• Two methods have been used to locate the centre of gravity:
• 1. Segmental/Suspension Method (In Motion)
• 2. Reaction Board (RB) Method (Motionless)
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Centre of Gravity (continues)
Single segmental objects have their Centre of Gravity directly above the base of support.
Sometimes the Centre of Gravity can be found outside the body. (Arc)
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Centre of Gravity (continues)
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Centre of Gravity (continues)
• Another Example:
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Centre of Gravity (continues)
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Stability• Definition of Stability:
• A resistance to a disruption in the equilibrium of the body
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Stability (continues)
• If a body in equilibrium is given a small displacement from its equilibrium position, one of three things will happen:
• It will keep going in the same direction (unstable equilibrium) or
• It will stop in the position it has been moved (neutral equilibrium) or
• It will return to its original position (stable equilibrium) 11
Newton’s First Law• A rotating body will continue to turn about its
axis of rotation with constant angular momentum unless an external couple or eccentric force is exerted upon it
• Also known as:The principle of conservation of angular momentum
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Newton’s Second Law
• The angular acceleration of a body is proportional to the torque causing it and takes place in the direction in which the torque acts
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Newton’s second law
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Newton’s Third Law
• For every torque that is exerted by one body on another, there is an equal and opposite torque exerted by the second body on the first
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End of Unit
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