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

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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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