The centre of gravity is the point at which the whole weight of an object can be considered to act and, thus, at which all parts of an object are in balance. The position of the centre of gravity varies according to the shape of the object. In objects with a regular shape, the centre of gravity coincides with its geometric centre. In objects with an irregular and variable shape (as in the human body), the centre of gravity cannot be defined easily and changes with every change in position of the body; it may not even lie within the physical substance of the body. The centre of gravity of a projectile in flight follows a fixed path, but body movements may raise or lower the body parts around the centre of gravity. In this way, it is possible to jump different heights even though the centre of gravity reaches the same height.
NOTE: The term centre of mass is often used interchangeably with centre of gravity, but they are physically different concepts. They happen to coincide in a uniform gravitational field, but where gravity is not uniform, centre of gravity refers to the mean location of the gravitational force acting on a body.
Websites:
http://en.wikipedia.org/wiki/Center_of_mass
http://www.grc.nasa.gov/WWW/K-12/airplane/cg.html
http://www.answers.com/topic/center-of-gravity
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