The effect of gravity, is such as to exert a force on each point of the mass of a body. Its line of action is the line connecting any one point of the mass with the centre of the earth. Its magnitude equals the weight of the mass imagined as concentrated at this point of the mass. As a result of the distance from the centre of the earth, all the lines of action of the forces of gravity are parallel. There is a resultant of the forces of gravity. For a certain position of a body, this resultant corresponds to a straight line inside the body. This straight line remains unchanged if the body is displaced a short distance in space, parallel to its long axis. It changes its position in the body, however, as soon as the long axis is turned. The straight line in the new position intersects the previous one at a particular point. The intersection point thus can be considered as the general point of application of all the resultants of the forces of gravity for any position of the body. This point is the centre of gravity of the body. The magnitude of the resultant is the weight of the body since the resultant equals the sum of the different forces of gravity. Consequently, we can imagine the weights of the different parts of the body as united at the centre of gravity of the body. The direction of the resultant is designated as the line of gravity. The line of gravity thus connects the centre of gravity and the centre of the earth. It does not change its absolute position in space as long as the centre of gravity is maintained, for example if the body is rotated any way about its centre of gravity.


Body Segment Supporting Point Unstable Equilibrium Plumb Line Erect Position 
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