Consider a thin flat plate of irregular shape (see figure 6.1a). The centroid of such a thin plate, or lamina, is that point at which the whole of the area of the plate may be assumed to be concentrated. The first moment of area of a lamina about some axis XX is given by Aӯ, where A is the total area of the lamina and ӯ is the distance of the centroid from the chosen axis XX.
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