Fig. 6.5 describes an area of constant thickness which is denoted by t. Therefore the mass of elemental i.e. dm is given by
dm = ptdA p, t are constants, dA =elemental area.
From equation (6.2), we have
x = ʃ x dA / A , y = ʃ y dA / A , z = ʃ z dA / A
In equation (6.6), ʃ x dA, ʃ y dA, ʃ z dA are referred as first moment of area.