Specific values can be obtained by decomposing the pressure prism into two parts, ABDE and BCD, as shown in Fig. However, the resultant force is still equal in magnitude to the volume of the pressure prism, and it passes through the centroid of the volume. In this instance, the cross section of the pressure prism is trapezoidal. This same graphical approach can be used for plane surfaces that do not extend up to the fluid surface as illustrated in Fig. This result can readily be shown to be consistent with that obtained from Eqs. For the volume under consideration the centroid is located along the vertical axis of symmetry of the surface, and at a distance of h/3 above the base (since the centroid of a triangle is located at h/3 above its base). The resultant force must pass through the centroid of the pressure prism.
The magnitude of the resultant fluid force is equal to the volume of the pressure prism and passes through its centroid. Graphical representation of hydrostatic forces on a vertical rectangular surface.
Pressure prism for vertical rectangular area. Where bh is the area of the rectangular surface, A.
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