Pressure is deŸned as the normal force per unit area exerted by a žuid (liquid or gas) on any surface. œe surface can be either a solid boundary in contact with the žuid or, for purposes of analysis, an imaginary plane drawn through the žuid. Only the component of the force normal to the surface needs to be considered for the determination of pressure. Tangential forces that give rise to shear and žuid motion will not be a relevant subject of discussion here. In the limit that the surface area approaches zero, the ratio of the di¥erential normal force tothe di¥erential area represents the pressure at apoint on the surface. Furthermore, if there is no shear in the žuid, the pressure at any point can be shown tobe independent of the orientation of the imaginary surface under consideration. Finally, it should be noted that pressure is not deŸned as a vector quantity and is therefore nondirectional.