1. The problem statement... The textbook I am using states that an idealized fluid cannot sustain a force tangential to its surface. Can anyone expound upon this argument? This statement is an introductory exploration of pressure and is cited as the reason that the force dF exerted by its surrounding is perpendicular to the surface bounding a fluid. Therefore, this force dF is parallel to the area vector dA. The pressure is then defined to be the ratio of these two quantities. Pressure being the ratio of force per unit area makes perfect sense, but this explanation is somewhat cloudy to me.