006 In which direction does fluid pressure act?


From the way the “water skin” “explodes” away, we can infer the direction in which the water was pressing against the balloon. It looks like at every point on the balloon surface, the pressure forces were acting perpendicular to the balloon surface.

We should be careful in distinguishing between PRESSURE and PRESSURE FORCES. At the microscopic level, because each fluid molecule pushes against all the surrounding molecules, pressure acts in all directions. At the macroscopic level, when a fluid encounters a surface, it exerts a pressure force perpendicularly into the surface.

Because pressure “acts in all directions”, pressure is not considered to be a vector quantity (which should act in only ONE direction). The pressure force acting on a surface, on the other hand, acts in one particular direction, and is considered to be a vector quantity. Meaning in the formula F=P.A, F is vector whereas P is scalar.

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