# PRESSURE IN A FLUID

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**PRESSURE IN A FLUID**

When a fluid (either liquid or gas) is at rest, it exerts a force perpendicular to any surface in contact with it, such as a container wall or a body immersed in the fluid. This is the force that you feel pressing on your legs when you dangle them in a swimming pool. While the fluid as a whole is at rest, the molecules that make up the fluid are in motion; the force exerted by the fluid is due to molecules colliding with their surroundings. If we think of an imaginary surface within the fluid, the fluid on the two sides of the surface exerts equal and opposite forces on the surface. (Otherwise, the surface would accelerate and the fluid would not remain at rest.) Consider a small surface of area dA

centered on a point in the fluid; the normal force exerted by the fluid on each side is dF.l (Fig. 14-1). We define the pressure p at that point as the normal force per unit area, that is, the ratio of dF.l to dA: