12.1. Pressure in Fluids http://www.ck12.org
Hydrostatic Pressure
Hydrostatic pressure is the pressure produced by a fluid under gravity when at equilibrium. Under hydrostatics
pressure, a fluid remains motionless. For example, a vertical column of water confined behind a dam experiences
hydrostatic pressure.
Figure12.5 shows a picture of Hoover Dam and a typical cross section of a dam. Have you ever noticed that dams
tend to be bottom heavy, that is, there is more reinforcement at the bottom of the dam than at the top of the dam?
Why do you suppose this is?
FIGURE 12.5
Try this simple experiment! Poke a hole near the bottom of an empty soft drink can and another hole in the center of
the can. Fill the can with water and water will flow out of each hole. But will the flow out of each hole have the same
horizontal extent? No. The flow will resemble what we see inFigure12.6. Water will flow out of the bottom hole at
a greater pressure (and velocity) than the top hole. The reason for this and the reason why dams are reinforced more
at their base is hydrostatic pressure. Pressure increases with depth.
FIGURE 12.6
Figure12.7 shows a column of water visualized as three “cubes” of water, one on top of the other. Each cube is
identical, with length, width, and height equal todand of weightmg. The entire column of cubes rests upon area
A. The normal force upon each cube increases with depth since each cube must support the weight of all the cubes
above it. The surface areaAremains constant but the force acting upon this area increases withmg, 2 mg, 3 mg, and
so on. We will show that the pressuresP 1 ,P 2 , andP 3 at the points shown inFigure12.7 increase with the height, if
the density of the fluid is constant. We defineP 0 =0.