Suppose the liquid in the manometer of Figure 9.8 is mercury (ρ = 1.36 × 10 4 kg/m 3) and the height h is measured to be 12.8 cm. Thus, the height difference h between surfaces of the liquid in a manometer is proportional to the gauge pressure in the region being measured. Equation (9-13) gives the difference P – P o between the absolute pressure and atmospheric pressure, and this difference is called gauge pressure. Equation (9-14) gives the absolute pressure P in the region of the fluid that has the pressure being measured. Of course, we could have obtained this result from Equation (9-6) directly. In which h is the difference, y o – y, in the heights of the liquid surfaces in the two columns. The standard value of atmospheric pressure is 1.013 × 10 5 Pa or about 2120 lb/ft 2 = 14.7 lb/in 2 (atmospheric pressure varies somewhat depending on elevation and weather conditions). Thus the pressure at points on the surface of the earth is called atmospheric pressure. We say “additional” because there is a pressure at the surface of the pool that results from the weight of the atmosphere pushing downward on each unit area of the surface. The force on each unit area of the pool bottom is greater than the force on each unit area of its surface because the bottom is supporting the additional weight of the water in the pool. The pressure at the bottom of a swimming pool is greater than at the top. Pressure units will be considered further after we have discussed the measurement of pressure.Īt points within a fluid at rest, pressure varies with vertical position. Many units of pressure are used in the sciences and engineering. The metric unit of pressure then is N/m 2 ≡ Pa (the pascal) the English unit is lb/ft 2. Pressure has units of force per unit area.

Thus the magnitude of the force on an infinitesimal surface at a point in a fluid has the same value regardless of the orientation of that surface.

Pressure is a physical property of a fluid at a point and must be independent of direction.

We have seen that fluids are isotropic, that the physical properties of the fluid at a point in the fluid cannot depend on direction.

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