What is Fluid Pressure and Force?
Fluid pressure is the force exerted by a fluid per unit area, increasing with depth due to overlying fluid weight. This principle governs dams, pipelines, submarine hulls, and hydraulic systems. Pressure forces on submerged surfaces depend on depth, area, and fluid density.
Hydrostatic pressure P = ρgh (pressure at depth h), and total pressure force on a surface is F = P·A or the integral of pressure over area.
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Step-by-step worked examples
Water at 10 m depth (ρ = 1000 kg/m³, g = 9.81 m/s²). Find pressure.
P = ρgh P = 1000 × 9.81 × 10 P = 98,100 Pa ≈ 98.1 kPa
Dam wall 50 m² area at average depth 8 m. Average pressure force?
P_avg = ρgh_avg = 1000 × 9.81 × 8 = 78,480 Pa F = P_avg × A = 78,480 × 50 = 3,924,000 N ≈ 3.9 MN
Submarine at 300 m depth (seawater ρ = 1025 kg/m³). Pressure?
P = 1025 × 9.81 × 300 P = 3,016,575 Pa ≈ 3.02 MPa ≈ 30 atm
Flashcards
Quick quiz
Q1.At 20 m depth in water, pressure is…
Q2.Hydrostatic pressure depends on…
Q3.Seawater vs freshwater at same depth, pressure is…
Q4.Total force on submerged flat plate…
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Common mistakes
Pressure depends on total fluid volume. — Correct: Pressure depends only on depth, not total volume.
Pressure is same everywhere in static fluid. — Correct: Pressure increases with depth; constant at same level.
Pressure force is F = P. — Correct: Force is F = P·A (pressure times area).
Air pressure at sea level is negligible. — Correct: Air pressure ≈ 101 kPa at sea level, significant.
FAQ
What is hydrostatic pressure?
Pressure due to fluid weight above a point; P = ρgh.
Why is pressure greater at dam bottom?
More water above means greater weight and higher pressure.
How do submarines withstand high pressure?
Strong hull materials and spherical shapes distribute pressure force evenly.
Is pressure scalar or vector?
Pressure is scalar (magnitude only); forces are vectors (normal to surface).