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Pressure; Pascal’s Principle. PRESSURE OF A FLUID. pressure = height of mercury column . air pressure. Barometer. Gauge pressure – pressure above the atmospheric pressure. absolute (total) pressure = gauge pressure + atmospheric press. Pressure is produced by the
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PRESSURE OF A FLUID pressure = height of mercury column air pressure Barometer
Gauge pressure – pressure above the atmospheric pressure. absolute (total) pressure = gauge pressure + atmospheric press.
Pressure is produced by the weight of the fluid above the surface.
force pressure = ------ area weight of fluid = ---------------- area mg = ----- area density · volume · g = ------------------------- area density · (area · height) · g = -------------------------------- area
pressure = density · height · g P = ρhg gauge pressure P = ρhg + P0 absolute pressure SI units: pressure: N/m2 = Pascals (Pa) density: kg/m3 height (depth): m
Standard atmospheric pressure 760 mm Hg ≈ 30 in Hg Calculate 1 atm in Pascals P = ρgh ρHg = 13.6 x 103 kg/m3 g = 9.8 N/kg h = 0.760 m P = 1.013 x 105 Pa
Why does a barometer use mercury and not water? If p = 1 atmosphere = 1.013 x 105 Pa ρ = 1.00 g/cm3 = 1.00 x 103 kg/m3 Find height. 10.3 m
Pressure does not depend on volume, only on height (depth). “Water seeks its own level”
Measure blood pressure at upper arm same height as heart
Pascal’s Principle – The pressure in an enclosed fluid is constant throughout the fluid. p1 = p2 F1 F2 --- = --- A1 A2
1 N ? A = 1 cm2 A = 50 cm2 p1 = p2 1 N F2 ------- = ------ 1 cm2 50 cm2 50 N
If piston on left moves 10 cm, what distance does piston on right move? work1 = work2 F1 d1 = F2 d2 (1) (10) = (50) d2 0.2 cm
Area of brake cylinder > area of brake line force of brake cylinder > force of brake pedal