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Pressure in Fluid Systems

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Unit 3 Pressure Pages Fluid Hydraulic System Pneumatic System Density Specific gravity Buoyant force Hydrometer Pressure PSI Atmospheric Pressure Absolute pressure Gage pressure Manometer

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Pressure in a Fluid System Unit 3 Review Page 53 #1-15

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Fluid Gas or liquid that conforms to the shape of the container Anything that flows

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Hydraulic system Fluid system that uses liquid as the fluid Pneumatic system Fluid system that uses air or gas as the fluid

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Why does a hot air balloon float? Why does motor oil rise to the top of water? Density Amount of matter in a given amount of substance = Mass/Volume

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Density SI measured in: Kg/m 3 or gm/cm 3 English measured in: Lbm/ft 3 or lb/ft 3

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Density What is the density of gold if you have a 1.036cm 3 piece that had a mass of 20grams? D=m/v D=20g/1.036cm 3 D=19.3g/cm 3

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Density What is the density of gold if you have a 3.108cm 3 piece that had a mass of 60grams? D=m/v D=60g/3.108cm 3 D=19.3g/cm 3

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Other Densities Platinum Diamond Chromium Tin (white) Tin (gray)

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Density What is the mass in grams of mercury with a volume of 1cm 3 ? D = m / v 13.6 g/cm 3 = x / 1cm g = x

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Density What is the mass in kilograms of balsa wood with a volume of 1m 3 ?.3g / cm 3 = x / 1,000,000cm 3 300,000 g = x 300 kg =x 1m 3 = __cm 3 1m 3 = 100cm x 100cm x 100cm = 1,000,000 cm 3 D = m / v.3g / cm 3 = x / 1m 3

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Specific Gravity Density of a substance divided by the density of water Because specific gravity is density/density the units cancel out and is written as a whole number

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Specific Gravity Copper has a density of 8.9g/cm 3 What is its specific gravity? Specific Gravity = density of substance = density of water S.G. = (8.9g/cm 3 ) / (1.0g/cm 3 ) S.G. = 8.9

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Buoyant Force The upward force on a substance from a fluid Will lead sink or float in water? Will it sink or float in mercury?

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Hydrometer Instrument that measures density or specific gravity of fluids Can you drown in quick sand?

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Pressure Force per unit area exerted by a fluid

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Force on Airplane Windows An airplane window has a surface area of 136 square inches. Air pressure inside the cabin is 12.3 lb/in 2 The force pushing on the window

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Pressure What happens to the pressure as we move away from the earth? home_runs_amp_holeinone.htm home_runs_amp_holeinone.htm

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Force on Airplane Windows An airplane window has a surface area of 144 square inches. Air pressure inside the cabin is 14.7 lb/in 2 Air pressure outside the window is 6.7 lb/in 2 The force pushing in the window The force pushing out the window Net force on window

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Inward force on window F = P x A F = (14.7lb/in 2 )(144in 2 ) F = lb F = P x A F = (6.7 lb/in 2 )(144in 2 ) F= 964.8lb Outward force on window

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Net Force on window The window is being pushed outward with a net force of 1152 lb. Net Force = Force out – Force in Net Force = lb – lb Net Force = 1152 lb

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Net Force on window If the plane rises to a higher altitude and the pressure outside the plane changes to 5.4 lb/in 2 How much stronger will the windows need to be in order to hold the pressure

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Inward force on window F = P x A F = (14.7lb/in 2 )(144in 2 ) F = lb F = P x A F = (5.4 lb/in 2 )(144in 2 ) F= 777.6lb Outward force on window

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Net Force on window The window was originally pushing outward with a net force of 1152 lb. Therefore it needs to hold more pounds of pressure ( – 1152) Net Force = Force out – Force in Net Force = lb – lb Net Force = lb

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Pressure Pressure acts equally in all direction at any point in a fluid and therefore it is a scalar

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Absolute vs. Gage Pressure When we fill a tire to 30lb/in 2 is that the absolute or the gage pressure? Atmospheric pressure = 14.7 lb/in 2

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Absolute Pressure Total pressure compared to a perfect vacuum Gage Pressure Pressure measured above atmospheric pressure G.P = Total pressure – atmospheric pressure

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Total Pressure Gage pressure is generally measured with a gage

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Pressure Tire gage reads 38lb/in 2 What is the atmospheric pressure? What is the gage pressure? What is the total pressure?

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Pressure Tire gage reads 38lb/in 2 What is the atmospheric pressure? What is the gage pressure? What is the total pressure?

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How does pressure change with depth? Where is the pressure greater the shallow end or the deep end? Why?

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Pressure increases with depth There is more water sitting on top of the deep end There is twice as much weight Twice as much force Twice as much pressure

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Relationship between pressure and depth

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Water Pressure Calculation Given: The height of the water in a storage tank is 100 ft above the valve. The weight density of water is 62.4 lb/ft 3 Find: The pressure at the valve in lb/ft 2

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Water Pressure Calculation P = pw x h P = (62.4 lb/ft3)(100ft) P = 6240 lb/ft 2 Given: 1 ft 2 = 144 in 2 Now find: Pressure in PSI

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Water Pressure Calculation P = pw x h P = (62.4 lb/ft3)(100ft) P = 6240 lb/ft 2 Given: 1 ft 2 = 144 in 2 p = (6240 lb/ft 2 )(1ft2/144in 2 ) P = 43.3 lb/in 2 (psi)

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Balanced pressure across the valve

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Unbalanced pressure across the valve

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Pressure on bottom does not depend on the size of the tank

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Pressure acts like forces Pressure is a prime mover Measuring Pressures Manometer – instrument used to measure fluid pressure

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Hydraulic lift Liquids are incompressible Air compressor increases the pressure to the fluid Large pushing force is exerted on the lifting piston

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Hydraulic jack? Large cylinder to a small cylinder Same pressure = more force in the smaller cylinder Small to large = allowable force but small increments?

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An enclosed fluid under pressure exerts that pressure throughout its volume and against any surface containing it. That's called 'Pascal's Principle', and allows a hydraulic lift to generate large amounts of FORCE from the application of a small FORCE. Assume a small piston (one square inch area) applies a weight of 1 lbs. to a confined hydraulic fluid. That provides a pressure of 1 lbs. per square inch throughout the fluid. If another larger piston with an area of 10 square inches is in contact with the fluid, that piston will feel a force of 1 lbs/square inch x 10 square inches = 10 lbs.

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So we can apply 1 lbs. to the small piston and get 10 lbs. of force to lift a heavy object with the large piston. Is this 'getting something for nothing'? Unfortunately, no. Just as a lever provides more force near the fulcrum in exchange for more distance further away, the hydraulic lift merely converts work (force x distance) at the smaller piston for the SAME work at the larger one. In the example, when the smaller piston moves a distance of 10 inches it displaces 10 cubic inch of fluid. That 10 cubic inch displaced at the 10 square inch piston moves it only 1 inch, so a small force and larger distance has been exchanged for a large force through a smaller distance.

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