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Pressure in Fluid Systems
3/25/2017 UNIT 3 Pressure in Fluid Systems
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Unit 3 Pressure Pages 43-60 Fluid Hydraulic System Pneumatic System
3/25/2017 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
3/25/2017 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
3/25/2017 Fluid Gas or liquid that conforms to the shape of the container “Anything that flows”
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Hydraulic system Pneumatic system
3/25/2017 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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3/25/2017 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: English measured in: Kg/m3 or gm/cm3
3/25/2017 Density SI measured in: Kg/m3 or gm/cm3 English measured in: Lbm/ft3 or lb/ft3
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3/25/2017 Density What is the density of gold if you have a 1.036cm3 piece that had a mass of 20grams? D=m/v D=20g/1.036cm3 D=19.3g/cm3
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3/25/2017 Density What is the density of gold if you have a 3.108cm3 piece that had a mass of 60grams? D=m/v D=60g/3.108cm3 D=19.3g/cm3
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Other Densities Platinum Diamond Chromium Tin (white) Tin (gray) 21.45
3/25/2017 Platinum Diamond Chromium Tin (white) Tin (gray) 21.45 7.15 7.265 5.769
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What is the mass in grams of mercury with a volume of 1cm3?
3/25/2017 Density What is the mass in grams of mercury with a volume of 1cm3? D = m / v 13.6 g/cm3 = x / 1cm3 13.6 g = x
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What is the mass in kilograms of balsa wood with a volume of 1m3?
3/25/2017 Density What is the mass in kilograms of balsa wood with a volume of 1m3? 1m3 = __cm3 1m3 = 100cm x 100cm x 100cm = 1,000,000 cm3 D = m / v .3g / cm3 = x / 1m3 .3g / cm3 = x / 1,000,000cm3 300,000 g = x 300 kg =x
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Density of a substance divided by the density of water
3/25/2017 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/cm3
3/25/2017 Specific Gravity Copper has a density of 8.9g/cm3 What is its specific gravity? Specific Gravity = density of substance = density of water S.G. = (8.9g/cm3) / (1.0g/cm3) S.G. = 8.9
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The upward force on a substance from a fluid
3/25/2017 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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3/25/2017 Hydrometer Instrument that measures density or specific gravity of fluids Can you drown in quick sand?
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Force per unit area exerted by a fluid
3/25/2017 Pressure Force per unit area exerted by a fluid
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Force on Airplane Windows
3/25/2017 Force on Airplane Windows An airplane window has a surface area of 136 square inches. Air pressure inside the cabin is 12.3 lb/in2 Find: The force pushing on the window
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What happens to the pressure as we move away from the earth?
3/25/2017 Pressure What happens to the pressure as we move away from the earth?
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Force on Airplane Windows
3/25/2017 Force on Airplane Windows An airplane window has a surface area of 144 square inches. Air pressure inside the cabin is 14.7 lb/in2 Air pressure outside the window is 6.7 lb/in2 Find: The force pushing in the window The force pushing out the window Net force on window
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Outward force on window
3/25/2017 Inward force on window F = P x A F = (6.7 lb/in2)(144in2) F= 964.8lb Outward force on window F = P x A F = (14.7lb/in2)(144in2) F = lb
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Net Force on window Net Force = Force out – Force in
3/25/2017 Net Force on window Net Force = Force out – Force in Net Force = lb – lb Net Force = 1152 lb The window is being pushed outward with a net force of 1152 lb.
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3/25/2017 Net Force on window If the plane rises to a higher altitude and the pressure outside the plane changes to 5.4 lb/in2 How much stronger will the windows need to be in order to hold the pressure
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Outward force on window
3/25/2017 F = P x A F = (14.7lb/in2)(144in2) F = lb Inward force on window F = P x A F = (5.4 lb/in2)(144in2) F= 777.6lb
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Net Force on window Net Force = Force out – Force in
3/25/2017 Net Force on window Net Force = Force out – Force in Net Force = lb – lb Net Force = lb The window was originally pushing outward with a net force of 1152 lb. Therefore it needs to hold more pounds of pressure ( – 1152)
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3/25/2017 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
3/25/2017 Absolute vs. Gage Pressure When we fill a tire to 30lb/in2 is that the absolute or the gage pressure? Atmospheric pressure = 14.7 lb/in2
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Absolute Pressure Gage Pressure
3/25/2017 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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Gage pressure is generally measured “with a gage”
3/25/2017 Gage pressure is generally measured “with a gage” Total Pressure
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Pressure Tire gage reads 38lb/in2 What is the atmospheric pressure?
3/25/2017 Pressure Tire gage reads 38lb/in2 What is the atmospheric pressure? What is the gage pressure? What is the total pressure?
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Pressure What is the atmospheric pressure? What is the gage pressure?
3/25/2017 Pressure Tire gage reads 38lb/in2 What is the atmospheric pressure? What is the gage pressure? What is the total pressure? 14.7 lb/in2 38 lb/in2 38 lb/in lb/in2 = 52.7 lb/in2
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How does pressure change with depth?
3/25/2017 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
3/25/2017 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
3/25/2017 Relationship between pressure and depth
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Water Pressure Calculation
3/25/2017 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/ft3 Find: The pressure at the valve in lb/ft2
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Water Pressure Calculation
3/25/2017 Water Pressure Calculation P = pw x h P = (62.4 lb/ft3)(100ft) P = 6240 lb/ft2 Given: 1 ft2 = 144 in2 Now find: Pressure in PSI
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Water Pressure Calculation
3/25/2017 Water Pressure Calculation P = pw x h P = (62.4 lb/ft3)(100ft) P = 6240 lb/ft2 Given: 1 ft2 = 144 in2 p = (6240 lb/ft2)(1ft2/144in2) P = 43.3 lb/in2 (psi)
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Balanced pressure across the valve
3/25/2017 Balanced pressure across the valve
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Unbalanced pressure across the valve
3/25/2017 Unbalanced pressure across the valve
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Pressure on bottom does not depend on the size of the tank
3/25/2017 Pressure on bottom does not depend on the size of the tank
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Pressure acts like forces
3/25/2017 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
3/25/2017 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
3/25/2017 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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3/25/2017 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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3/25/2017 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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