# Pressure in Fluid Systems

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Pressure in Fluid Systems
3/25/2017 UNIT 3 Pressure in Fluid Systems

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

Pressure in a Fluid System
3/25/2017 Pressure in a Fluid System Unit 3 Review Page 53 #1-15

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”

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

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

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

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

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

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

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

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

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?

Force per unit area exerted by a fluid
3/25/2017 Pressure Force per unit area exerted by a fluid

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

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?

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

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

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.

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

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

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)

3/25/2017 Pressure Pressure acts equally in all direction at any point in a fluid and therefore it is a scalar

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

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

Gage pressure is generally measured “with a gage”
3/25/2017 Gage pressure is generally measured “with a gage” Total Pressure

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?

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

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?

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

Relationship between pressure and depth
3/25/2017 Relationship between pressure and depth

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

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

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)

Balanced pressure across the valve
3/25/2017 Balanced pressure across the valve

Unbalanced pressure across the valve
3/25/2017 Unbalanced pressure across the valve

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

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

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

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?

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.

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.