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Pressure inPressure in
Fluid SystemsFluid Systems
Unit 3 Pressure Pages 43-60 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
Unit 3 Review
Page 53 #1-15
Fluid
Gas or liquid that conforms to the shape of the container
“Anything that flows”
Hydraulic system
Fluid system that uses liquid as the fluid
Pneumatic system
Fluid system that uses air or gas as the fluid
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: Kg/m3 or gm/cm3
English measured in: Lbm/ft3 or lb/ft3
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
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
Other Densities Platinum Diamond Chromium Tin (white) Tin (gray)
21.45 3.5-3.53 7.15 7.265 5.769
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
Density
What is the mass in kilograms of balsa wood with a volume of 1m3?
.3g / cm3 = x / 1,000,000cm3
300,000 g = x 300 kg =x
1m3 = __cm3
1m3 = 100cm x 100cm x 100cm
= 1,000,000 cm3
D = m / v .3g / cm3 = x / 1m3
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
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
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?
Hydrometer
Instrument that measures density or
specific gravity of fluids
Can you drown in quick sand?
Pressure Force per unit area exerted by a fluid
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
The force pushing on the window
Pressure What happens to the
pressure as we move away from the earth?
http://www.sciencedaily.com/videos/2006/1201-home_runs_amp_holeinone.htm
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
The force pushing in the window The force pushing out the window
Net force on window
Inward force on window
F = P x A F = (14.7lb/in2)(144in2)
F = 2116.8 lb
F = P x A F = (6.7 lb/in2)(144in2)
F= 964.8lb
Outward force on window
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 = 2116.8 lb – 964.8 lb
Net Force = 1152 lb
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
Inward force on window
F = P x A F = (14.7lb/in2)(144in2)
F = 2116.8 lb
F = P x A F = (5.4 lb/in2)(144in2)
F= 777.6lb
Outward force on window
Net Force on window
The window was originally pushing outward with a net force of 1152 lb.
Therefore it needs to hold 187.2 more pounds of pressure (1339.2 – 1152)
Net Force = Force out – Force in Net Force = 2116.8 lb – 777.6 lb
Net Force = 1339.2 lb
Pressure Pressure acts equally in all
direction at any point in a fluid and therefore it is a scalar
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
Total pressure compared to a perfect vacuum
Gage Pressure Pressure measured above
atmospheric pressure
G.P = Total pressure – atmospheric pressure
Total Pressure
Gage pressure is generally
measured “with a gage”
Pressure
Tire gage reads 38lb/in2
What is the atmospheric pressure? What is the gage pressure? What is the total pressure?
Pressure Tire gage reads 38lb/in2
What is the atmospheric pressure?
What is the gage pressure?
What is the total pressure?
How does pressure change with depth? Where is the pressure greater the
shallow end or the deep end?
Why?
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
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
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
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
Unbalanced pressure across the valve
Pressure on bottom does not depend on the size of the tank
Pressure acts like forces
Pressure is a prime mover
Measuring Pressures
Manometer – instrument used to measure fluid pressure
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 Same pressure = more force in the smaller
cylinder Small to large = allowable force but small
increments?
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.
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.