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

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

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Fluid system that uses liquid as the fluid
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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?
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Amount of matter in a given amount of substance
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= Mass/Volume

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

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

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

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Platinum
Diamond
Chromium
Tin (white)
Tin (gray)
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21.45
3.5-3.53
7.15
7.265
5.769

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

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

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

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Copper has a density of
8.9g/cm 3
What is its specific gravity?
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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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The upward force on a substance from a fluid
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Will lead sink or float in water?
Will it sink or float in mercury?

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Instrument that measures density or specific gravity of fluids
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Can you sink or float in quick sand?
(mythbusters
)

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Force per unit area exerted by a fluid

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

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What happens to the pressure as we move away from the earth?
 http://www.sciencedaily.com/videos/2006/1201home_runs_amp_holeinone.htm

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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
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The force pushing in the window
The force pushing out the window
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Net force on window

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F = P x A
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F = (6.7 lb/in 2 )(144in 2 )
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F= 964.8lb
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F = P x A
F = (14.7lb/in 2 )(144in 2 )
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F = 2116.8 lb

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Net Force = Force out – Force in
Net Force = 2116.8 lb – 964.8 lb
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Net Force = 1152 lb
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The window is being pushed outward with a net force of
1152 lb.

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

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F = P x A
F = (14.7lb/in 2 )(144in 2 )
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F = 2116.8 lb
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F = P x A
F = (5.4 lb/in 2 )(144in 2 )
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F= 777.6lb

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Net Force = Force out – Force in
Net Force = 2116.8 lb – 777.6 lb
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Net Force = 1339.2 lb
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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)

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

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

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

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Gage pressure is generally measured “with a gage”

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

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

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Where is the pressure greater the shallow end or the deep end?
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Why?

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There is more water sitting on top of the deep end
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There is more weight on top
Which means more force
Which means more pressure

Relationship between pressure and depth

Water Pressure Calculation
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Given:
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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
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Find:
The pressure at the valve in lb/ft 2

Water Pressure Calculation
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P = p w x h
P = (62.4 lb/ft 3 )(100ft)
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P = 6240 lb/ft 2
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Given: 1 ft 2 = 144 in 2
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Now find:
Pressure in PSI

Water Pressure Calculation
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P = pw x h
P = (62.4 lb/ft3)(100ft)
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P = 6240 lb/ft 2
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Given: 1 ft 2 = 144 in 2
 p = (6240 lb/ft 2 )(1ft2/144in 2 )
P = 43.3 lb/in 2 (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
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Pressure is a prime mover
Measuring Pressures
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Manometer – instrument used to measure fluid pressure

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

Hydraulic jack?
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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.