1. Pressure in Fluid Systems
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UNIT 3
Pressure in
Fluid Systems

2. Unit 3 Pressure Pages 43-60 Fluid Hydraulic System Pneumatic System
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Fluid
Hydraulic System
Pneumatic System
Density
Specific gravity
Buoyant force
Hydrometer
Pressure
PSI
Atmospheric Pressure
Absolute pressure
Gage pressure
Manometer

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

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

5. Hydraulic system Pneumatic system
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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

6. 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

7. Density SI measured in: English measured in: Kg/m3 or gm/cm3
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Density
SI measured in:
Kg/m3 or gm/cm3
English measured in:
Lbm/ft3 or lb/ft3

8. 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

9. 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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11. Other Densities Platinum Diamond Chromium Tin (white) Tin (gray) 21.45
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Platinum
Diamond
Chromium
Tin (white)
Tin (gray)
21.45
7.15
7.265
5.769

12. What is the mass in grams of mercury with a volume of 1cm3?
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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

13. What is the mass in kilograms of balsa wood with a volume of 1m3?
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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

14. Density of a substance divided by the density of water
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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

15. Specific Gravity Copper has a density of 8.9g/cm3
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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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17. The upward force on a substance from a fluid
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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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19. 3/25/2017
Hydrometer
Instrument that measures density or specific gravity of fluids
Can you drown in quick sand?

20. Force per unit area exerted by a fluid
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Pressure
Force per unit area exerted by a fluid

21. Force on Airplane Windows
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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/in2
Find:
The force pushing on the window

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

23. Force on Airplane Windows
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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/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

24. Outward force on window
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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

25. Net Force on window Net Force = Force out – Force in
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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.

26. 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

27. Outward force on window
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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

28. Net Force on window Net Force = Force out – Force in
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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)

29. 3/25/2017
Pressure
Pressure acts equally in all direction at any point in a fluid and therefore it is a scalar

30. Absolute vs. Gage Pressure
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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

31. Absolute Pressure Gage Pressure
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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

32. Gage pressure is generally measured “with a gage”
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Gage pressure is generally measured “with a gage”
Total Pressure

33. Pressure Tire gage reads 38lb/in2 What is the atmospheric pressure?
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Pressure
Tire gage reads 38lb/in2
What is the atmospheric pressure?
What is the gage pressure?
What is the total pressure?

34. Pressure What is the atmospheric pressure? What is the gage pressure?
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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

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

36. Pressure increases with depth
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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

37. Relationship between pressure and depth
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Relationship between pressure and depth

38. Water Pressure Calculation
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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/ft3
Find:
The pressure at the valve in lb/ft2

39. Water Pressure Calculation
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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

40. Water Pressure Calculation
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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)

41. Balanced pressure across the valve
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Balanced pressure across the valve

42. Unbalanced pressure across the valve
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Unbalanced pressure across the valve

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

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

45. Hydraulic lift Liquids are incompressible
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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

46. Hydraulic jack? Large cylinder to a small cylinder
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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?

47. 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.

48. 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.