Pressure in

Fluid 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

Uses liquid as the fluid

Pneumatic system

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 on the window The force pushing out on 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 30lb/in2

What is the atmospheric pressure? What is the gage pressure? What is the total pressure?

Pressure Tire gage reads 30lb/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)

Hydraulic lift Liquids are incompressible

Air compressor increases the

pressure to the fluid

Large pushing force is exerted

on the lifting piston

Pressure acts like forces

Pressure is a prime mover

Measuring Pressures

Manometer – instrument used to measure fluid pressure

Balanced pressure across the valve

Unbalanced pressure across the valve

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

Hydraulic jack?

Large cylinder to a small cylinder Same pressure = more force in the smaller

cylinder Small to large = allowable force but small

increments?