Sub Unit 1.2 – Fluid ForceSub Unit 1.2 – Fluid Force

Pressure in Fluid SystemsPressure in Fluid Systems

ObjectivesObjectives Describe the four states of matter.Describe the four states of matter. Define density and pressureDefine density and pressure Explain why pressure in a fluid depends on depth Explain why pressure in a fluid depends on depth

in the fluidin the fluid Explain why an object submerged in a fluid Explain why an object submerged in a fluid

experiences a buoyant forceexperiences a buoyant force Predict whether an object will sink or float in a Predict whether an object will sink or float in a

given fluid.given fluid. Explain how a force can be multiplied in a Explain how a force can be multiplied in a

hydraulic lift.hydraulic lift. Explain where atmospheric pressure comes from.Explain where atmospheric pressure comes from. Describe how a barometer measures atmospheric Describe how a barometer measures atmospheric

pressurepressure Explain the difference between absolute and gage Explain the difference between absolute and gage

pressure.pressure.

States of MatterStates of Matter

Matter can exist in four states:Matter can exist in four states:– Solid (ice)Solid (ice)– Liquid (water)Liquid (water)– Gas (steam vapor)Gas (steam vapor)– Plasma (Extremely hot ionized atoms)Plasma (Extremely hot ionized atoms)

FluidsFluids

Fluids are materials that can flow, has no Fluids are materials that can flow, has no definite shape of its own, and conforms to definite shape of its own, and conforms to the shape of its container.the shape of its container.

LiquidsLiquids GassesGasses Fluid systems use both liquids (hydraulic) Fluid systems use both liquids (hydraulic)

and gasses (pneumatic) to operate and gasses (pneumatic) to operate mechanical devices.mechanical devices.

City Water SystemCity Water System

Hydraulic system under pressure

Density and PressureDensity and Pressure

Density is how much mass is contained in a Density is how much mass is contained in a given amount of space.given amount of space.

Amount of matter per unit of volume.Amount of matter per unit of volume.

(rho)

UnitsUnits

EnglishEnglish SISI

ExampleExample

The mass The mass of 1 cmof 1 cm33 of of water has a water has a mass of mass of 1g; 1g; therefore therefore the density the density is 1g/1cmis 1g/1cm33..

Weight DensityWeight Density Weight Density is the comparison Weight Density is the comparison

of an object’s weight to it’s volumeof an object’s weight to it’s volume

w

Units

Weight Density of WaterWeight Density of Water

Water has a weight density of Water has a weight density of 62.4 lb/ft62.4 lb/ft33..

PressurePressure

A force applied over a surface is pressure.A force applied over a surface is pressure.

English SI

Units

Pressure UnitsPressure Units

N/mN/m22 = 1 Pascal (Pa) = 1 Pascal (Pa) 1000 Pa = 1kilopascal (kPa)1000 Pa = 1kilopascal (kPa) lb/inlb/in22 = psi (pounds per square inch) = psi (pounds per square inch)

Pressure and DepthPressure and Depth

Pressure increase with depth because of the Pressure increase with depth because of the additional weight of the fluid above. additional weight of the fluid above.

w

Buoyancy and Archimedes’ Buoyancy and Archimedes’ PrinciplePrinciple

PPtoptop = = ww x h x h PPbottom bottom = = ww x (h + d) x (h + d) F = P x A F = P x A

– FFtoptop = P = Ptoptop x A = ( x A = (ww x h) x A x h) x A

– FFbottom bottom = P= Pbottombottom x A = [ x A = [ww x (h+d)] x A x (h+d)] x A

FFbuoyant buoyant = F= Fbottombottom – F – Ftop top = = ww Ad Ad Ad = VAd = Vbrick brick

FFbuoyant buoyant = = ww x V x Vbrickbrick = weight of water displaced = weight of water displaced

Archimedes’ PrincipleArchimedes’ Principle

An object immersed in a fluid has an An object immersed in a fluid has an upward force exerted on it equal to the upward force exerted on it equal to the weight of the fluid displaced by the object.weight of the fluid displaced by the object.

Note: the buoyant force is based on the Note: the buoyant force is based on the weight of the fluid displaced not on the weight of the fluid displaced not on the weight of the object.weight of the object.

Pascal’s PrinciplePascal’s Principle

A change in pressure at any point in a A change in pressure at any point in a confined fluid is transmitted undiminished confined fluid is transmitted undiminished throughout the fluid.throughout the fluid.

P = F / A orP = F / A or F = P x AF = P x A A = A = rr22

Atmospheric PressureAtmospheric Pressure The weight of the air above an area.The weight of the air above an area. At sea level, a column of air extending up At sea level, a column of air extending up

through the atmosphere, with a cross through the atmosphere, with a cross sectional area of 1msectional area of 1m22, encloses about 10,000 , encloses about 10,000 kg of air.kg of air.

This air weighs about 1 x 10This air weighs about 1 x 1055 N N Therefore, atmospheric pressure is about 10Therefore, atmospheric pressure is about 1055

Pa or 100kPa at sea level.Pa or 100kPa at sea level. Decreases with altitudeDecreases with altitude This is why your ears pop (equalization)This is why your ears pop (equalization)

Atmospheric PressureAtmospheric Pressure

Barometer – instrument used for measuring Barometer – instrument used for measuring atmospheric pressure.atmospheric pressure.

At sea level the average atmospheric At sea level the average atmospheric pressure is 101.3 kPa = 760mm of mercury pressure is 101.3 kPa = 760mm of mercury = one atmosphere = 14.7 psi = 2117 lb/ft= one atmosphere = 14.7 psi = 2117 lb/ft22

Absolute and Gage PressureAbsolute and Gage Pressure Absolute pressure is the total pressure measured Absolute pressure is the total pressure measured

above zero (perfect vacuum).above zero (perfect vacuum). Gage pressure is the pressure measured above Gage pressure is the pressure measured above

atmospheric pressure.atmospheric pressure. Absolute pressure = gage pressure + atmospheric pressureAbsolute pressure = gage pressure + atmospheric pressure Suppose a tire gage measures the pressure of a tire Suppose a tire gage measures the pressure of a tire

to be 30 psi;to be 30 psi; Absolute pressure = 30 psi + 14.7 psi = 44.7 psiAbsolute pressure = 30 psi + 14.7 psi = 44.7 psi The air inside the tire pushes out with a pressure of The air inside the tire pushes out with a pressure of

44.7 psi. The atmosphere pushes in with a pressure 44.7 psi. The atmosphere pushes in with a pressure of 14.7 psi. The difference is 30 psi – the gage of 14.7 psi. The difference is 30 psi – the gage pressure.pressure.

Pressure is a Prime MoverPressure is a Prime Mover

Pressure acts like a force to cause Pressure acts like a force to cause movement.movement.

EquilibriumEquilibrium

SummarySummary Matter can exist in four states: solid, liquid, gas, and plasma. Matter can exist in four states: solid, liquid, gas, and plasma. Liquids and gases are called fluids.Liquids and gases are called fluids. The density of a substance is its mass per unit volume. The density of a substance is its mass per unit volume. The density ofThe density of water is 1g/cmwater is 1g/cm33.. Weight density is weight per unit volume.Weight density is weight per unit volume. Pressure is force divided by the area over which the force acts. Pressure is force divided by the area over which the force acts. We treat pressure as a scalar. We treat pressure as a scalar. In SI units, pressure is measured in pascals, where 1 Pa= 1 N/mIn SI units, pressure is measured in pascals, where 1 Pa= 1 N/m Pressure increases with depth in a fluid. Pressure increases with depth in a fluid.

SummarySummary For a given fluid, the pressure does not depend on the size For a given fluid, the pressure does not depend on the size

or shape of the container.or shape of the container. When an object is submerged in a fluid, an upward force is When an object is submerged in a fluid, an upward force is

exerted on the object caused by the pressure difference exerted on the object caused by the pressure difference between the top and the bottom of the object. This force is between the top and the bottom of the object. This force is called a buoyant force.called a buoyant force.

The buoyant force exerted on a submerged object equals the The buoyant force exerted on a submerged object equals the weight of the fluid displaced by the object.weight of the fluid displaced by the object.

A pressure applied to a confined fluid is transmitted A pressure applied to a confined fluid is transmitted throughout the fluid.throughout the fluid.

Atmospheric pressure is caused by the weight of the air Atmospheric pressure is caused by the weight of the air above a given area. above a given area.

Atmospheric pressure can be measured with a barometer.Atmospheric pressure can be measured with a barometer. Absolute pressure is the sum of the gage pressure and Absolute pressure is the sum of the gage pressure and

atmospheric pressure.atmospheric pressure.