Physical Science Unit: Forces in Fluids. What is Pressure? Related to the word press Earth’s gravity pulls downward Due to gravity, your feet exert

Physical SciencePhysical Science

Unit: Forces in Unit: Forces in FluidsFluids

What is Pressure?What is Pressure?

Related to the word pressRelated to the word press Earth’s gravity pulls downwardEarth’s gravity pulls downward Due to gravity, your feet exert a Due to gravity, your feet exert a

force on the surface of Earth over force on the surface of Earth over an area the size of your feetan area the size of your feet

Pressure = Force / AreaPressure = Force / Area Pressure: a force pushing on a surfacePressure: a force pushing on a surface Pressure = Force / areaPressure = Force / area Unit of measure for Pressure is the Pascal: Unit of measure for Pressure is the Pascal:

1Pa = 1N/m1Pa = 1N/m22

Remember 1 N = 1kg m/ secRemember 1 N = 1kg m/ sec22

Pressure and AreaPressure and Area

The amount of pressure you exert The amount of pressure you exert depends on the area over which you depends on the area over which you exert force.exert force. Larger area = less pressure exertedLarger area = less pressure exerted

PressurePressure

Can be calculated by dividing the force Can be calculated by dividing the force exerted by a fluid by the total area over exerted by a fluid by the total area over which the force actswhich the force acts

When force is measured in Newtons (N) When force is measured in Newtons (N) and area is measured in square and area is measured in square centimeters (cmcentimeters (cm22), ), pressure is pressure is measured in Newtons per square measured in Newtons per square centimeter (N/cmcentimeter (N/cm22))

Calculating PressureCalculating Pressure

Pressure = Pressure = ForceForce

AreaArea

Measured in Measured in

Pascals – 1 N/m²Pascals – 1 N/m²

PressurePressure

The pressure a fluid exerts is due to the The pressure a fluid exerts is due to the fact that the fluid is made of particles that fact that the fluid is made of particles that have weight and motion.have weight and motion.

The weight of the particles in a fluid The weight of the particles in a fluid cause them to push against objectscause them to push against objects

Pressure ProblemsPressure Problems

What is the force water applies to your hand What is the force water applies to your hand when you dive to bottom of a swimming pool? when you dive to bottom of a swimming pool? Assume your hand has an area of 0.008 m2 Assume your hand has an area of 0.008 m2 and the water pressure is 120,000 Pa. and the water pressure is 120,000 Pa.

Solution:Solution:

P = F/A, F = P ·A, A = F/P, P = F/A, F = P ·A, A = F/P, F = (.008) x (120,000) = 960 NF = (.008) x (120,000) = 960 N


Your brand new car weighs 16,170 N and you need to Your brand new car weighs 16,170 N and you need to get your first oil change. You pull it onto the hydraulic get your first oil change. You pull it onto the hydraulic lift, which has an area of 5005 cm2. The mechanic lift, which has an area of 5005 cm2. The mechanic applies a force to a small piston that has an area of 6.5 applies a force to a small piston that has an area of 6.5 cm2. What force does he apply to lift your car? cm2. What force does he apply to lift your car?

Solution:Solution: P = F/A, F = P ·A, A = F/P, P = F/A, F = P ·A, A = F/P,

SolutionSolution

new car weighs 16,170 N

an area of 5005 cm2an area of 65 cm2

Divide larger area by smaller

5005/65 = 770

Divide power by area

16170/ 770 = 21 N

Fluid PressureFluid Pressure

FluidFluid is a material that can easily is a material that can easily flowflow LiquidsLiquids GasesGases

Air and heliumAir and helium


FluidFluid is a material that can easily flow. is a material that can easily flow. All Liquids and Gases are fluids.All Liquids and Gases are fluids.

In a fluid, all of the forces exerted by the In a fluid, all of the forces exerted by the individual particles combine to make up individual particles combine to make up the pressure exerted by the fluid.the pressure exerted by the fluid.

Fluid PressureFluid Pressure Fluid is a substance that can flow easily.Fluid is a substance that can flow easily.

Scientifically liquids & gases are considered Scientifically liquids & gases are considered “fluids”“fluids”

In fluids, molecules are constantly moving in In fluids, molecules are constantly moving in all directionsall directions

.. Pressure = Force / AreaPressure = Force / Area

Pascal's Vase - demonstrating that depth, not

shape, determines fluid pressure...

As a molecule moves and collides w/ As a molecule moves and collides w/ a surface, it exerts a force on that a surface, it exerts a force on that surfacesurface

All of the forces exerted by the All of the forces exerted by the individual molecules are added individual molecules are added together to make up the pressure together to make up the pressure exerted by the fluidexerted by the fluid


What Causes Fluid What Causes Fluid PressurePressure

In a fluid, all of the forces exerted In a fluid, all of the forces exerted by the individual particles combine by the individual particles combine to make up the pressure exerted by to make up the pressure exerted by the fluid.the fluid.

Air PressureAir Pressure

Air exerts pressure because it has mass.Air exerts pressure because it has mass.

Because the force of gravity pulls down Because the force of gravity pulls down on the mass of air, the air has weight.on the mass of air, the air has weight.

Weight of the air is the force that Weight of the air is the force that produces air pressure or atmospheric produces air pressure or atmospheric pressure.pressure.

Air PressureAir Pressure Air pressure Air pressure is the result of is the result of

the the weight of a column of air weight of a column of air pushing down on an area.pushing down on an area.• 14.7 lbs/inch14.7 lbs/inch22 ( 10.13 N/cm ( 10.13 N/cm22))• 1013.25 millibars1013.25 millibars• 1013.25 hPa (hecto Pascals)1013.25 hPa (hecto Pascals)• 29.92 inches of Hg29.92 inches of Hg

1 in x 1 in square column of air weighs 14.7 lbs at sea level

Air exerts a balanced force when fluid is NOT moving: the

pressure pushing down on your hand is balanced by the pressure

pushing up on your hand

Balanced PressureBalanced Pressure

When you hold out your hand, you are When you hold out your hand, you are holding up air. 1,000 Newtons of air, holding up air. 1,000 Newtons of air, which is about the same weight as that of which is about the same weight as that of a large washing machine!a large washing machine!

..

What keeps air pressure What keeps air pressure from crushing you?from crushing you?

The fluids in your body exert pressureThe fluids in your body exert pressure The air pressure (outside your body) and The air pressure (outside your body) and

your body pressure are equalyour body pressure are equal

At high AltitudesAt high Altitudes

There are fewer particles of air in a given There are fewer particles of air in a given areaarea

So air pressure decreasesSo air pressure decreases

Unbalanced PressureUnbalanced Pressure

What happens when air pressure What happens when air pressure becomes unbalanced?becomes unbalanced?

Variations in Fluid Variations in Fluid PressurePressure Elevation – the distance above sea level. As altitude increases Air pressure

decreases As air pressure decreases, so does

density. As water depth increases water

pressure increases Water 800x more dense than air, so

pressure increases dramatically w/ depth – every 33 ft in depth adds 1 “atmosphere “ of pressure

SuctionSuction

Is the result of unequal air pressureIs the result of unequal air pressure Ex operating a vacuum cleanerEx operating a vacuum cleaner

Variations in Atmospheric Variations in Atmospheric Pressure and Elevation.Pressure and Elevation.

Popping in ear is caused by changing air Popping in ear is caused by changing air pressurepressure

Measuring PressureMeasuring Pressure

BarometerBarometer measures atmospheric measures atmospheric pressurepressure

Archimedes PrincipleArchimedes Principle The buoyant force on an object is equal to the The buoyant force on an object is equal to the

weight of the fluid displaced by the object.weight of the fluid displaced by the object. The buoyant force is opposite (pushes up) to the The buoyant force is opposite (pushes up) to the

force of gravity (pulls down)force of gravity (pulls down)

Archimedes PrincipleArchimedes Principle

Can be stated in terms of density: an Can be stated in terms of density: an object will float in a fluid if the density of object will float in a fluid if the density of the object is less than the density of the the object is less than the density of the fluidfluid

DensityDensity

Comparing densities, you can predict Comparing densities, you can predict whether an object will float or sink in a fluid.whether an object will float or sink in a fluid.

If object is more dense than fluid - it sinks If object is more dense than fluid - it sinks (obviously!)(obviously!)

If object is less dense than fluid - it floats. If object is less dense than fluid - it floats. (no, duh...)(no, duh...)

An object with a density equal to that of the An object with a density equal to that of the fluid floats at a constant depth.fluid floats at a constant depth.

DensityDensity

Density = Mass / Density = Mass / VolumeVolume

Density of water = Density of water = 1.01.0

Changing density Changing density can explain why an can explain why an object (like a object (like a submarine) floats or submarine) floats or sinks.sinks.

Density of hot air BalloonDensity of hot air Balloon

The density of hot air is less than that of The density of hot air is less than that of cold air, which is why hot air balloons can cold air, which is why hot air balloons can riserise

BuoyancyBuoyancy

Buoyancy Buoyancy is the ability to float. is the ability to float.

Buoyant ForceBuoyant Force

Buoyant force is when water and other Buoyant force is when water and other fluids exert an upward force.fluids exert an upward force.

The buoyant force acts in the direction The buoyant force acts in the direction opposite to the force of gravity, so it opposite to the force of gravity, so it makes an object feel lighter.makes an object feel lighter.

The less dense the object is, the The less dense the object is, the greater the buoyant force it greater the buoyant force it experiences.experiences.



BuoyancyBuoyancy

When an object is placed in a fluid, it When an object is placed in a fluid, it displaces some of the fluid, the amount displaces some of the fluid, the amount displaced equals the volume of the objectdisplaced equals the volume of the object

Ice cube float in water because the buoyant force on it is equal to the weight of water displaced

Same is true for submarines


An object weighs 36 g in air and has a volume of 8.0 cm3. What will be its apparent weight when immersed in water?

Solution: When immersed in water, the object is buoyed up by the mass of the water it displaces, which of course is the mass of 8 cm3 of water. Taking the density of water as unity, the upward (buoyancy) force is just 8 g.

The apparent weight will be (36 g) – (8 g) = 28 g.

Transmitting Pressure in a Transmitting Pressure in a FluidFluid

In the 1600s, Blaise Pascal In the 1600s, Blaise Pascal developed a principle to explain developed a principle to explain how pressure is transmitted in a how pressure is transmitted in a fluidfluid Fluid exerts pressure on Fluid exerts pressure on

any surface it touches.any surface it touches.

Pascal’s Pascal’s PrinciplePrinciple When force is applied to a When force is applied to a

CONFINED FLUID, an CONFINED FLUID, an increase in pressure is increase in pressure is transmitted equally to ALL transmitted equally to ALL parts of the fluid.parts of the fluid.

Pascal’s PrinciplePascal’s Principle

Pascal’s principle states that Pascal’s principle states that pressure increases by the same pressure increases by the same amount throughout an enclosed or amount throughout an enclosed or confined fluidconfined fluid When force is applied to a confined When force is applied to a confined

fluid, the change in pressure is fluid, the change in pressure is transmitted equally to all parts of the transmitted equally to all parts of the fluid.fluid.

Pascal’s Principle at Pascal’s Principle at WorkWork

Pascal’s Principle and Pascal’s Principle and Hydraulics Hydraulics

Hydraulic devices use the principle that Hydraulic devices use the principle that pressure is transmitted equally in all pressure is transmitted equally in all directions throughout a liquid.directions throughout a liquid.

Hydraulic SystemsHydraulic Systems

Hydraulic system uses liquids to Hydraulic system uses liquids to transmit pressure and multiply force in transmit pressure and multiply force in a confined fluid.a confined fluid. A hydraulic system multiplies force by A hydraulic system multiplies force by

applying the force to a small surface area. applying the force to a small surface area. The increase in pressure is then transmitted The increase in pressure is then transmitted to another part of the confined fluid, which to another part of the confined fluid, which pushes on a larger surface area.pushes on a larger surface area. Because hydraulic systems use fluids to transmit Because hydraulic systems use fluids to transmit

pressure, hydraulic systems have few moving pressure, hydraulic systems have few moving parts that can jam, break or wear down. parts that can jam, break or wear down.

Hydraulic LiftsHydraulic Lifts

Hydraulic LiftsHydraulic Lifts

Bernoulli’s PrincipleBernoulli’s Principle The pressure exerted by a moving stream of a fluid The pressure exerted by a moving stream of a fluid

is less than the pressure of the surrounding fluid.is less than the pressure of the surrounding fluid. The faster the fluid moves, the less pressure it The faster the fluid moves, the less pressure it

exerts on the surface of the objectexerts on the surface of the object

Bernoulli's principleBernoulli's principle

A fluid tends to move from an area of low A fluid tends to move from an area of low pressure to an area of high pressure. pressure to an area of high pressure.

Bernoulli's principle helps explain how Bernoulli's principle helps explain how planes fly.planes fly.

It also explains how an atomizer works, It also explains how an atomizer works, why smoke rises up a chimney, and how why smoke rises up a chimney, and how a flying disk glides through the air.a flying disk glides through the air.

When an airplane is in level flight, When an airplane is in level flight, lift is equal to the weight of the lift is equal to the weight of the planeplane

High pressure below the wing create lift

The planes propellers produce the thrust to move the plane forward The plane’s steam-lined

Shape reduces dragOn the plane

Air pressure aboveThe wing is low

Plane FlightPlane Flight

An airplanes wing is designed to take An airplanes wing is designed to take advantage of Bernoulli’s principleadvantage of Bernoulli’s principle

The plane rises because the plane's wing The plane rises because the plane's wing is curved. This curve causes the air is curved. This curve causes the air above the plane to move faster than the above the plane to move faster than the air below it, causing it to liftair below it, causing it to lift

Documents

Physical Science Unit: Forces in Fluids. What is Pressure? Related to the word press Earth’s gravity pulls downward Due to gravity, your feet exert