FluidsFluids

Pressure in LiquidsPressure in Liquids

A liquid exerts a pressure against the A liquid exerts a pressure against the bottom of its containerbottom of its container P = P = ForceForce

AreaArea

But it also exerts a force against the But it also exerts a force against the container’s side, - container’s side, - At any point within a At any point within a liquid, the forces that produce pressure liquid, the forces that produce pressure are exerted equally in all directionsare exerted equally in all directions

How Pressure Within a Fluid How Pressure Within a Fluid VariesVaries• In a still fluid, as you go In a still fluid, as you go

deeper in the fluid, more deeper in the fluid, more is on top of you so the is on top of you so the pressure increases with pressure increases with depth.depth.

• Compare the pressure of Compare the pressure of two two different fluidsdifferent fluids at the at the same depthsame depth : The : The pressure is greater in the pressure is greater in the more dense fluid – so more dense fluid – so density of the fluid affects density of the fluid affects the pressure it exertsthe pressure it exerts

DensityDensity

• Density if the “puffyness” of an object. Density if the “puffyness” of an object. Density = Density = MassMass

VolumeVolume• If an object is submerged in a fluid and If an object is submerged in a fluid and

it is less dense than the fluid, it will it is less dense than the fluid, it will float on top, if more dense, it will sink, float on top, if more dense, it will sink, and if it is the same density it will stay and if it is the same density it will stay within the fluid where ever you put itwithin the fluid where ever you put it

BouyancyBouyancy

• Submerge something under a liquid and the Submerge something under a liquid and the object pushes the liquid out of the way object pushes the liquid out of the way ((displacesdisplaces it), the it), the VOLUME of the displaced VOLUME of the displaced liquid = the VOLUME of the object.liquid = the VOLUME of the object.

• But notice something else: The object But notice something else: The object seems to weigh less in the liquid than on seems to weigh less in the liquid than on land – WHY? Because the fluid exerts an land – WHY? Because the fluid exerts an upward forceupward force (called the Buoyant Force)(called the Buoyant Force) on on the submerged object that helps you lift if the submerged object that helps you lift if against gravityagainst gravity

Archimedes’ PrincipleArchimedes’ Principle

• An immersed object is buoyed up by a force An immersed object is buoyed up by a force equal to the equal to the weight of the fluid it displacesweight of the fluid it displaces

• It does It does NOTNOT (except in a special (except in a special circumstance)circumstance) equal the weight of the object equal the weight of the object

• The Special CircumstanceThe Special Circumstance - The Principle of - The Principle of Floatation: A Floating object displaces a Floatation: A Floating object displaces a weight of fluid equal to its own weight OR weight of fluid equal to its own weight OR when an object is immersed and the weight when an object is immersed and the weight of the fluid displaced just equals the weight of the fluid displaced just equals the weight of the object, the object will sink no further of the object, the object will sink no further and just floatand just float

THINK !THINK !

• If a 5 kg box with a volume of 3 L is If a 5 kg box with a volume of 3 L is submerged in water, how much submerged in water, how much VOLUMEVOLUME of of water does it displace? _____water does it displace? _____

• If water has a density of 1, ( 1 kg = 1 L) how If water has a density of 1, ( 1 kg = 1 L) how much does this displaced water much does this displaced water WEIGHWEIGH? _____? _____

• What is the What is the buoyant forcebuoyant force on the box on the box ________?________?

• What is the What is the apparent weightapparent weight of the box of the box ( Apparent weight = weight on land (F( Apparent weight = weight on land (Fg) - ) - buoyant force (Fbuoyant force (FB) _________) _________

Pascal’s PrinciplePascal’s Principle

• Changes in pressure Changes in pressure at any point in an at any point in an enclosed fluid at rest enclosed fluid at rest are transmitted are transmitted undiminished to all undiminished to all points in the fluid points in the fluid and act in all and act in all directions.directions.

• Example is the Example is the hydraulic lift or presshydraulic lift or press

Our AtmosphereOur Atmosphere

Is made of Is made of 78 % N78 % N22

21 % O21 % O22

1 % Argon1 % Argon

.03 % CO.03 % CO22

90% of the air exists below 20 km (12 90% of the air exists below 20 km (12 miles) miles)

Atmospheric PressureAtmospheric Pressure

• Gasses like those in our atmosphere Gasses like those in our atmosphere ARE compressible, so the density of a ARE compressible, so the density of a gas (and our atmosphere) varies with gas (and our atmosphere) varies with depthdepth

• Air density at sea level is much Air density at sea level is much higher than on top of a mountain.higher than on top of a mountain.

• SO – if density is greater at the SO – if density is greater at the bottom of the atmosphere, what bottom of the atmosphere, what about air pressure? It is greater too.about air pressure? It is greater too.

Atmospheric Pressure is Atmospheric Pressure is significant!significant!

• The air above us has mass and weight. The air above us has mass and weight. It pushes down at 100,000 N /m It pushes down at 100,000 N /m2 2 (mat (mat demo)demo)

• We don’t notice it because we were We don’t notice it because we were born here but it can crush things (can born here but it can crush things (can demo). demo).

• This significant pressure is why straws, This significant pressure is why straws, pumps in wells, and barometers work.pumps in wells, and barometers work.

Measuring pressureMeasuring pressure

• Pressure can be measured with a Pressure can be measured with a barometerbarometer or a pressure gage. They work or a pressure gage. They work differently and measure different things!differently and measure different things!

• The fluid in a barometer is The fluid in a barometer is pushed upward pushed upward (Science never sucks) by the downward (Science never sucks) by the downward force of the atmosphere.force of the atmosphere.

• A gage (like a tire gage) measures the A gage (like a tire gage) measures the DIFFERENCE DIFFERENCE between the internal pressure between the internal pressure in the container and the atmospheric in the container and the atmospheric pressure outside the container.pressure outside the container.

The GAS LawsThe GAS Laws

• Boyle’s Boyle’s P1V1= P2V2P1V1= P2V2

• CharlesCharles V1 V1 = = V2 V2

T1T1 T2T2

• Gay-LusaccGay-Lusacc P1P1 == P2P2

T1T1 T2T2

Bernoulli’s PrincipleBernoulli’s Principle

• Pressure within a Pressure within a MOVING MOVING fluid changes with speed – as the fluid changes with speed – as the speed of the fluid increases the speed of the fluid increases the internal pressureinternal pressure drops drops

• Explains lift forces like airplane wings. The air moving over Explains lift forces like airplane wings. The air moving over the top of the wing is the top of the wing is going fastergoing faster, it has , it has less pressureless pressure. This . This lets the lets the slower slower moving air below the wing (which has moving air below the wing (which has more more pressurepressure) to push up on the bottom of the wing – giving it lift.) to push up on the bottom of the wing – giving it lift.