L-14 Fluids [3]L-14 Fluids [3]Why things floatWhy things floatFluids in Motion Fluids in Motion Fluid Dynamics Fluid Dynamics

–HydrodynamicsHydrodynamics–AerodynamicsAerodynamics

Buoyancy – why things float

• The trick is to keep the water on the outside of the ship, and

• to avoid hitting icebergs (which also float), and• are easy to miss since 90 % of it is submerged.

TITANIC

Buoyant Force

W

PTopA

PBottomA

submerged objectthat has a massdensity ρO

The density of thewater is ρW

hF = P A

Pressure increases with depth

Archimedes’ Principle• PBottomA > PTopA

• A buoyant force FB equal to the weight of displaced water is exerted on a submerged object.

• The object sinks to the level where FB = W

WFB

Archimedes principle The buoyant force on an object in

a fluid equals the weight of the fluid which it displaces.

– this works for objects in water– helium balloons (density of He = 0.18 kg/m3,

about 7 times less dense than air)– hot air balloons the density of hot air is lower than the density of cool air so the weight of the cool air that is displaced is higher than the weight of the balloon

Will it float?• The buoyant force is always there whether the

object floats or not• The object will float if the buoyant force is

enough to support the object’s weight• The object will displace just enough water so

that the buoyant force = its weight• If it displaces as much water as possible and this

does not match its weight, it will sink.• Objects that have a density less than water will

always float- when fully submerged, they weigh less than the water, so the water supports them

Floating objects

lighter object heavier object too heavy

The weight of displacedwater is less than theweight of the object

Oil Tankers

emptytanker

fulltanker

Floating in a cup of water

Only a thin layer ofwater around the hullis needed for the shipto float!

Why does ice float?

• Water, the most plentiful substance on earth is also one of the most unusual in its behavior in that it expands when it freezes.

• Since it expands the density of ice is slightly less than the density of water (917 kg/ m3 as compared to 1000 kg/ m3 for water). So the part of the iceberg above the surface is less than 10% of the total volume.

Place your bets!

When the ice cube melts will:1)the water spill out, or2)the water level stay the same, or3)the level go down ????????

Answer: The level stays the same.Ice is less dense than water, so thatthe volume occupied by the ice is exactly big enough to hold the volume of melted water that wasnot submerged!

ice cube

Fluid Flow• The physics of fluid

flow was worked out by Daniel Bernoulli

• He was born in Switzerland in 1700

• He was one of 5 brothers and came from a large family of mathematicians and scientists.

Pressure increases with depth, so the speed of water leakingfrom the bottom hole islarger than that from the higher ones.

fluid flow example – leaky cup

How do we measure fluid flow?• We see how much comes out

in some time interval• Time how long it takes to fill the

bucket, say 30 seconds• the flow rate is then 1 bucket

say per 30 seconds• in other words volume per unit

time• gallons per min (gpm), liters/s,

cubic feet per min (cfm), gpf, or m3/s volume flow rate

Volume flow rate

• If the water comes out of a tube of cross sectional area A with a flow speed u the volume flow rate is

• volume flow rate = u A (m/s m2)

• To measure u just see how long it takes to fill a gallon jug from a hose and measure the diameter of the hose.

m3/s

Mass flow rate

• We could also measure how much mass comes out per unit time – kg/s for example

• if you are using a fluid of density coming out of a hose of cross sectional area A with speed v the mass flow rate is

• mass flow rate = u A

What makes water flow?

• gravity• by placing the water

up high the pressure at the bottom is high enough to supply water to all parts of town that are lower than the tower

Stanton, IAMontgomery Co.

Pressure differences

P2 P1

a pressure difference must be establishedacross the ends of the pipe to push the wateralong. P2 must be greater than P1

This pressure difference can be set up by a water pump.

Water does not disappear!• If water goes in one end of a pipe it must

come out the other end (if there are no leaks of course. Sounds obvious, but it has a number of interesting consequences!

v1, A1 v2, A2

This applies to pipes that have constrictions also.

Continuity of flow• since whatever goes in must come out we

have that the incoming flow rate – outgoing flow rate or

• v1 A1 = v2 A2

• thus the fluid in the narrow part of the tube must flow FASTER that the fluid on the left.

• Cardiologists use this to determine if arteries might be clogged.

Other examples - the nozzle effect

• you use this principle whenever you hold your finger over the end of the hose to make the water spray farther.

An amazing thing about moving fluids

• The pressure in a moving fluid is less than the pressure in a fluid at rest! this is Bernoulli's principle.

• Where a fluid moves faster its pressure is lower, where it moves slower, its pressure is higher.

• As we will see, this is the principle that makes airplanes work.

The Venturi Meter

windBernoulli appliesto householdplumbing too!

When the wind isreally blowing, watch the water level in thetoilet go up and down

air vent

sewer

roof

Prairie dogs know how to useBernoulli's principle

“atomizers”

• fine droplets of liquid (not atoms) are sprayed from this device using the Bernoulli effect

END HERE

Hot air balloon• The ideal gas law tells is

that when a gas is heated, its density goes down

• so the air density inside the balloon is less than the density of cold air on the outside.

• The cold air exerts an upward buoyant force on the balloon.