Chapter 7 Section 1Fluids and Pressure

Students will: describe how fluids exert pressure

Analyze how atmospheric pressure varies with depthGive examples of fluids flowing from high to low pressure

Vocabulary:FluidPressurePascalAtmospheric Pressure

FluidsALL can flow ALL can take the shape of its containerFluids include liquid and gases.

Remember all matter is either: a solid, liquid, or a gasGases are what makes up the air we breath.

For Example: Oxygen is a gasParticles move easily past each otherSolids are NOT a fluid because they can NOT flow,

can NOT take the shape of its container , and their particles DO NOT move easily past each other

PressureThe amount of force exerted on a given areaThe SI Unit for pressure is pascal (symbol,

Pa)Fluids exert pressure evenly in all directions

Example: The air you blow into a bubble exerts pressure

evenly in all directions. So, the bubble expands in all directions.

Pressure = force area

REMEMBER THIS??A force is a push or a pull exerted on an

object in order to change the motion of the object.

Pressure is a FORCEInertia is NOT a FORCEWeight is a FORCEMass is NOT a FORCEMomentum is NOT a FORCEFriction is a FORCEGravitational Force is a FORCEAir Resistance is a FORCEAcceleration is NOT a FORCE

Atmospheric pressureLook at figure 3 on page 182Atmospheric pressure changes as you travel

through the atmosphere.The further DOWN through the atmosphere

you go, the GREATER the pressure is.Pressure varies depending on depth

Sea level has the greatest atmospheric pressure in the figure 3

For example,As you go from the top of a mountain to sea

level, the pressure increases.

Water PressureIncreases as depth increasesA diver feels more pressure the deeper he swims

because more water above the diver is being pulled by Earth’s gravitational force AND the atmospheric pressure presses down on the water, so the total pressure on the diver includes water pressure and atmospheric pressure

Water exerts more pressure than air, because water is more dense than air.Remember:

Density is the amount of matter in a given volume. Density=Mass/Volume D=m/v

Example: A diver 10 m underwater would feel twice as much

pressure than if he was just standing on the beach’s surface.

Pressure Difference and Fluid Flow***Fluids flow from areas of HIGH

pressure to LOW pressure***Example:

When a fluid flows from “Area A” to “Area B”, that means “Area A” has a HIGHER PRESSURE than “Area B”

With tornadoes, the air pressure outside of the tornado is higher than the pressureinside the tornado. This pressure difference causes air to enter into the tornado.

Now Do:Chapter 7 Section 1 Review in your notebookDo numbers 3-8 in your SNBNo, you do not have to write the questions.

Answers to Chapter 7 Section 1

3. B- Fluids include liquids and gases4. Particles in the fluid collide with the side

of the container. The force of the collisions creates pressure on the container.

5. You aren’t crushed by atmospheric pressure because the fluids inside your body exert pressure that works against atmospheric pressure.

6. Atmospheric pressure increases as depth increases because at lower levels of the atmosphere, there is more air above that is being pulled down by gravitational force

Answers Continued7. Examples of fluids flowing from high

pressure to low pressure are drinking through a straw, breathing, and squeezing toothpaste from a tube.

8. Pressure = force/areaPressure= 2.4 N/0.012 m2 Pressure = 200 Pa

Chapter 7 Section 2Buoyant ForceStudents will:

Explain the relationship between fluid pressure and buoyant force

Predict whether an object will float or sink in a fluid

Analyze the role of density in an object’s ability to float

Explain how the overall density of an object can be changed

VocabularyBuoyant forceArchimedes’ principle

Buoyant ForceIs the upward force that fluids exert on all

matter.A liquid exerts a buoyant force on an object

that increases as the density of the fluid increases.

In a fluid, buoyant forces exists because the pressure is greater at the BOTTOM of an object than the pressure at the topLook at figure 1 on page 186

Archimedes’ principleThe principle that states that the buoyant

force on an object in a fluid is an upward force equal to the weight of the volume of fluid that the object displaces.

ONLY the weight of the displaced fluid determines the buoyant force on an object.

The weight of the object DOES NOT affect buoyant force.

Weight vs. Buoyant forceRefer to figure 2 page 187If the weight of the water an object displaces is

equal to the weight of the object, the object FLOATSExample: A fish is suspended in the water

If the weight of the water an object displaces is less than the weight of the object, the object SINKSExample: a rock sinks

If the weight of the water an object displaces is more than to the weight of the object, the object BUOYED UPExample: a duck would be buoyed up after a diveBuoyed up means “pushed up in water”

Floating, Sinking, and DensityA rock has more mass per volume than water has.Mass per unit of volume is densityThe rock sinks because it is more dense than the

water is.The duck floats because it is less dense than the

water is.Most substances don’t float in air because most

substances are denser than air.There are only a FEW substances that are LESS

dense than air.Example: Helium is 7 times less dense than air, thus

helium is used in balloons to make them “float in air”

Changing overall densityChanging Shape

Ships float because of their shape- see fig.5, pg.189

Shaping the steel into a hollow form increases the volume occupied be the same mass. The overall density of the ship is reduced.

Changing MassBallast tanks are devices used by submarines

to control density.Changing Volume

Most fishes have an organ called a swim bladder that allows them to adjust their overall density

Chapter 7 Section 3Fluids and MotionStudents will:Describe the relationship between

pressure and fluid speed.Analyze the roles of lift, thrust, and

wing size in flight.Explain Pascal’s principle.Describe drag, and explain how it

affects lift.

VocabularyBernoulli’s PrincipleLiftThrustDragPascal’s Principle

Fluid Speed and PressureBernoulli’s Principle

States that the pressure in a fluid decreases as the fluid’s velocity increases

So, the faster the fluid’s speed is, the lower the pressure.

Factors that Affect FlightAccording to Bernoulli’s principle, the fast-

moving air above the wing exerts less pressure than the slow-moving air below the wing.

The greater pressure below the wing exerts an upward force (LIFT: the upward force on an airplane wing from air flow)Wing size, speed, and turbulence affect

lift!High-performance jets need SMALL wingsGliders need large wings

The forward force produced by a plane’s engine is called THRUST.THRUST INCREASE LIFT!Jets have a lot of thrust, but gliders have

none

Drag and Motion in FluidsDRAG is the force that opposes or restricts motion in a fluid

TURBULENCE is an irregular or unpredictable flow of fluids

Airplanes reduce drag by using wing flaps.

Birds reduce drag by adjusting their wing feathers.

Pascal’s PrincipleAccording to Pascal, changes in water

pressure will be transmitted equally through an enclosed fluid.

Pascal’s Principle is used by hydraulic devices to move or lift objectsHydraulic devices can multiply forcesWhen breaks are used to stop a car, Pascal’s Principle is in effect.