CHAPTER 4

Forces in Fluids

4.1 Fluid Pressure

• Objectives–Define pressure and explain how it

is measured.–Identify examples of fluid

pressure.–Predict the effects of fluid

pressure in land and water environments.

Fluid• Name as many fluids as you can

think of in 60 seconds…

• Air is a fluid. A fluid is any substance that can flow.

• People often think of fluids as liquids, such as water or oil. But a gas, like air, is also a fluid.

If you squeeze a bicycle tire, you feel an opposing

force.

The force comes from billions of air particles

striking the inside of the tire producing pressure.

What happens if you put too much air in the tire?

Too much air pressure in the tire will cause it to burst.

Pressure

Pressure

• Any force (including any fluid force) exerted on a surface is called pressure.

• To calculate pressure, you divide the force by the area over which it is applied.

Pressure• Remember force is measured in• Write the equation: pressure = force area• The official SI unit of pressure, the

pascal (Pa), is equal to Newtons per square meter or 1 N/m2

?????NEWTONS

Pressure• What is the pressure exerted by a 10 kg

backpack put onto a rectangular table? The table is 0.5 m2.

• F=ma m=10kg a=9.8m/s2

• F=100N• P= force P=(100) area (0.5)

P= 50 N/m2 or 50 Pa

Fluid Pressure in the Environment

• Air and water are the most common fluids on the earth's surface. As you move up or down in air or water, the pressure around you changes.

• The change may be too small for you to notice. However, your eardrums sometimes detect small pressure changes.

Pressure and the Atmosphere

• Gas (air) particles are pulled toward the earth by the force of gravity.

• The weight of air causes atmospheric pressure.

• Over the earth's entire surface, the

average pressure produced by the atmosphere is about 100,000 Pa.

Atmospheric Pressure at Different Places

MountainsAs you climb a mountain, air

pressure steadily

decreases.

AIR PRESSURE CHANGE

Altitude is the most important factor in surface

airpressure.

Sea LevelThe average

surface pressure measured at sea

level is 101, 300 Pa.

Atmospheric Pressure at Different Places

Edge of SpaceAt an altitude of

150 km, air contains so few gas

particles that the pressure is almost

ZERO.

OZONE LAYER

The air pressure in the ozone layer is less

than 100 Pa.

AIRCRAFTJets travel at an altitude of about 11km. The air

pressure there is only about 25,000 Pa.

Pressure and the Ocean• The particles that

make up water are packed more closely together than the particles in air.

• Water is denser than air.

• So, water exerts more force per unit area than air does.

Water Pressure Beneath the Ocean

Intertidal Zone

Near shore, the water level changes with the tides. In one

day, the pressure can vary as much as

200, 000 Pa.

Continental Shelf

The shelf is up to 200 m deep.

Organisms living on the deepest part must withstand

water pressure of 2 million Pa.

Mid-OceanAt depths of up to 1 km, organisms can withstand a pressure of 10 million Pa.

TrenchThe trenches are the deepest parts of the sea. Pressures can exceed 100 millionPa.

Water Pressure Beneath the Ocean

Pressure and the Ocean• .

DO NOW

1. Page 90 – Read

Answer:Follow the path of pressure through the brake system. What happens if there is a break in the fluid line?

The brakes would fail. The lack of fluid pressure would prevent

the fluid in the cylinders from pushing against the pistons, and the brake pads would not push against the wheel.

Check & Explain pg. 90

1. What is fluid pressure?

2. Give two examples of fluid pressure.

4. Predict What changes in both air and water pressure occur on your body as you leap from a diving board?

4.2 Buoyancy

4.2 Buoyancy

• Objectives–Explain buoyant force.–Compare and contrast density

and buoyancy.–Predict whether an object will

float in water.

Unbalanced Forces

Buoyant Force

• The downward force is gravity.• The upward force acting on the iceberg

is the buoyant force.• The buoyant force opposes gravity.

Two forces act on an iceberg.

Buoyant Force• If the weight of a

submerged object is greater than the buoyant force, the object will sink.

• If the weight is less than the buoyant force, the object will rise to the surface and float.

• If the weight is equal to the buoyant force, the submerged object will remain at any level. The forces are balanced!!

Buoyant Force• To understand buoyant

force, think about what happens to the water level in a glass when you add an ice cube…

• The ice displaces, some of the water.

• The weight of the water that is displaced is equal to the buoyant force. The greater the volume displaced, the greater the buoyant force !

DO NOW

The volume of each of these three cubes is the same.

Why is each cube at a different level in the water?1 - the buoyant force acting upon the block is greater than the downward force of gravity. These unbalanced forces result in an upward force that causes the block to float.

2 - the block's weight is greater than the buoyant force. A downward force results, and the block sinks.

3 - the downward force of gravity equals the upward buoyant force, no movement occurs. The forces balance each other.

1

2

3

ARCHIMEDE‘S PRINCIPLEMore than 2,000 years ago, the Greek mathematician and inventor Archimedes observed a relationship between the buoyant force and the fluid displaced by an object.

According to legend, when he stepped into his bathtub, he noticed that the water level rose. He later reasoned that the weight of the fluid displaced would be equal to the buoyant force.

ARCHIMEDE‘S PRINCIPLEFor example, if an object immersed in water displaces a volume of waterthat weighs 4.9 N, then a buoyant force of 4.9 N acts on the object.

Archimedes' principle applies to all fluids, including air.http://www.youtube.com/watch?v=eQsmq3Hu9HA

Density and BuoyancyTo predict whether an object will sink or float, you

need to consider its density.

DENSITY = MASS / VOLUME

The density of water is 1 g/cm3. Any object with a density greater than 1 g/cm3 will sink in water.

The chest sinks because its density is greater than that of

water.

Density and Buoyancy

http://www.youtube.com/watch?v=MzsORE0ae10&feature=related

Check & Explain pg. 95

1. Describe the forces acting on a floating object.

Buoyant force (upward force) and gravity (downward force).

2. What is Archimedes' principle?The weight of the fluid displaced by an object is equal to

the buoyant force acting on it

3. Compare and Contrast: How are density and buoyancy

similar? How are they different.Density is a property of matter. Buoyancy is a force that acts on matter in a fluid to overcome gravity. Buoyancy depends on the relative densities of both the object and the fluid.

4.3 Forces in Moving Fluids

4.3 Forces in Moving Fluids

• Objectives–Describe Bernoulli 's principle.–Explain how airplanes and birds fly.–Generalize about things that fly.– Infer how an object's shape affects its

movement through a fluid.

Pressure Differences in Moving Fluids

Air is a fluid. When a fluid moves, its pressure drops.

• As the student blows across the paper, he creates a low-pressure region above it.

• The air beneath the paper is not affected by the blowing, therefore, its pressure remains the same.

• The pressure is greater on the bottom of the paper.

• The unbalanced pressures produce an upward force on the paper.

Bernoulli's Principle

In the 1700s, Swiss scientist Daniel Bernoulli studied the relationship

between moving fluids and pressure.

He observed that as the speed of a fluid

increases, the pressure in the fluid decreases. This concept is known as Bernoulli's Principle

Using Pressure Differences for Flight

• The special shape of an airplane wing is called an airfoil.

• An airfoil has a curved upper surface and a relatively flat-bottom surface.

As air moves past an airfoil, the airfoil's shape creates a pressure

difference.

Airplane Lift

The difference between the pressure on the airfoil's upper and lower surfaces results in an upward force, called lift.

When the lift is greater than the plane's weight, the plane rises.

When the lift is less than the plane's weight, the plane sinks..

• The force that pushes the plane forward is called thrust.

• The shape and movement of the propeller blades produce thrust.

Airplane Thrust

As the blades spin, they create a low-pressure region in front of the propeller and

a high pressure region behind it, pushing the plane

forward.

Drag

• The forward motion of an aircraft is slowed by an opposing force called drag.

• Like friction, drag opposes the movement of objects.

• Disturbances in the flow of air or other fluids increase drag.

By designing streamlined aircraft, drag is reduced.

Bird Flight

• Like the wing of an airplane, a birds wing is an airfoil.

• Air passing over the birds wing creates lift.

• A bird flaps its wing producing thrust.

Bird SkeletonBird bones have

pockets of air to make them light

Co-operative Learning Activity

101

GROUP 1GROUP 3

GROUP 2

1. Planes, birds, and boats are streamlined to reduce ________ .

2. The _________ on an object is equal to the weight of the fluid the object displaces.

3. Force per unit area is _________ .4. The shape of an airplane wing is a(n)

________ .5. Any substance that can flow is

a(n)_________ .6. The force that moves a plane forward is

_________ .• 7. As air moves over an airfoil. ___ is

generated.

Check Your Vocabulary

DRAG

Buoyant force

pressure airfoil

fluid

Lift

thrust

Pair each numbered word with a vocabulary term.

8. Friction 9. Wing DRAG AIRFOIL

10. Float 11. Pascal buoyant force pressure

12. Flow 13. Up fluid LIFT

14. Forward 15. Area thrust pressure

16. Pressure 17. Sink lift buoyant force

Check Your Vocabulary